fix: fix disconnect when selecting firmware for dfu

README.md

@@ -4,7 +4,7 @@ A new Flutter project.
 
 ## Operational Docs
 
-- [DFU v1 Operator Guide](docs/dfu-v1-operator-guide.md)
+- [Bootloader OTA Operator Guide](docs/bootloader-ota-operator-guide.md)
 
 ## Getting Started
 

docs/bootloader-ota-operator-guide.md

@@ -0,0 +1,62 @@
+# Bootloader OTA Operator Guide
+
+This guide explains the Universal Shifters single-slot bootloader update flow in `abawo_bt_app`.
+
+## App-Side Flow
+
+1. Connect to the target button and open **Device Details**.
+2. In **Firmware Update**, select a local raw application `.bin` with **Select Firmware**.
+3. The app validates size and vector table before enabling the update.
+4. Review file metadata: size, session id, CRC32, app start, image version, and reset vector.
+5. Tap **Start Update** and keep the phone close to the button.
+6. The app sends `EnterDfu` to the running application, waits for reset, and connects to `US-DFU`.
+7. The app sends bootloader `START`; this erases the active app slot.
+8. The app transfers offset-based frames and tracks bootloader `expected_offset`.
+9. The app sends `FINISH`, waits for final OK, then waits for the bootloader reset.
+10. Success is shown only after the updated app reconnects and status verification passes.
+
+## Image Requirements
+
+- File extension must be `.bin`.
+- Image must be at least 8 bytes and no larger than `0x3F000` bytes (252 KiB).
+- Image bytes must start at application address `0x00030000`.
+- Initial stack pointer must be aligned and within `0x20000000..=0x20010000`.
+- Reset vector must have the Thumb bit set and point inside the image after the first two vector words.
+- Flags are always `0`; encrypted/signed update flags are not supported by the current bootloader.
+- Image version is currently sent as `0` unless a later packaging flow provides it.
+
+## Operational Notes
+
+- Single-slot update is destructive after bootloader `START`; the previous app is erased before image transfer.
+- If transfer fails after `START`, recovery is through bootloader DFU or external reflash.
+- Gear writes and **Connect Button to Bike** stay disabled while OTA is running.
+- If BLE drops during transfer, retry promptly while the bootloader is still advertising `US-DFU`.
+- Cancel after `START` sends bootloader `ABORT` and leaves the device in bootloader/recovery flow.
+
+## Troubleshooting
+
+| Symptom in app | Likely cause | Operator action |
+| --- | --- | --- |
+| Invalid stack pointer or reset vector | `.bin` is not a raw app image for `0x00030000` | Rebuild/export the application image from the correct linker layout. |
+| Could not connect to bootloader DFU mode | Phone did not find `US-DFU` after app reset | Move closer, retry, and verify the device is advertising `US-DFU`. |
+| Timed out waiting for bootloader DFU status | Status indication/read did not arrive | Reconnect to `US-DFU` and retry. |
+| Bootloader status `bounds error` | Image length or app start rejected | Use a valid app image no larger than `0x3F000` bytes (252 KiB). |
+| Bootloader status `CRC error` | Full-image CRC did not match flash contents | Re-export or re-download the `.bin`, then retry. |
+| Bootloader status `vector table error` | Bootloader rejected the written vector table | Rebuild firmware for app start `0x00030000`. |
+| Bootloader status `flash error` | Flash erase/write/read failed | Retry once; if repeated, service or externally reflash the device. |
+| Bootloader status `boot metadata error` | Bootloader could not persist boot metadata | Treat as service risk; retry reflash, then return device if repeated. |
+| Updated app did not reconnect | New app did not boot/confirm or reconnect window expired | Scan for `US-DFU`; if present, retry OTA with a known-good image. |
+| Updated app reconnected but verification failed | Normal app status read failed | Reconnect manually and verify status; retry only if the device is still in bootloader or unusable. |
+
+Escalate with app logs, device identifier, firmware filename/hash, and observed bootloader status when a known-good image repeatedly fails.
+
+## Manual QA Checklist
+
+- [ ] Happy path: select valid `.bin`, enter bootloader, transfer, finish, reboot, reconnect, completed.
+- [ ] Image validation: invalid extension, empty file, too-small file, too-large file, invalid SP, invalid reset vector.
+- [ ] UI state gating: gear ratio save and trainer assignment remain disabled during OTA.
+- [ ] Queue-full/status recovery: app sends `GET_STATUS` and resumes from returned offset.
+- [ ] Cancel path: cancel after `START` sends `[ABORT, session]` and shows canceled state.
+- [ ] Bootloader status errors: CRC/vector/flash/metadata statuses show actionable messages.
+- [ ] Reconnect timeout: no updated app reconnect produces a clear failure message.
+- [ ] Regression check: after successful update, status and firmware telemetry still load normally.

docs/dfu-v1-operator-guide.md

@@ -1,62 +0,0 @@
-# DFU v1 Operator Guide
-
-This guide explains how to run and support firmware updates for Universal Shifters in `abawo_bt_app`.
-
-## App-Side Flow (Operator)
-
-1. Connect to the target button and open **Device Details**.
-2. In **Firmware Update**, select a local `.bin` file with **Select Firmware**.
-3. Confirm file metadata is shown (size, session id, CRC32), then tap **Start Update**.
-4. Monitor progress:
-   - Phase text: `Sending START`, `Waiting for ACK`, `Transferring`, `Finalizing`
-   - Progress bar and bytes sent
-   - Last ACK sequence (`0x..`)
-5. During `Finalizing`, expect a brief disconnect while the device reboots.
-6. The app attempts reconnect + reachability verification automatically.
-7. Success is only shown after reconnect verification passes.
-
-Operational notes:
-- Keep the phone near the button for the full transfer.
-- Keep this screen open until completion.
-- Gear writes and "Connect Button to Bike" are disabled during DFU by design.
-
-## Troubleshooting Matrix
-
-| Symptom in app | Likely cause | Operator action |
-| --- | --- | --- |
-| Preflight fails with MTU too low | Negotiated MTU below minimum required for 64-byte frames (`>=67`) | Reconnect BLE, retry update, and reduce RF interference/distance. |
-| `Timed out waiting for initial DFU ACK after START` | ACK indications not enabled/received, or unstable link | Disconnect/reconnect button, retry update, keep device nearby. |
-| `Upload stalled: no ACK progress ...` | Packet loss or weak BLE link; missing frame prevents cumulative ACK movement | Move closer, reduce interference, retry update; app will rewind and resend from last ACK while running. |
-| `Received malformed ACK indication` | Corrupted/unexpected ACK payload from transport path | Reconnect and retry. If repeatable, capture logs and firmware version for investigation. |
-| `Device did not perform the expected post-FINISH reset disconnect` | Device did not reset after FINISH, or disconnect event was missed | Retry update once. If repeatable, treat as firmware-side finalize/reset issue. |
-| `Device did not reconnect after DFU reset` | Reboot happened but reconnect window expired | Manually reconnect in app and retry update with strong signal. |
-| `post-update verification failed` or verification timeout | Device reconnected but status read failed in verification step | Reconnect and verify normal status manually; retry update only if needed. |
-| Transfer reaches end but completion never succeeds; ACK does not advance after FINISH | Likely CRC mismatch (or device rejected FINISH completeness/integrity checks) | Re-export/re-download firmware `.bin`, reselect file, retry. Do not power cycle mid-transfer. |
-
-Escalate with logs when the same firmware + device repeatedly fails after one clean retry.
-
-## DFU v1 Limitations and Roadmap
-
-Current v1 limitations:
-- The app verifies reachability after reconnect, but **cannot strictly compare old/new firmware version** yet (no version characteristic exposed by device).
-- `START.flags` supports encrypted/signed modes, but the app currently runs plain `.bin` updates and does **not** perform signed/encrypted payload validation.
-
-Roadmap direction:
-- Add device firmware version characteristic and enforce strict version progression checks in-app.
-- Add signed update manifest verification before upload acceptance.
-- Add encrypted payload transport mode and key management flow.
-
-## Manual QA Checklist (Release Validation)
-
-Run on at least one known-good button and firmware image.
-
-- [ ] **Happy path**: Select valid `.bin` -> start -> transfer -> reboot/disconnect -> reconnect -> completed.
-- [ ] **UI state gating**: During DFU, gear ratio save and "Connect Button to Bike" controls stay disabled.
-- [ ] **Cancel path**: Start update, cancel mid-transfer, confirm terminal `canceled` state and safe recovery.
-- [ ] **Preflight MTU failure**: Force low-MTU environment; confirm clear failure message and no transfer start.
-- [ ] **Stalled ACK handling**: In degraded RF conditions, verify retries/rewind behavior and bounded failure messaging.
-- [ ] **Reconnect timeout handling**: Simulate slow/no reconnect after FINISH; confirm explicit reconnect timeout error.
-- [ ] **Bad file validation**: Confirm non-`.bin` and empty file selections are rejected with actionable messages.
-- [ ] **Regression check**: After update attempt (success/failure), reconnect normally and verify status reads still work.
-
-If a checklist item fails, attach app logs, device identifier, firmware filename/hash, and observed phase/error text.

lib/controller/bluetooth.dart

@@ -14,6 +14,13 @@ part 'bluetooth.g.dart';
 
 final log = Logger('BluetoothController');
 
+final backgroundBluetoothDisconnectSuppressionCountProvider =
+    StateProvider<int>((ref) => 0);
+
+final backgroundBluetoothDisconnectSuppressedProvider = Provider<bool>((ref) {
+  return ref.watch(backgroundBluetoothDisconnectSuppressionCountProvider) > 0;
+});
+
 @Riverpod(keepAlive: true)
 FlutterReactiveBle reactiveBle(Ref ref) {
   ref.keepAlive();

lib/main.dart

@@ -57,6 +57,9 @@ class _AbawoBtAppState extends ConsumerState<AbawoBtApp>
   void didChangeAppLifecycleState(AppLifecycleState state) {
     if (state == AppLifecycleState.hidden ||
         state == AppLifecycleState.paused) {
+      if (ref.read(backgroundBluetoothDisconnectSuppressedProvider)) {
+        return;
+      }
       unawaited(_disconnectBluetoothForBackground());
     }
   }

lib/model/firmware_file_selection.dart

@@ -1,21 +1,29 @@
 import 'dart:typed_data';
 
-class DfuV1FirmwareMetadata {
+class BootloaderDfuFirmwareMetadata {
-  const DfuV1FirmwareMetadata({
+  const BootloaderDfuFirmwareMetadata({
     required this.totalLength,
     required this.crc32,
+    required this.appStart,
+    required this.imageVersion,
     required this.sessionId,
     required this.flags,
+    required this.vectorStackPointer,
+    required this.vectorReset,
   });
 
   final int totalLength;
   final int crc32;
+  final int appStart;
+  final int imageVersion;
   final int sessionId;
   final int flags;
+  final int vectorStackPointer;
+  final int vectorReset;
 }
 
-class DfuV1PreparedFirmware {
+class BootloaderDfuPreparedFirmware {
-  const DfuV1PreparedFirmware({
+  const BootloaderDfuPreparedFirmware({
     required this.fileName,
     required this.fileBytes,
     required this.metadata,
@@ -25,7 +33,7 @@ class DfuV1PreparedFirmware {
   final String fileName;
   final String? filePath;
   final Uint8List fileBytes;
-  final DfuV1FirmwareMetadata metadata;
+  final BootloaderDfuFirmwareMetadata metadata;
 }
 
 enum FirmwareSelectionFailureReason {
@@ -33,6 +41,9 @@ enum FirmwareSelectionFailureReason {
   malformedSelection,
   unsupportedExtension,
   emptyFile,
+  imageTooSmall,
+  imageTooLarge,
+  invalidVectorTable,
   readFailed,
 }
 
@@ -52,7 +63,7 @@ class FirmwareFileSelectionResult {
     this.failure,
   });
 
-  final DfuV1PreparedFirmware? firmware;
+  final BootloaderDfuPreparedFirmware? firmware;
   final FirmwareSelectionFailure? failure;
 
   bool get isSuccess => firmware != null;
@@ -60,7 +71,8 @@ class FirmwareFileSelectionResult {
   bool get isCanceled =>
       failure?.reason == FirmwareSelectionFailureReason.canceled;
 
-  static FirmwareFileSelectionResult success(DfuV1PreparedFirmware firmware) {
+  static FirmwareFileSelectionResult success(
+      BootloaderDfuPreparedFirmware firmware) {
     return FirmwareFileSelectionResult._(firmware: firmware);
   }
 

lib/model/shifter_types.dart

@@ -18,7 +18,7 @@ const String universalShifterDfuControlCharacteristicUuid =
     '0993826f-0ee4-4b37-9614-d13ecba40008';
 const String universalShifterDfuDataCharacteristicUuid =
     '0993826f-0ee4-4b37-9614-d13ecba40009';
-const String universalShifterDfuAckCharacteristicUuid =
+const String universalShifterDfuStatusCharacteristicUuid =
     '0993826f-0ee4-4b37-9614-d13ecba4000a';
 const String ftmsServiceUuid = '00001826-0000-1000-8000-00805f9b34fb';
 const String batteryServiceUuid = '0000180f-0000-1000-8000-00805f9b34fb';
@@ -36,14 +36,23 @@ bool isFtmsUuid(Uuid uuid) {
 const int universalShifterDfuOpcodeStart = 0x01;
 const int universalShifterDfuOpcodeFinish = 0x02;
 const int universalShifterDfuOpcodeAbort = 0x03;
+const int universalShifterDfuOpcodeGetStatus = 0x04;
 
 const int universalShifterDfuFrameSizeBytes = 64;
-const int universalShifterDfuFramePayloadSizeBytes = 63;
+const int universalShifterBootloaderDfuDataHeaderSizeBytes = 9;
+const int universalShifterBootloaderDfuMaxPayloadSizeBytes =
+    universalShifterDfuFrameSizeBytes -
+        universalShifterBootloaderDfuDataHeaderSizeBytes;
+const int universalShifterBootloaderDfuStatusSizeBytes = 6;
 const int universalShifterAttWriteOverheadBytes = 3;
-const int universalShifterDfuMinimumMtu =
-    universalShifterDfuFrameSizeBytes + universalShifterAttWriteOverheadBytes;
 const int universalShifterDfuPreferredMtu = 128;
 
+const int universalShifterDfuAppStart = 0x00030000;
+const int universalShifterDfuAppSlotSizeBytes = 0x0003F000;
+const int universalShifterDfuMinimumImageLengthBytes = 8;
+const int universalShifterDfuRamStart = 0x20000000;
+const int universalShifterDfuRamEnd = 0x20010000;
+
 const int universalShifterDfuFlagEncrypted = 0x01;
 const int universalShifterDfuFlagSigned = 0x02;
 const int universalShifterDfuFlagNone = 0x00;
@@ -64,9 +73,14 @@ const int trainerScanDeviceFlagConnectable = 0x08;
 enum DfuUpdateState {
   idle,
   starting,
-  waitingForAck,
+  enteringBootloader,
+  connectingBootloader,
+  waitingForStatus,
+  erasing,
   transferring,
   finishing,
+  rebooting,
+  verifying,
   completed,
   aborted,
   failed,
@@ -105,18 +119,20 @@ class DfuUpdateProgress {
     required this.state,
     required this.totalBytes,
     required this.sentBytes,
-    required this.lastAckedSequence,
+    required this.expectedOffset,
     required this.sessionId,
     required this.flags,
+    this.bootloaderStatus,
     this.errorMessage,
   });
 
   final DfuUpdateState state;
   final int totalBytes;
   final int sentBytes;
-  final int lastAckedSequence;
+  final int expectedOffset;
   final int sessionId;
   final DfuUpdateFlags flags;
+  final DfuBootloaderStatus? bootloaderStatus;
   final String? errorMessage;
 
   double get fractionComplete {
@@ -134,59 +150,47 @@ class DfuUpdateProgress {
       state == DfuUpdateState.failed;
 }
 
-enum DfuPreflightFailureReason {
+enum DfuBootloaderStatusCode {
-  deviceNotConnected,
+  ok(0x00),
-  wrongConnectedDevice,
+  parseError(0x01),
-  mtuRequestFailed,
+  stateError(0x02),
-  mtuTooLow,
+  boundsError(0x03),
+  crcError(0x04),
+  flashError(0x05),
+  unsupportedError(0x06),
+  vectorError(0x07),
+  queueFull(0x08),
+  bootMetadataError(0x09),
+  unknown(-1);
+
+  const DfuBootloaderStatusCode(this.value);
+
+  final int value;
+
+  static DfuBootloaderStatusCode fromRaw(int value) {
+    for (final code in values) {
+      if (code.value == value) {
+        return code;
+      }
+    }
+    return DfuBootloaderStatusCode.unknown;
+  }
 }
 
-class DfuPreflightResult {
+class DfuBootloaderStatus {
-  const DfuPreflightResult._({
+  const DfuBootloaderStatus({
-    required this.requestedMtu,
+    required this.code,
-    required this.requiredMtu,
+    required this.rawCode,
-    required this.negotiatedMtu,
+    required this.sessionId,
-    required this.failureReason,
+    required this.expectedOffset,
-    required this.message,
   });
 
-  final int requestedMtu;
+  final DfuBootloaderStatusCode code;
-  final int requiredMtu;
+  final int rawCode;
-  final int? negotiatedMtu;
+  final int sessionId;
-  final DfuPreflightFailureReason? failureReason;
+  final int expectedOffset;
-  final String? message;
 
-  bool get canStart => failureReason == null;
+  bool get isOk => code == DfuBootloaderStatusCode.ok;
-
-  static DfuPreflightResult ready({
-    required int requestedMtu,
-    required int negotiatedMtu,
-    int requiredMtu = universalShifterDfuMinimumMtu,
-  }) {
-    return DfuPreflightResult._(
-      requestedMtu: requestedMtu,
-      requiredMtu: requiredMtu,
-      negotiatedMtu: negotiatedMtu,
-      failureReason: null,
-      message: null,
-    );
-  }
-
-  static DfuPreflightResult failed({
-    required int requestedMtu,
-    required DfuPreflightFailureReason failureReason,
-    required String message,
-    int requiredMtu = universalShifterDfuMinimumMtu,
-    int? negotiatedMtu,
-  }) {
-    return DfuPreflightResult._(
-      requestedMtu: requestedMtu,
-      requiredMtu: requiredMtu,
-      negotiatedMtu: negotiatedMtu,
-      failureReason: failureReason,
-      message: message,
-    );
-  }
 }
 
 class ShifterErrorInfo {
@@ -253,7 +257,8 @@ enum UniversalShifterCommand {
   stopScan(0x02),
   connectToDevice(0x03),
   disconnect(0x04),
-  turnOff(0x05);
+  turnOff(0x05),
+  enterDfu(0x06);
 
   const UniversalShifterCommand(this.value);
   final int value;

lib/pages/device_details_page.dart

@@ -80,12 +80,12 @@ class _DeviceDetailsPageState extends ConsumerState<DeviceDetailsPage> {
   late final FirmwareFileSelectionService _firmwareFileSelectionService;
   FirmwareUpdateService? _firmwareUpdateService;
   StreamSubscription<DfuUpdateProgress>? _firmwareProgressSubscription;
-  DfuV1PreparedFirmware? _selectedFirmware;
+  BootloaderDfuPreparedFirmware? _selectedFirmware;
   DfuUpdateProgress _dfuProgress = const DfuUpdateProgress(
     state: DfuUpdateState.idle,
     totalBytes: 0,
     sentBytes: 0,
-    lastAckedSequence: 0xFF,
+    expectedOffset: 0,
     sessionId: 0,
     flags: DfuUpdateFlags(),
   );
@@ -99,9 +99,14 @@ class _DeviceDetailsPageState extends ConsumerState<DeviceDetailsPage> {
     }
     switch (_dfuProgress.state) {
       case DfuUpdateState.starting:
-      case DfuUpdateState.waitingForAck:
+      case DfuUpdateState.enteringBootloader:
+      case DfuUpdateState.connectingBootloader:
+      case DfuUpdateState.waitingForStatus:
+      case DfuUpdateState.erasing:
       case DfuUpdateState.transferring:
       case DfuUpdateState.finishing:
+      case DfuUpdateState.rebooting:
+      case DfuUpdateState.verifying:
         return true;
       case DfuUpdateState.idle:
       case DfuUpdateState.completed:
@@ -492,10 +497,6 @@ class _DeviceDetailsPageState extends ConsumerState<DeviceDetailsPage> {
   }
 
   Future<FirmwareUpdateService?> _ensureFirmwareUpdateService() async {
-    final shifter = _shifterService;
-    if (shifter == null) {
-      return null;
-    }
     if (_firmwareUpdateService != null) {
       return _firmwareUpdateService;
     }
@@ -508,7 +509,7 @@ class _DeviceDetailsPageState extends ConsumerState<DeviceDetailsPage> {
 
     final service = FirmwareUpdateService(
       transport: ShifterFirmwareUpdateTransport(
-        shifterService: shifter,
+        shifterService: _shifterService,
         bluetoothController: bluetooth,
         buttonDeviceId: widget.deviceAddress,
       ),
@@ -526,7 +527,7 @@ class _DeviceDetailsPageState extends ConsumerState<DeviceDetailsPage> {
         }
         if (progress.state == DfuUpdateState.completed) {
           _firmwareUserMessage =
-              'Firmware update completed. The button rebooted and reconnected.';
+              'Firmware update completed. The bootloader rebooted into the updated app and verification passed.';
         }
         if (progress.state == DfuUpdateState.aborted) {
           _firmwareUserMessage = 'Firmware update canceled.';
@@ -548,7 +549,19 @@ class _DeviceDetailsPageState extends ConsumerState<DeviceDetailsPage> {
       _firmwareUserMessage = null;
     });
 
-    final result = await _firmwareFileSelectionService.selectAndPrepareDfuV1();
+    final suppressionCount = ref.read(
+      backgroundBluetoothDisconnectSuppressionCountProvider.notifier,
+    );
+    suppressionCount.state += 1;
+
+    final FirmwareFileSelectionResult result;
+    try {
+      result =
+          await _firmwareFileSelectionService.selectAndPrepareBootloaderDfu();
+    } finally {
+      suppressionCount.state =
+          suppressionCount.state <= 0 ? 0 : suppressionCount.state - 1;
+    }
     if (!mounted) {
       return;
     }
@@ -558,7 +571,7 @@ class _DeviceDetailsPageState extends ConsumerState<DeviceDetailsPage> {
       if (result.isSuccess) {
         _selectedFirmware = result.firmware;
         _firmwareUserMessage =
-            'Selected ${result.firmware!.fileName}. Ready to start update.';
+            'Validated ${result.firmware!.fileName}. Ready for bootloader update.';
       } else if (!result.isCanceled) {
         _firmwareUserMessage = result.failure?.message;
       }
@@ -579,7 +592,6 @@ class _DeviceDetailsPageState extends ConsumerState<DeviceDetailsPage> {
       return;
     }
 
-    await _startStatusStreamingIfNeeded();
     final updater = await _ensureFirmwareUpdateService();
     if (!mounted) {
       return;
@@ -595,12 +607,14 @@ class _DeviceDetailsPageState extends ConsumerState<DeviceDetailsPage> {
     setState(() {
       _isStartingFirmwareUpdate = true;
       _firmwareUserMessage =
-          'Starting update. Keep this screen open and stay near the button.';
+          'Requesting bootloader mode. Keep this screen open and stay near the button.';
     });
 
     final result = await updater.startUpdate(
       imageBytes: firmware.fileBytes,
       sessionId: firmware.metadata.sessionId,
+      appStart: firmware.metadata.appStart,
+      imageVersion: firmware.metadata.imageVersion,
       flags: DfuUpdateFlags.fromRaw(firmware.metadata.flags),
     );
 
@@ -626,13 +640,23 @@ class _DeviceDetailsPageState extends ConsumerState<DeviceDetailsPage> {
       case DfuUpdateState.idle:
         return 'Idle';
       case DfuUpdateState.starting:
-        return 'Sending START command';
+        return 'Preparing update';
-      case DfuUpdateState.waitingForAck:
+      case DfuUpdateState.enteringBootloader:
-        return 'Waiting for ACK from button';
+        return 'Requesting bootloader mode';
+      case DfuUpdateState.connectingBootloader:
+        return 'Connecting to bootloader';
+      case DfuUpdateState.waitingForStatus:
+        return 'Waiting for bootloader status';
+      case DfuUpdateState.erasing:
+        return 'Starting destructive bootloader update';
       case DfuUpdateState.transferring:
-        return 'Transferring firmware frames';
+        return 'Transferring firmware image';
       case DfuUpdateState.finishing:
-        return 'Finalizing update and waiting for reboot/reconnect';
+        return 'Finalizing bootloader update';
+      case DfuUpdateState.rebooting:
+        return 'Waiting for updated app reboot';
+      case DfuUpdateState.verifying:
+        return 'Verifying updated app';
       case DfuUpdateState.completed:
         return 'Update completed';
       case DfuUpdateState.aborted:
@@ -652,10 +676,6 @@ class _DeviceDetailsPageState extends ConsumerState<DeviceDetailsPage> {
     return '${(bytes / (1024 * 1024)).toStringAsFixed(2)} MB';
   }
 
-  String _hexByte(int value) {
-    return '0x${(value & 0xFF).toRadixString(16).padLeft(2, '0').toUpperCase()}';
-  }
-
   Future<void> _manualReconnect() async {
     if (_isManualReconnectRunning || _isFirmwareUpdateBusy) {
       return;
@@ -726,7 +746,7 @@ class _DeviceDetailsPageState extends ConsumerState<DeviceDetailsPage> {
       ScaffoldMessenger.of(context).showSnackBar(
         const SnackBar(
           content: Text(
-              'Firmware update is running. Keep this screen open until it completes.'),
+              'Firmware update is running. Do not close this screen or the app until it completes.'),
         ),
       );
       return;
@@ -739,6 +759,21 @@ class _DeviceDetailsPageState extends ConsumerState<DeviceDetailsPage> {
     context.go('/devices');
   }
 
+  void _dismissFirmwareFullscreen() {
+    setState(() {
+      _dfuProgress = const DfuUpdateProgress(
+        state: DfuUpdateState.idle,
+        totalBytes: 0,
+        sentBytes: 0,
+        expectedOffset: 0,
+        sessionId: 0,
+        flags: DfuUpdateFlags(),
+      );
+      _firmwareUserMessage = null;
+      _selectedFirmware = null;
+    });
+  }
+
   void _showStatusHistory() {
     showModalBottomSheet<void>(
       context: context,
@@ -858,12 +893,30 @@ class _DeviceDetailsPageState extends ConsumerState<DeviceDetailsPage> {
         currentConnectionStatus == ConnectionStatus.connected;
     final hasDeviceAccess =
         isCurrentConnected && _shifterService != null && _latestStatus != null;
+    final canUseFirmwareUpdate = hasDeviceAccess;
     final canSelectFirmware =
-        hasDeviceAccess && !_isSelectingFirmware && !_isFirmwareUpdateBusy;
+        canUseFirmwareUpdate && !_isSelectingFirmware && !_isFirmwareUpdateBusy;
-    final canStartFirmware = hasDeviceAccess &&
+    final canStartFirmware = canUseFirmwareUpdate &&
         !_isSelectingFirmware &&
         !_isFirmwareUpdateBusy &&
         _selectedFirmware != null;
+    if (_isFirmwareUpdateBusy ||
+        (_dfuProgress.state != DfuUpdateState.idle &&
+            _dfuProgress.state != DfuUpdateState.completed &&
+            _dfuProgress.state != DfuUpdateState.failed)) {
+      return _FirmwareUpdateFullscreen(
+        progress: _dfuProgress,
+        selectedFirmware: _selectedFirmware,
+        phaseText: _dfuPhaseText(_dfuProgress.state),
+        statusText: _firmwareUserMessage,
+        formattedProgressBytes:
+            '${_formatBytes(_dfuProgress.sentBytes)} / ${_formatBytes(_dfuProgress.totalBytes)}',
+        expectedOffsetHex:
+            '0x${_dfuProgress.expectedOffset.toRadixString(16).padLeft(8, '0').toUpperCase()}',
+        onDismiss: _dismissFirmwareFullscreen,
+      );
+    }
+
     return PopScope(
       canPop: false,
       onPopInvokedWithResult: (bool didPop, bool? result) {
@@ -893,8 +946,26 @@ class _DeviceDetailsPageState extends ConsumerState<DeviceDetailsPage> {
                     status: _latestStatus,
                   ),
                   const SizedBox(height: 20),
-                  if (hasDeviceAccess) ...[
+                  if (canUseFirmwareUpdate) ...[
+                    _FirmwareUpdateCard(
+                      selectedFirmware: _selectedFirmware,
+                      progress: _dfuProgress,
+                      isSelecting: _isSelectingFirmware,
+                      isStarting: _isStartingFirmwareUpdate,
+                      canSelect: canSelectFirmware,
+                      canStart: canStartFirmware,
+                      phaseText: _dfuPhaseText(_dfuProgress.state),
+                      statusText: _firmwareUserMessage,
+                      formattedProgressBytes:
+                          '${_formatBytes(_dfuProgress.sentBytes)} / ${_formatBytes(_dfuProgress.totalBytes)}',
+                      onSelectFirmware: _selectFirmwareFile,
+                      onStartUpdate: _startFirmwareUpdate,
+                      expectedOffsetHex:
+                          '0x${_dfuProgress.expectedOffset.toRadixString(16).padLeft(8, '0').toUpperCase()}',
+                    ),
                     const SizedBox(height: 16),
+                  ],
+                  if (hasDeviceAccess) ...[
                     _StatusBanner(
                       status: _latestStatus,
                       onTap: _showStatusHistory,
@@ -910,22 +981,16 @@ class _DeviceDetailsPageState extends ConsumerState<DeviceDetailsPage> {
                     const SizedBox(height: 16),
                     _TrainerConnectionCard(
                       status: _latestStatus,
-                      onAssign:
+                      onAssign: _connectButtonToBike,
-                          _isFirmwareUpdateBusy ? null : _connectButtonToBike,
                       onShowStatusConsole: _showStatusHistory,
                     ),
                     const SizedBox(height: 16),
-                    Opacity(
+                    GearRatioEditorCard(
-                      opacity: _isFirmwareUpdateBusy ? 0.6 : 1,
-                      child: AbsorbPointer(
-                        absorbing: _isFirmwareUpdateBusy,
-                        child: GearRatioEditorCard(
                       ratios: _gearRatios,
                       defaultGearIndex: _defaultGearIndex,
                       isLoading: _isGearRatiosLoading,
                       errorText: _gearRatiosError,
-                          onRetry:
+                      onRetry: _loadGearRatios,
-                              _isFirmwareUpdateBusy ? null : _loadGearRatios,
                       onSave: _saveGearRatios,
                       presets: const [
                         GearRatioPreset(
@@ -993,29 +1058,11 @@ class _DeviceDetailsPageState extends ConsumerState<DeviceDetailsPage> {
                         ),
                       ],
                     ),
-                      ),
-                    ),
-                    const SizedBox(height: 16),
-                    _FirmwareUpdateCard(
-                      selectedFirmware: _selectedFirmware,
-                      progress: _dfuProgress,
-                      isSelecting: _isSelectingFirmware,
-                      isStarting: _isStartingFirmwareUpdate,
-                      canSelect: canSelectFirmware,
-                      canStart: canStartFirmware,
-                      phaseText: _dfuPhaseText(_dfuProgress.state),
-                      statusText: _firmwareUserMessage,
-                      formattedProgressBytes:
-                          '${_formatBytes(_dfuProgress.sentBytes)} / ${_formatBytes(_dfuProgress.totalBytes)}',
-                      onSelectFirmware: _selectFirmwareFile,
-                      onStartUpdate: _startFirmwareUpdate,
-                      ackSequenceHex: _hexByte(_dfuProgress.lastAckedSequence),
-                    ),
                   ] else if (isCurrentConnected) ...[
                     _PairingRequiredCard(
                       isChecking: _isPairingCheckRunning,
                       errorText: _pairingError,
-                      onRetry: _isFirmwareUpdateBusy ? null : _retryPairing,
+                      onRetry: _retryPairing,
                       onOpenBluetoothSettings: _openPairingSettings,
                     ),
                   ] else ...[
@@ -1056,12 +1103,12 @@ class _FirmwareUpdateCard extends StatelessWidget {
     required this.phaseText,
     required this.statusText,
     required this.formattedProgressBytes,
-    required this.ackSequenceHex,
+    required this.expectedOffsetHex,
     required this.onSelectFirmware,
     required this.onStartUpdate,
   });
 
-  final DfuV1PreparedFirmware? selectedFirmware;
+  final BootloaderDfuPreparedFirmware? selectedFirmware;
   final DfuUpdateProgress progress;
   final bool isSelecting;
   final bool isStarting;
@@ -1070,7 +1117,7 @@ class _FirmwareUpdateCard extends StatelessWidget {
   final String phaseText;
   final String? statusText;
   final String formattedProgressBytes;
-  final String ackSequenceHex;
+  final String expectedOffsetHex;
   final Future<void> Function() onSelectFirmware;
   final Future<void> Function() onStartUpdate;
 
@@ -1082,9 +1129,33 @@ class _FirmwareUpdateCard extends StatelessWidget {
 
   bool get _showRebootExpectation {
     return progress.state == DfuUpdateState.finishing ||
+        progress.state == DfuUpdateState.rebooting ||
+        progress.state == DfuUpdateState.verifying ||
         progress.state == DfuUpdateState.completed;
   }
 
+  String? get _bootloaderStatusText {
+    final status = progress.bootloaderStatus;
+    if (status == null) {
+      return null;
+    }
+    final codeLabel = switch (status.code) {
+      DfuBootloaderStatusCode.ok => 'OK',
+      DfuBootloaderStatusCode.parseError => 'parse error',
+      DfuBootloaderStatusCode.stateError => 'state error',
+      DfuBootloaderStatusCode.boundsError => 'bounds error',
+      DfuBootloaderStatusCode.crcError => 'CRC error',
+      DfuBootloaderStatusCode.flashError => 'flash error',
+      DfuBootloaderStatusCode.unsupportedError => 'unsupported flags',
+      DfuBootloaderStatusCode.vectorError => 'vector table error',
+      DfuBootloaderStatusCode.queueFull => 'queue full',
+      DfuBootloaderStatusCode.bootMetadataError => 'boot metadata error',
+      DfuBootloaderStatusCode.unknown =>
+        'unknown 0x${status.rawCode.toRadixString(16).padLeft(2, '0').toUpperCase()}',
+    };
+    return 'Bootloader status: $codeLabel, session ${status.sessionId}, expected offset 0x${status.expectedOffset.toRadixString(16).padLeft(8, '0').toUpperCase()}';
+  }
+
   @override
   Widget build(BuildContext context) {
     final theme = Theme.of(context);
@@ -1109,7 +1180,7 @@ class _FirmwareUpdateCard extends StatelessWidget {
             ),
             const SizedBox(height: 8),
             Text(
-              'Select a firmware image, review the transfer state, and start the update when ready.',
+              'Select a raw app image for the single-slot bootloader. Once START is accepted, the active app slot is erased.',
               style: theme.textTheme.bodyMedium?.copyWith(
                 color: colorScheme.onSurface.withValues(alpha: 0.68),
               ),
@@ -1168,6 +1239,11 @@ class _FirmwareUpdateCard extends StatelessWidget {
                       'Size: ${selectedFirmware!.fileBytes.length} bytes  |  Session: ${selectedFirmware!.metadata.sessionId}  |  CRC32: 0x${selectedFirmware!.metadata.crc32.toRadixString(16).padLeft(8, '0').toUpperCase()}',
                       style: theme.textTheme.bodySmall,
                     ),
+                    const SizedBox(height: 2),
+                    Text(
+                      'App start: 0x${selectedFirmware!.metadata.appStart.toRadixString(16).padLeft(8, '0').toUpperCase()}  |  Image version: ${selectedFirmware!.metadata.imageVersion}  |  Reset: 0x${selectedFirmware!.metadata.vectorReset.toRadixString(16).padLeft(8, '0').toUpperCase()}',
+                      style: theme.textTheme.bodySmall,
+                    ),
                   ],
                 ],
               ),
@@ -1183,14 +1259,21 @@ class _FirmwareUpdateCard extends StatelessWidget {
               ),
               const SizedBox(height: 6),
               Text(
-                '${progress.percentComplete}%  •  $formattedProgressBytes  •  Last ACK $ackSequenceHex',
+                '${progress.percentComplete}%  •  $formattedProgressBytes  •  Expected offset $expectedOffsetHex',
                 style: theme.textTheme.bodySmall,
               ),
+              if (_bootloaderStatusText != null) ...[
+                const SizedBox(height: 4),
+                Text(
+                  _bootloaderStatusText!,
+                  style: theme.textTheme.bodySmall,
+                ),
+              ],
             ],
             if (_showRebootExpectation) ...[
               const SizedBox(height: 8),
               Text(
-                'Expected behavior: the button reboots after FINISH, disconnects briefly, then reconnects.',
+                'Expected behavior: after FINISH, the bootloader verifies the image, resets, and the updated app confirms itself before reconnecting.',
                 style: theme.textTheme.bodySmall?.copyWith(
                   color: colorScheme.primary,
                   fontWeight: FontWeight.w600,
@@ -1873,3 +1956,244 @@ class _OverviewMetricTile extends StatelessWidget {
     );
   }
 }
+
+class _FirmwareUpdateFullscreen extends StatelessWidget {
+  const _FirmwareUpdateFullscreen({
+    required this.progress,
+    required this.selectedFirmware,
+    required this.phaseText,
+    required this.statusText,
+    required this.formattedProgressBytes,
+    required this.expectedOffsetHex,
+    required this.onDismiss,
+  });
+
+  final DfuUpdateProgress progress;
+  final BootloaderDfuPreparedFirmware? selectedFirmware;
+  final String phaseText;
+  final String? statusText;
+  final String formattedProgressBytes;
+  final String expectedOffsetHex;
+  final VoidCallback onDismiss;
+
+  bool get _isTerminal =>
+      progress.state == DfuUpdateState.completed ||
+      progress.state == DfuUpdateState.failed;
+
+  bool get _isRunning => !_isTerminal && progress.state != DfuUpdateState.idle;
+
+  String? get _bootloaderStatusText {
+    final status = progress.bootloaderStatus;
+    if (status == null) {
+      return null;
+    }
+    final codeLabel = switch (status.code) {
+      DfuBootloaderStatusCode.ok => 'OK',
+      DfuBootloaderStatusCode.parseError => 'parse error',
+      DfuBootloaderStatusCode.stateError => 'state error',
+      DfuBootloaderStatusCode.boundsError => 'bounds error',
+      DfuBootloaderStatusCode.crcError => 'CRC error',
+      DfuBootloaderStatusCode.flashError => 'flash error',
+      DfuBootloaderStatusCode.unsupportedError => 'unsupported flags',
+      DfuBootloaderStatusCode.vectorError => 'vector table error',
+      DfuBootloaderStatusCode.queueFull => 'queue full',
+      DfuBootloaderStatusCode.bootMetadataError => 'boot metadata error',
+      DfuBootloaderStatusCode.unknown =>
+        'unknown 0x${status.rawCode.toRadixString(16).padLeft(2, '0').toUpperCase()}',
+    };
+    return '$codeLabel  •  session ${status.sessionId}  •  offset 0x${status.expectedOffset.toRadixString(16).padLeft(8, '0').toUpperCase()}';
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    final colorScheme = theme.colorScheme;
+    final isFailed = progress.state == DfuUpdateState.failed;
+
+    return PopScope(
+      canPop: false,
+      child: Scaffold(
+        backgroundColor: colorScheme.surface,
+        body: SafeArea(
+          child: Column(
+            children: [
+              if (_isRunning)
+                Container(
+                  width: double.infinity,
+                  padding:
+                      const EdgeInsets.symmetric(horizontal: 20, vertical: 12),
+                  color: colorScheme.errorContainer,
+                  child: Row(
+                    children: [
+                      Icon(Icons.warning_amber_rounded,
+                          color: colorScheme.onErrorContainer, size: 20),
+                      const SizedBox(width: 10),
+                      Expanded(
+                        child: Text(
+                          'Do not close the app, lock the phone, or move away from the button.',
+                          style: theme.textTheme.bodySmall?.copyWith(
+                            color: colorScheme.onErrorContainer,
+                            fontWeight: FontWeight.w600,
+                          ),
+                        ),
+                      ),
+                    ],
+                  ),
+                ),
+              Expanded(
+                child: SingleChildScrollView(
+                  padding: const EdgeInsets.fromLTRB(24, 32, 24, 24),
+                  child: Column(
+                    crossAxisAlignment: CrossAxisAlignment.center,
+                    children: [
+                      Icon(
+                        _isTerminal
+                            ? (isFailed
+                                ? Icons.error_outline_rounded
+                                : Icons.check_circle_outline_rounded)
+                            : Icons.system_update_alt_rounded,
+                        size: 56,
+                        color: _isTerminal
+                            ? (isFailed
+                                ? colorScheme.error
+                                : colorScheme.primary)
+                            : colorScheme.primary,
+                      ),
+                      const SizedBox(height: 16),
+                      Text(
+                        _isTerminal
+                            ? (isFailed ? 'Update failed' : 'Update completed')
+                            : 'Updating firmware',
+                        style: theme.textTheme.headlineSmall?.copyWith(
+                          fontWeight: FontWeight.w700,
+                        ),
+                      ),
+                      const SizedBox(height: 8),
+                      Text(
+                        phaseText,
+                        style: theme.textTheme.bodyLarge?.copyWith(
+                          color: colorScheme.onSurface.withValues(alpha: 0.72),
+                        ),
+                      ),
+                      if (selectedFirmware != null) ...[
+                        const SizedBox(height: 12),
+                        Text(
+                          '${selectedFirmware!.fileName}  •  ${_formatBytes(selectedFirmware!.fileBytes.length)}',
+                          style: theme.textTheme.bodySmall,
+                        ),
+                      ],
+                      const SizedBox(height: 24),
+                      if (_isRunning) ...[
+                        LinearProgressIndicator(
+                          value: progress.totalBytes > 0
+                              ? progress.fractionComplete
+                              : null,
+                          minHeight: 12,
+                          borderRadius: BorderRadius.circular(999),
+                        ),
+                        const SizedBox(height: 12),
+                        Text(
+                          '${progress.percentComplete}%  •  $formattedProgressBytes',
+                          style: theme.textTheme.bodyMedium?.copyWith(
+                            fontWeight: FontWeight.w600,
+                          ),
+                        ),
+                      ],
+                      if (progress.state == DfuUpdateState.finishing ||
+                          progress.state == DfuUpdateState.rebooting ||
+                          progress.state == DfuUpdateState.verifying) ...[
+                        const SizedBox(height: 20),
+                        Container(
+                          width: double.infinity,
+                          padding: const EdgeInsets.all(14),
+                          decoration: BoxDecoration(
+                            color: colorScheme.primaryContainer
+                                .withValues(alpha: 0.36),
+                            borderRadius: BorderRadius.circular(14),
+                          ),
+                          child: Row(
+                            children: [
+                              SizedBox(
+                                width: 18,
+                                height: 18,
+                                child: CircularProgressIndicator(
+                                    strokeWidth: 2, color: colorScheme.primary),
+                              ),
+                              const SizedBox(width: 10),
+                              Expanded(
+                                child: Text(
+                                  'Bootloader is verifying, resetting, and booting the new app. Keep the screen open.',
+                                  style: theme.textTheme.bodySmall?.copyWith(
+                                    color: colorScheme.primary,
+                                    fontWeight: FontWeight.w600,
+                                  ),
+                                ),
+                              ),
+                            ],
+                          ),
+                        ),
+                      ],
+                      if (_bootloaderStatusText != null) ...[
+                        const SizedBox(height: 12),
+                        Text(
+                          _bootloaderStatusText!,
+                          style: theme.textTheme.bodySmall?.copyWith(
+                            color:
+                                colorScheme.onSurface.withValues(alpha: 0.56),
+                          ),
+                        ),
+                      ],
+                      if (statusText != null &&
+                          statusText!.trim().isNotEmpty) ...[
+                        const SizedBox(height: 20),
+                        Container(
+                          width: double.infinity,
+                          padding: const EdgeInsets.all(14),
+                          decoration: BoxDecoration(
+                            color: isFailed
+                                ? colorScheme.errorContainer
+                                : colorScheme.surfaceContainerHighest,
+                            borderRadius: BorderRadius.circular(14),
+                          ),
+                          child: Text(
+                            statusText!,
+                            style: theme.textTheme.bodyMedium?.copyWith(
+                              color: isFailed
+                                  ? colorScheme.onErrorContainer
+                                  : null,
+                            ),
+                          ),
+                        ),
+                      ],
+                      if (_isTerminal) ...[
+                        const SizedBox(height: 32),
+                        SizedBox(
+                          width: double.infinity,
+                          child: FilledButton.icon(
+                            onPressed: onDismiss,
+                            icon: Icon(isFailed
+                                ? Icons.arrow_back_rounded
+                                : Icons.check_rounded),
+                            label: Text(
+                              isFailed ? 'Back to device' : 'Done',
+                            ),
+                          ),
+                        ),
+                      ],
+                    ],
+                  ),
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+
+  String _formatBytes(int bytes) {
+    if (bytes < 1024) return '$bytes B';
+    if (bytes < 1024 * 1024) return '${(bytes / 1024).toStringAsFixed(1)} KB';
+    return '${(bytes / (1024 * 1024)).toStringAsFixed(2)} MB';
+  }
+}

lib/service/dfu_protocol.dart

@@ -2,112 +2,170 @@ import 'dart:typed_data';
 
 import 'package:abawo_bt_app/model/shifter_types.dart';
 
-const int _startPayloadLength = 11;
+const int _startPayloadLength = 19;
 
-class DfuStartPayload {
+class BootloaderDfuStartPayload {
-  const DfuStartPayload({
+  const BootloaderDfuStartPayload({
     required this.totalLength,
     required this.imageCrc32,
+    this.appStart = universalShifterDfuAppStart,
+    this.imageVersion = 0,
     required this.sessionId,
     required this.flags,
   });
 
   final int totalLength;
   final int imageCrc32;
+  final int appStart;
+  final int imageVersion;
   final int sessionId;
   final int flags;
 }
 
-class DfuDataFrame {
+class BootloaderDfuDataFrame {
-  const DfuDataFrame({
+  const BootloaderDfuDataFrame({
-    required this.sequence,
+    required this.sessionId,
     required this.offset,
     required this.payloadLength,
     required this.bytes,
   });
 
-  final int sequence;
+  final int sessionId;
   final int offset;
   final int payloadLength;
   final Uint8List bytes;
 }
 
-class DfuProtocol {
+class BootloaderDfuProtocol {
-  const DfuProtocol._();
+  const BootloaderDfuProtocol._();
 
-  static Uint8List encodeStartPayload(DfuStartPayload payload) {
+  static const int crc32Initial = 0xFFFFFFFF;
+  static const int _crc32PolynomialReflected = 0xEDB88320;
+
+  static Uint8List encodeStartPayload(BootloaderDfuStartPayload payload) {
     final data = ByteData(_startPayloadLength);
     data.setUint8(0, universalShifterDfuOpcodeStart);
     data.setUint32(1, payload.totalLength, Endian.little);
     data.setUint32(5, payload.imageCrc32, Endian.little);
-    data.setUint8(9, payload.sessionId);
+    data.setUint32(9, payload.appStart, Endian.little);
-    data.setUint8(10, payload.flags);
+    data.setUint32(13, payload.imageVersion, Endian.little);
+    data.setUint8(17, payload.sessionId & 0xFF);
+    data.setUint8(18, payload.flags & 0xFF);
     return data.buffer.asUint8List();
   }
 
-  static Uint8List encodeFinishPayload() {
+  static Uint8List encodeFinishPayload(int sessionId) {
-    return Uint8List.fromList([universalShifterDfuOpcodeFinish]);
+    return Uint8List.fromList([
+      universalShifterDfuOpcodeFinish,
+      sessionId & 0xFF,
+    ]);
   }
 
-  static Uint8List encodeAbortPayload() {
+  static Uint8List encodeAbortPayload(int sessionId) {
-    return Uint8List.fromList([universalShifterDfuOpcodeAbort]);
+    return Uint8List.fromList([
+      universalShifterDfuOpcodeAbort,
+      sessionId & 0xFF,
+    ]);
   }
 
-  static List<DfuDataFrame> buildDataFrames(
+  static Uint8List encodeGetStatusPayload() {
-    List<int> imageBytes, {
+    return Uint8List.fromList([universalShifterDfuOpcodeGetStatus]);
-    int startSequence = 0,
+  }
+
+  static BootloaderDfuDataFrame buildDataFrame({
+    required List<int> imageBytes,
+    required int sessionId,
+    required int offset,
+    int payloadSize = universalShifterBootloaderDfuMaxPayloadSizeBytes,
   }) {
-    final frames = <DfuDataFrame>[];
+    if (offset < 0 || offset >= imageBytes.length) {
-    var seq = _asU8(startSequence);
+      throw RangeError.range(offset, 0, imageBytes.length - 1, 'offset');
-    var offset = 0;
+    }
-    while (offset < imageBytes.length) {
+    if (payloadSize <= 0 ||
+        payloadSize > universalShifterBootloaderDfuMaxPayloadSizeBytes) {
+      throw RangeError.range(
+        payloadSize,
+        1,
+        universalShifterBootloaderDfuMaxPayloadSizeBytes,
+        'payloadSize',
+      );
+    }
+
     final remaining = imageBytes.length - offset;
-      final chunkLength = remaining < universalShifterDfuFramePayloadSizeBytes
+    final payloadLength = remaining < payloadSize ? remaining : payloadSize;
-          ? remaining
+    final payloadEnd = offset + payloadLength;
-          : universalShifterDfuFramePayloadSizeBytes;
+    final payload = imageBytes.sublist(offset, payloadEnd);
-
+    final frame = Uint8List(
-      final frame = Uint8List(universalShifterDfuFrameSizeBytes);
+      universalShifterBootloaderDfuDataHeaderSizeBytes + payloadLength,
-      frame[0] = seq;
-      frame.setRange(1, 1 + chunkLength, imageBytes, offset);
-
-      frames.add(
-        DfuDataFrame(
-          sequence: seq,
-          offset: offset,
-          payloadLength: chunkLength,
-          bytes: frame,
-        ),
     );
 
-      offset += chunkLength;
+    frame[0] = sessionId & 0xFF;
-      seq = nextSequence(seq);
+    final data = ByteData.view(frame.buffer);
+    data.setUint32(1, offset, Endian.little);
+    data.setUint32(5, crc32(payload), Endian.little);
+    frame.setRange(
+      universalShifterBootloaderDfuDataHeaderSizeBytes,
+      universalShifterBootloaderDfuDataHeaderSizeBytes + payloadLength,
+      payload,
+    );
+
+    return BootloaderDfuDataFrame(
+      sessionId: sessionId & 0xFF,
+      offset: offset,
+      payloadLength: payloadLength,
+      bytes: frame,
+    );
   }
 
+  static List<BootloaderDfuDataFrame> buildDataFrames({
+    required List<int> imageBytes,
+    required int sessionId,
+    int payloadSize = universalShifterBootloaderDfuMaxPayloadSizeBytes,
+  }) {
+    final frames = <BootloaderDfuDataFrame>[];
+    var offset = 0;
+    while (offset < imageBytes.length) {
+      final frame = buildDataFrame(
+        imageBytes: imageBytes,
+        sessionId: sessionId,
+        offset: offset,
+        payloadSize: payloadSize,
+      );
+      frames.add(frame);
+      offset += frame.payloadLength;
+    }
     return frames;
   }
 
-  static int nextSequence(int sequence) {
+  static int maxPayloadSizeForMtu(int negotiatedMtu) {
-    return _asU8(sequence + 1);
+    final writePayloadBytes =
+        negotiatedMtu - universalShifterAttWriteOverheadBytes;
+    final availablePayload =
+        writePayloadBytes - universalShifterBootloaderDfuDataHeaderSizeBytes;
+    if (availablePayload <= 0) {
+      return 0;
+    }
+    if (availablePayload > universalShifterBootloaderDfuMaxPayloadSizeBytes) {
+      return universalShifterBootloaderDfuMaxPayloadSizeBytes;
+    }
+    return availablePayload;
   }
 
-  static int rewindSequenceFromAck(int acknowledgedSequence) {
+  static DfuBootloaderStatus parseStatusPayload(List<int> payload) {
-    return nextSequence(acknowledgedSequence);
+    if (payload.length != universalShifterBootloaderDfuStatusSizeBytes) {
+      throw const FormatException(
+          'DFU status payload must be exactly 6 bytes.');
     }
-
+    final data = ByteData.sublistView(Uint8List.fromList(payload));
-  static int sequenceDistance(int from, int to) {
+    final rawCode = data.getUint8(0);
-    return _asU8(to - from);
+    return DfuBootloaderStatus(
+      code: DfuBootloaderStatusCode.fromRaw(rawCode),
+      rawCode: rawCode,
+      sessionId: data.getUint8(1),
+      expectedOffset: data.getUint32(2, Endian.little),
+    );
   }
 
-  static int parseAckPayload(List<int> payload) {
-    if (payload.length != 1) {
-      throw const FormatException('ACK payload must be exactly 1 byte.');
-    }
-    return _asU8(payload.first);
-  }
-
-  static const int crc32Initial = 0xFFFFFFFF;
-  static const int _crc32PolynomialReflected = 0xEDB88320;
-
   static int crc32Update(int crc, List<int> bytes) {
     var next = crc & 0xFFFFFFFF;
     for (final byte in bytes) {
@@ -130,8 +188,4 @@ class DfuProtocol {
   static int crc32(List<int> bytes) {
     return crc32Finalize(crc32Update(crc32Initial, bytes));
   }
-
-  static int _asU8(int value) {
-    return value & 0xFF;
-  }
 }

lib/service/firmware_file_selection_service.dart

@@ -75,7 +75,7 @@ class FirmwareFileSelectionService {
   final FirmwareFilePicker _filePicker;
   final SessionIdGenerator _sessionIdGenerator;
 
-  Future<FirmwareFileSelectionResult> selectAndPrepareDfuV1() async {
+  Future<FirmwareFileSelectionResult> selectAndPrepareBootloaderDfu() async {
     final FirmwarePickerSelection? selection;
     try {
       selection = await _filePicker.pickFirmwareFile();
@@ -127,15 +127,32 @@ class FirmwareFileSelectionService {
       );
     }
 
-    final metadata = DfuV1FirmwareMetadata(
+    final imageValidationFailure = _validateBootloaderImage(
+      selection.fileBytes,
+      fileName,
+    );
+    if (imageValidationFailure != null) {
+      return FirmwareFileSelectionResult.failed(imageValidationFailure);
+    }
+
+    final vectorStackPointer = _readLeU32(selection.fileBytes, 0);
+    final vectorReset = _readLeU32(selection.fileBytes, 4);
+
+    final sessionId = _normalizeSessionId(_sessionIdGenerator());
+
+    final metadata = BootloaderDfuFirmwareMetadata(
       totalLength: selection.fileBytes.length,
-      crc32: DfuProtocol.crc32(selection.fileBytes),
+      crc32: BootloaderDfuProtocol.crc32(selection.fileBytes),
-      sessionId: _sessionIdGenerator() & 0xFF,
+      appStart: universalShifterDfuAppStart,
+      imageVersion: 0,
+      sessionId: sessionId,
       flags: universalShifterDfuFlagNone,
+      vectorStackPointer: vectorStackPointer,
+      vectorReset: vectorReset,
     );
 
     return FirmwareFileSelectionResult.success(
-      DfuV1PreparedFirmware(
+      BootloaderDfuPreparedFirmware(
         fileName: fileName,
         filePath: selection.filePath,
         fileBytes: selection.fileBytes,
@@ -148,7 +165,64 @@ class FirmwareFileSelectionService {
     return fileName.toLowerCase().endsWith('.bin');
   }
 
+  FirmwareSelectionFailure? _validateBootloaderImage(
+    Uint8List imageBytes,
+    String fileName,
+  ) {
+    if (imageBytes.length < universalShifterDfuMinimumImageLengthBytes) {
+      return FirmwareSelectionFailure(
+        reason: FirmwareSelectionFailureReason.imageTooSmall,
+        message:
+            'Selected firmware file "$fileName" is too small for a bootloader application image. Need at least $universalShifterDfuMinimumImageLengthBytes bytes.',
+      );
+    }
+
+    if (imageBytes.length > universalShifterDfuAppSlotSizeBytes) {
+      return FirmwareSelectionFailure(
+        reason: FirmwareSelectionFailureReason.imageTooLarge,
+        message:
+            'Selected firmware file "$fileName" is ${imageBytes.length} bytes, which exceeds the $universalShifterDfuAppSlotSizeBytes byte application slot.',
+      );
+    }
+
+    final vectorStackPointer = _readLeU32(imageBytes, 0);
+    final vectorReset = _readLeU32(imageBytes, 4);
+    final resetAddress = vectorReset & ~0x1;
+    final imageEnd = universalShifterDfuAppStart + imageBytes.length;
+
+    if (vectorStackPointer < universalShifterDfuRamStart ||
+        vectorStackPointer > universalShifterDfuRamEnd ||
+        (vectorStackPointer & 0x3) != 0) {
+      return FirmwareSelectionFailure(
+        reason: FirmwareSelectionFailureReason.invalidVectorTable,
+        message:
+            'Selected firmware file "$fileName" has an invalid initial stack pointer (0x${vectorStackPointer.toRadixString(16).padLeft(8, '0').toUpperCase()}).',
+      );
+    }
+
+    if ((vectorReset & 0x1) == 0 ||
+        resetAddress < universalShifterDfuAppStart + 8 ||
+        resetAddress >= imageEnd) {
+      return FirmwareSelectionFailure(
+        reason: FirmwareSelectionFailureReason.invalidVectorTable,
+        message:
+            'Selected firmware file "$fileName" has an invalid reset vector (0x${vectorReset.toRadixString(16).padLeft(8, '0').toUpperCase()}). Ensure the image starts at application address 0x${universalShifterDfuAppStart.toRadixString(16).padLeft(8, '0').toUpperCase()}.',
+      );
+    }
+
+    return null;
+  }
+
+  int _readLeU32(Uint8List bytes, int offset) {
+    return ByteData.sublistView(bytes).getUint32(offset, Endian.little);
+  }
+
+  static int _normalizeSessionId(int sessionId) {
+    final normalized = sessionId & 0xFF;
+    return normalized == 0 ? 1 : normalized;
+  }
+
   static int _randomSessionId() {
-    return Random.secure().nextInt(256);
+    return Random.secure().nextInt(255) + 1;
   }
 }

lib/service/shifter_service.dart

@@ -10,29 +10,15 @@ final _log = Logger('ShifterService');
 
 class ShifterService {
   ShifterService({
-    BluetoothController? bluetooth,
+    required BluetoothController bluetooth,
     required this.buttonDeviceId,
-    DfuPreflightBluetoothAdapter? dfuPreflightBluetooth,
+  }) : _bluetooth = bluetooth;
-  })  : _bluetooth = bluetooth,
-        _dfuPreflightBluetooth =
-            dfuPreflightBluetooth ?? _BluetoothDfuPreflightAdapter(bluetooth!) {
-    if (bluetooth == null && dfuPreflightBluetooth == null) {
-      throw ArgumentError(
-        'Either bluetooth or dfuPreflightBluetooth must be provided.',
-      );
-    }
-  }
 
-  final BluetoothController? _bluetooth;
+  final BluetoothController _bluetooth;
   final String buttonDeviceId;
-  final DfuPreflightBluetoothAdapter _dfuPreflightBluetooth;
 
   BluetoothController get _requireBluetooth {
-    final bluetooth = _bluetooth;
+    return _bluetooth;
-    if (bluetooth == null) {
-      throw StateError('Bluetooth controller is not available.');
-    }
-    return bluetooth;
   }
 
   final StreamController<CentralStatus> _statusController =
@@ -243,72 +229,6 @@ class ShifterService {
     );
   }
 
-  Future<Result<DfuPreflightResult>> runDfuPreflight({
-    int requestedMtu = universalShifterDfuPreferredMtu,
-  }) async {
-    final currentConnection = _dfuPreflightBluetooth.currentConnectionState;
-    final connectionStatus = currentConnection.$1;
-    final connectedDeviceId = currentConnection.$2;
-
-    if (connectionStatus != ConnectionStatus.connected ||
-        connectedDeviceId == null) {
-      return Ok(
-        DfuPreflightResult.failed(
-          requestedMtu: requestedMtu,
-          failureReason: DfuPreflightFailureReason.deviceNotConnected,
-          message:
-              'No button connection is active. Connect the target button, then retry the firmware update.',
-        ),
-      );
-    }
-
-    if (connectedDeviceId != buttonDeviceId) {
-      return Ok(
-        DfuPreflightResult.failed(
-          requestedMtu: requestedMtu,
-          failureReason: DfuPreflightFailureReason.wrongConnectedDevice,
-          message:
-              'Connected to a different button ($connectedDeviceId). Reconnect to $buttonDeviceId before starting the firmware update.',
-        ),
-      );
-    }
-
-    final mtuResult = await _dfuPreflightBluetooth.requestMtuAndGetValue(
-      buttonDeviceId,
-      mtu: requestedMtu,
-    );
-    if (mtuResult.isErr()) {
-      return Ok(
-        DfuPreflightResult.failed(
-          requestedMtu: requestedMtu,
-          failureReason: DfuPreflightFailureReason.mtuRequestFailed,
-          message:
-              'Could not negotiate BLE MTU for DFU. Move closer to the button and retry. Details: ${mtuResult.unwrapErr()}',
-        ),
-      );
-    }
-
-    final negotiatedMtu = mtuResult.unwrap();
-    if (negotiatedMtu < universalShifterDfuMinimumMtu) {
-      return Ok(
-        DfuPreflightResult.failed(
-          requestedMtu: requestedMtu,
-          negotiatedMtu: negotiatedMtu,
-          failureReason: DfuPreflightFailureReason.mtuTooLow,
-          message:
-              'Negotiated MTU $negotiatedMtu is too small for 64-byte DFU frames. Need at least $universalShifterDfuMinimumMtu bytes MTU (64-byte frame + $universalShifterAttWriteOverheadBytes-byte ATT overhead). Reconnect and retry.',
-        ),
-      );
-    }
-
-    return Ok(
-      DfuPreflightResult.ready(
-        requestedMtu: requestedMtu,
-        negotiatedMtu: negotiatedMtu,
-      ),
-    );
-  }
-
   void startStatusNotifications() {
     if (_statusSubscription != null) {
       return;
@@ -369,32 +289,6 @@ class ShifterService {
   }
 }
 
-abstract interface class DfuPreflightBluetoothAdapter {
-  (ConnectionStatus, String?) get currentConnectionState;
-  Future<Result<int>> requestMtuAndGetValue(
-    String deviceId, {
-    required int mtu,
-  });
-}
-
-class _BluetoothDfuPreflightAdapter implements DfuPreflightBluetoothAdapter {
-  const _BluetoothDfuPreflightAdapter(this._bluetooth);
-
-  final BluetoothController _bluetooth;
-
-  @override
-  (ConnectionStatus, String?) get currentConnectionState =>
-      _bluetooth.currentConnectionState;
-
-  @override
-  Future<Result<int>> requestMtuAndGetValue(
-    String deviceId, {
-    required int mtu,
-  }) {
-    return _bluetooth.requestMtuAndGetValue(deviceId, mtu: mtu);
-  }
-}
-
 class GearRatiosData {
   const GearRatiosData({
     required this.ratios,

test/service/dfu_preflight_test.dart

@@ -1,137 +0,0 @@
-import 'package:abawo_bt_app/controller/bluetooth.dart';
-import 'package:abawo_bt_app/model/shifter_types.dart';
-import 'package:abawo_bt_app/service/shifter_service.dart';
-import 'package:anyhow/anyhow.dart';
-import 'package:flutter_test/flutter_test.dart';
-
-void main() {
-  group('ShifterService.runDfuPreflight', () {
-    test('fails when no active button connection exists', () async {
-      final adapter = _FakeDfuPreflightBluetoothAdapter(
-        currentConnectionState: (ConnectionStatus.disconnected, null),
-        mtuResult: Ok(128),
-      );
-      final service = ShifterService(
-        buttonDeviceId: 'target-device',
-        dfuPreflightBluetooth: adapter,
-      );
-
-      final result = await service.runDfuPreflight();
-
-      expect(result.isOk(), isTrue);
-      final preflight = result.unwrap();
-      expect(preflight.canStart, isFalse);
-      expect(preflight.failureReason,
-          DfuPreflightFailureReason.deviceNotConnected);
-      expect(adapter.requestMtuCallCount, 0);
-    });
-
-    test('fails when connected to a different button', () async {
-      final adapter = _FakeDfuPreflightBluetoothAdapter(
-        currentConnectionState: (ConnectionStatus.connected, 'wrong-device'),
-        mtuResult: Ok(128),
-      );
-      final service = ShifterService(
-        buttonDeviceId: 'target-device',
-        dfuPreflightBluetooth: adapter,
-      );
-
-      final result = await service.runDfuPreflight();
-
-      expect(result.isOk(), isTrue);
-      final preflight = result.unwrap();
-      expect(preflight.canStart, isFalse);
-      expect(preflight.failureReason,
-          DfuPreflightFailureReason.wrongConnectedDevice);
-      expect(adapter.requestMtuCallCount, 0);
-    });
-
-    test('fails when MTU negotiation fails', () async {
-      final adapter = _FakeDfuPreflightBluetoothAdapter(
-        currentConnectionState: (ConnectionStatus.connected, 'target-device'),
-        mtuResult: bail('adapter rejected mtu request'),
-      );
-      final service = ShifterService(
-        buttonDeviceId: 'target-device',
-        dfuPreflightBluetooth: adapter,
-      );
-
-      final result = await service.runDfuPreflight(requestedMtu: 247);
-
-      expect(result.isOk(), isTrue);
-      final preflight = result.unwrap();
-      expect(preflight.canStart, isFalse);
-      expect(
-          preflight.failureReason, DfuPreflightFailureReason.mtuRequestFailed);
-      expect(preflight.message, contains('adapter rejected mtu request'));
-      expect(adapter.requestedMtuValues, [247]);
-    });
-
-    test('fails when negotiated MTU is too low for 64-byte frame writes',
-        () async {
-      final adapter = _FakeDfuPreflightBluetoothAdapter(
-        currentConnectionState: (ConnectionStatus.connected, 'target-device'),
-        mtuResult: Ok(universalShifterDfuMinimumMtu - 1),
-      );
-      final service = ShifterService(
-        buttonDeviceId: 'target-device',
-        dfuPreflightBluetooth: adapter,
-      );
-
-      final result = await service.runDfuPreflight();
-
-      expect(result.isOk(), isTrue);
-      final preflight = result.unwrap();
-      expect(preflight.canStart, isFalse);
-      expect(preflight.failureReason, DfuPreflightFailureReason.mtuTooLow);
-      expect(preflight.negotiatedMtu, universalShifterDfuMinimumMtu - 1);
-      expect(preflight.requiredMtu, universalShifterDfuMinimumMtu);
-    });
-
-    test('passes when connected to target and MTU is sufficient', () async {
-      final adapter = _FakeDfuPreflightBluetoothAdapter(
-        currentConnectionState: (ConnectionStatus.connected, 'target-device'),
-        mtuResult: Ok(128),
-      );
-      final service = ShifterService(
-        buttonDeviceId: 'target-device',
-        dfuPreflightBluetooth: adapter,
-      );
-
-      final result = await service.runDfuPreflight();
-
-      expect(result.isOk(), isTrue);
-      final preflight = result.unwrap();
-      expect(preflight.canStart, isTrue);
-      expect(preflight.failureReason, isNull);
-      expect(preflight.negotiatedMtu, 128);
-      expect(preflight.requestedMtu, universalShifterDfuPreferredMtu);
-    });
-  });
-}
-
-class _FakeDfuPreflightBluetoothAdapter
-    implements DfuPreflightBluetoothAdapter {
-  _FakeDfuPreflightBluetoothAdapter({
-    required this.currentConnectionState,
-    required Result<int> mtuResult,
-  }) : _mtuResult = mtuResult;
-
-  @override
-  final (ConnectionStatus, String?) currentConnectionState;
-
-  final Result<int> _mtuResult;
-
-  int requestMtuCallCount = 0;
-  final List<int> requestedMtuValues = <int>[];
-
-  @override
-  Future<Result<int>> requestMtuAndGetValue(
-    String deviceId, {
-    required int mtu,
-  }) async {
-    requestMtuCallCount += 1;
-    requestedMtuValues.add(mtu);
-    return _mtuResult;
-  }
-}

test/service/dfu_protocol_test.dart

@@ -3,28 +3,28 @@ import 'package:abawo_bt_app/service/dfu_protocol.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
-  group('DfuProtocol CRC32', () {
+  group('BootloaderDfuProtocol CRC32', () {
     test('matches known vector', () {
-      final crc = DfuProtocol.crc32('123456789'.codeUnits);
+      final crc = BootloaderDfuProtocol.crc32('123456789'.codeUnits);
       expect(crc, 0xCBF43926);
     });
   });
 
-  group('DfuProtocol control payload encoding', () {
+  group('BootloaderDfuProtocol control payload encoding', () {
-    test('encodes START payload with exact 11-byte LE layout', () {
+    test('encodes START payload with exact 19-byte LE layout', () {
-      final payload = DfuProtocol.encodeStartPayload(
+      final payload = BootloaderDfuProtocol.encodeStartPayload(
-        const DfuStartPayload(
+        const BootloaderDfuStartPayload(
           totalLength: 0x1234,
           imageCrc32: 0x89ABCDEF,
+          appStart: universalShifterDfuAppStart,
+          imageVersion: 0x10203040,
           sessionId: 0x22,
-          flags: universalShifterDfuFlagEncrypted,
+          flags: universalShifterDfuFlagNone,
         ),
       );
 
-      expect(payload.length, 11);
+      expect(payload.length, 19);
-      expect(
+      expect(payload, [
-        payload,
-        [
         universalShifterDfuOpcodeStart,
         0x34,
         0x12,
@@ -34,66 +34,127 @@ void main() {
         0xCD,
         0xAB,
         0x89,
+        0x00,
+        0x00,
+        0x03,
+        0x00,
+        0x40,
+        0x30,
+        0x20,
+        0x10,
         0x22,
-          universalShifterDfuFlagEncrypted,
+        universalShifterDfuFlagNone,
-        ],
+      ]);
+    });
+
+    test('encodes FINISH, ABORT, and GET_STATUS payloads', () {
+      expect(
+        BootloaderDfuProtocol.encodeFinishPayload(0x12),
+        [universalShifterDfuOpcodeFinish, 0x12],
+      );
+      expect(
+        BootloaderDfuProtocol.encodeAbortPayload(0x34),
+        [universalShifterDfuOpcodeAbort, 0x34],
+      );
+      expect(
+        BootloaderDfuProtocol.encodeGetStatusPayload(),
+        [universalShifterDfuOpcodeGetStatus],
       );
     });
-
-    test('encodes FINISH and ABORT payloads as one byte', () {
-      expect(
-          DfuProtocol.encodeFinishPayload(), [universalShifterDfuOpcodeFinish]);
-      expect(
-          DfuProtocol.encodeAbortPayload(), [universalShifterDfuOpcodeAbort]);
-    });
   });
 
-  group('DfuProtocol data frame building', () {
+  group('BootloaderDfuProtocol data frame building', () {
-    test('builds 64-byte frames and handles final partial payload', () {
+    test('builds offset frames with payload CRC and variable final length', () {
-      final image = List<int>.generate(80, (index) => index);
+      final image = List<int>.generate(60, (index) => index);
-      final frames = DfuProtocol.buildDataFrames(image);
+      final frames = BootloaderDfuProtocol.buildDataFrames(
+        imageBytes: image,
+        sessionId: 0x7A,
+      );
 
       expect(frames.length, 2);
 
-      expect(frames[0].sequence, 0);
+      expect(frames[0].sessionId, 0x7A);
       expect(frames[0].offset, 0);
-      expect(frames[0].payloadLength, universalShifterDfuFramePayloadSizeBytes);
+      expect(frames[0].payloadLength,
+          universalShifterBootloaderDfuMaxPayloadSizeBytes);
       expect(frames[0].bytes.length, universalShifterDfuFrameSizeBytes);
-      expect(frames[0].bytes.sublist(1, 64), image.sublist(0, 63));
+      expect(frames[0].bytes[0], 0x7A);
+      expect(frames[0].bytes.sublist(1, 5), [0, 0, 0, 0]);
+      expect(
+        frames[0].bytes.sublist(5, 9),
+        _leU32Bytes(BootloaderDfuProtocol.crc32(image.sublist(0, 55))),
+      );
+      expect(frames[0].bytes.sublist(9), image.sublist(0, 55));
 
-      expect(frames[1].sequence, 1);
+      expect(frames[1].offset, 55);
-      expect(frames[1].offset, 63);
+      expect(frames[1].payloadLength, 5);
-      expect(frames[1].payloadLength, 17);
+      expect(frames[1].bytes.length, 14);
-      expect(frames[1].bytes.length, universalShifterDfuFrameSizeBytes);
+      expect(frames[1].bytes.sublist(1, 5), [55, 0, 0, 0]);
-      expect(frames[1].bytes.sublist(1, 18), image.sublist(63, 80));
+      expect(frames[1].bytes.sublist(9), image.sublist(55));
     });
 
-    test('uses deterministic wrapping sequence numbers from custom start', () {
+    test('uses caller supplied payload size for low-MTU links', () {
-      final image = List<int>.generate(
+      final image = List<int>.generate(15, (index) => index);
-          3 * universalShifterDfuFramePayloadSizeBytes,
+      final frames = BootloaderDfuProtocol.buildDataFrames(
-          (index) => index & 0xFF);
+        imageBytes: image,
+        sessionId: 0x01,
+        payloadSize: 4,
+      );
 
-      final frames = DfuProtocol.buildDataFrames(image, startSequence: 0xFE);
+      expect(frames.map((frame) => frame.payloadLength), [4, 4, 4, 3]);
+      expect(frames.map((frame) => frame.offset), [0, 4, 8, 12]);
+    });
 
-      expect(frames.length, 3);
+    test('calculates safe payload size from negotiated MTU', () {
-      expect(frames[0].sequence, 0xFE);
+      expect(
-      expect(frames[1].sequence, 0xFF);
+        BootloaderDfuProtocol.maxPayloadSizeForMtu(64),
-      expect(frames[2].sequence, 0x00);
+        universalShifterBootloaderDfuMaxPayloadSizeBytes - 3,
+      );
+      expect(BootloaderDfuProtocol.maxPayloadSizeForMtu(23), 11);
+      expect(BootloaderDfuProtocol.maxPayloadSizeForMtu(12), 0);
+      expect(
+        BootloaderDfuProtocol.maxPayloadSizeForMtu(128),
+        universalShifterBootloaderDfuMaxPayloadSizeBytes,
+      );
     });
   });
 
-  group('DfuProtocol sequence and ACK helpers', () {
+  group('BootloaderDfuProtocol status parsing', () {
-    test('wraps sequence values and computes ack+1 rewind', () {
+    test('parses bootloader status payload', () {
-      expect(DfuProtocol.nextSequence(0x00), 0x01);
+      final status = BootloaderDfuProtocol.parseStatusPayload(
-      expect(DfuProtocol.nextSequence(0xFF), 0x00);
+        [0x00, 0x22, 0x78, 0x56, 0x34, 0x12],
+      );
 
-      expect(DfuProtocol.rewindSequenceFromAck(0x05), 0x06);
+      expect(status.code, DfuBootloaderStatusCode.ok);
-      expect(DfuProtocol.rewindSequenceFromAck(0xFF), 0x00);
+      expect(status.rawCode, 0x00);
+      expect(status.sessionId, 0x22);
+      expect(status.expectedOffset, 0x12345678);
+      expect(status.isOk, isTrue);
     });
 
-    test('computes wrapping sequence distance', () {
+    test('preserves unknown status codes', () {
-      expect(DfuProtocol.sequenceDistance(250, 2), 8);
+      final status = BootloaderDfuProtocol.parseStatusPayload(
-      expect(DfuProtocol.sequenceDistance(1, 1), 0);
+        [0xFE, 0x00, 0x00, 0x00, 0x00, 0x00],
+      );
+
+      expect(status.code, DfuBootloaderStatusCode.unknown);
+      expect(status.rawCode, 0xFE);
+    });
+
+    test('rejects malformed status payloads', () {
+      expect(
+        () => BootloaderDfuProtocol.parseStatusPayload(const [0, 1]),
+        throwsFormatException,
+      );
     });
   });
 }
+
+List<int> _leU32Bytes(int value) {
+  return [
+    value & 0xFF,
+    (value >> 8) & 0xFF,
+    (value >> 16) & 0xFF,
+    (value >> 24) & 0xFF,
+  ];
+}

test/service/firmware_file_selection_service_test.dart

@@ -2,34 +2,40 @@ import 'dart:typed_data';
 
 import 'package:abawo_bt_app/model/firmware_file_selection.dart';
 import 'package:abawo_bt_app/model/shifter_types.dart';
+import 'package:abawo_bt_app/service/dfu_protocol.dart';
 import 'package:abawo_bt_app/service/firmware_file_selection_service.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
   group('FirmwareFileSelectionService', () {
-    test('prepares v1 metadata for selected .bin firmware', () async {
+    test('prepares bootloader metadata for selected .bin firmware', () async {
+      final image = _validBootloaderImage();
       final service = FirmwareFileSelectionService(
         filePicker: _FakeFirmwareFilePicker(
           selection: FirmwarePickerSelection(
             fileName: 'firmware.BIN',
             filePath: '/tmp/firmware.BIN',
-            fileBytes: Uint8List.fromList(<int>[1, 2, 3, 4]),
+            fileBytes: image,
           ),
         ),
         sessionIdGenerator: () => 0x1AB,
       );
 
-      final result = await service.selectAndPrepareDfuV1();
+      final result = await service.selectAndPrepareBootloaderDfu();
       expect(result.isSuccess, isTrue);
 
       final firmware = result.firmware!;
       expect(firmware.fileName, 'firmware.BIN');
       expect(firmware.filePath, '/tmp/firmware.BIN');
-      expect(firmware.fileBytes, <int>[1, 2, 3, 4]);
+      expect(firmware.fileBytes, image);
-      expect(firmware.metadata.totalLength, 4);
+      expect(firmware.metadata.totalLength, image.length);
-      expect(firmware.metadata.crc32, 0xB63CFBCD);
+      expect(firmware.metadata.crc32, BootloaderDfuProtocol.crc32(image));
+      expect(firmware.metadata.appStart, universalShifterDfuAppStart);
+      expect(firmware.metadata.imageVersion, 0);
       expect(firmware.metadata.sessionId, 0xAB);
       expect(firmware.metadata.flags, universalShifterDfuFlagNone);
+      expect(firmware.metadata.vectorStackPointer, 0x20001000);
+      expect(firmware.metadata.vectorReset, 0x00030009);
     });
 
     test('returns canceled result when user dismisses picker', () async {
@@ -37,7 +43,7 @@ void main() {
         filePicker: _FakeFirmwareFilePicker(selection: null),
       );
 
-      final result = await service.selectAndPrepareDfuV1();
+      final result = await service.selectAndPrepareBootloaderDfu();
 
       expect(result.isSuccess, isFalse);
       expect(result.isCanceled, isTrue);
@@ -49,12 +55,12 @@ void main() {
         filePicker: _FakeFirmwareFilePicker(
           selection: FirmwarePickerSelection(
             fileName: 'firmware.hex',
-            fileBytes: Uint8List.fromList(<int>[1]),
+            fileBytes: _validBootloaderImage(),
           ),
         ),
       );
 
-      final result = await service.selectAndPrepareDfuV1();
+      final result = await service.selectAndPrepareBootloaderDfu();
 
       expect(result.isSuccess, isFalse);
       expect(result.failure?.reason,
@@ -71,31 +77,124 @@ void main() {
         ),
       );
 
-      final result = await service.selectAndPrepareDfuV1();
+      final result = await service.selectAndPrepareBootloaderDfu();
 
       expect(result.isSuccess, isFalse);
       expect(result.failure?.reason, FirmwareSelectionFailureReason.emptyFile);
     });
 
+    test('rejects images that are too small for a vector table', () async {
+      final service = FirmwareFileSelectionService(
+        filePicker: _FakeFirmwareFilePicker(
+          selection: FirmwarePickerSelection(
+            fileName: 'firmware.bin',
+            fileBytes: Uint8List.fromList(<int>[1, 2, 3, 4]),
+          ),
+        ),
+      );
+
+      final result = await service.selectAndPrepareBootloaderDfu();
+
+      expect(result.isSuccess, isFalse);
+      expect(
+          result.failure?.reason, FirmwareSelectionFailureReason.imageTooSmall);
+    });
+
+    test('rejects images larger than the application slot', () async {
+      final image = Uint8List(universalShifterDfuAppSlotSizeBytes + 1);
+      _writeLeU32(image, 0, 0x20001000);
+      _writeLeU32(image, 4, 0x00030009);
+      final service = FirmwareFileSelectionService(
+        filePicker: _FakeFirmwareFilePicker(
+          selection: FirmwarePickerSelection(
+            fileName: 'firmware.bin',
+            fileBytes: image,
+          ),
+        ),
+      );
+
+      final result = await service.selectAndPrepareBootloaderDfu();
+
+      expect(result.isSuccess, isFalse);
+      expect(
+          result.failure?.reason, FirmwareSelectionFailureReason.imageTooLarge);
+    });
+
+    test('accepts image exactly at application slot size', () async {
+      final image = Uint8List(universalShifterDfuAppSlotSizeBytes);
+      _writeLeU32(image, 0, 0x20001000);
+      _writeLeU32(image, 4, 0x00030009);
+      final service = FirmwareFileSelectionService(
+        filePicker: _FakeFirmwareFilePicker(
+          selection: FirmwarePickerSelection(
+            fileName: 'firmware.bin',
+            fileBytes: image,
+          ),
+        ),
+      );
+
+      final result = await service.selectAndPrepareBootloaderDfu();
+
+      expect(result.isSuccess, isTrue);
+      expect(result.firmware?.metadata.totalLength,
+          universalShifterDfuAppSlotSizeBytes);
+    });
+
+    test('rejects images with invalid vector table', () async {
+      final image = _validBootloaderImage();
+      _writeLeU32(image, 0, 0x10001000);
+      final service = FirmwareFileSelectionService(
+        filePicker: _FakeFirmwareFilePicker(
+          selection: FirmwarePickerSelection(
+            fileName: 'firmware.bin',
+            fileBytes: image,
+          ),
+        ),
+      );
+
+      final result = await service.selectAndPrepareBootloaderDfu();
+
+      expect(result.isSuccess, isFalse);
+      expect(result.failure?.reason,
+          FirmwareSelectionFailureReason.invalidVectorTable);
+    });
+
     test('generates session id per run', () async {
       var nextSession = 9;
       final service = FirmwareFileSelectionService(
         filePicker: _FakeFirmwareFilePicker(
           selection: FirmwarePickerSelection(
             fileName: 'firmware.bin',
-            fileBytes: Uint8List.fromList(<int>[10]),
+            fileBytes: _validBootloaderImage(),
           ),
         ),
         sessionIdGenerator: () => nextSession++,
       );
 
-      final first = await service.selectAndPrepareDfuV1();
+      final first = await service.selectAndPrepareBootloaderDfu();
-      final second = await service.selectAndPrepareDfuV1();
+      final second = await service.selectAndPrepareBootloaderDfu();
 
       expect(first.firmware?.metadata.sessionId, 9);
       expect(second.firmware?.metadata.sessionId, 10);
     });
 
+    test('normalizes generated zero session id to one', () async {
+      final service = FirmwareFileSelectionService(
+        filePicker: _FakeFirmwareFilePicker(
+          selection: FirmwarePickerSelection(
+            fileName: 'firmware.bin',
+            fileBytes: _validBootloaderImage(),
+          ),
+        ),
+        sessionIdGenerator: () => 0,
+      );
+
+      final result = await service.selectAndPrepareBootloaderDfu();
+
+      expect(result.isSuccess, isTrue);
+      expect(result.firmware?.metadata.sessionId, 1);
+    });
+
     test('maps picker read failure to explicit validation error', () async {
       final service = FirmwareFileSelectionService(
         filePicker: _FakeFirmwareFilePicker(
@@ -104,7 +203,7 @@ void main() {
         ),
       );
 
-      final result = await service.selectAndPrepareDfuV1();
+      final result = await service.selectAndPrepareBootloaderDfu();
 
       expect(result.isSuccess, isFalse);
       expect(result.failure?.reason, FirmwareSelectionFailureReason.readFailed);
@@ -113,6 +212,18 @@ void main() {
   });
 }
 
+Uint8List _validBootloaderImage() {
+  final image = Uint8List(16);
+  _writeLeU32(image, 0, 0x20001000);
+  _writeLeU32(image, 4, 0x00030009);
+  return image;
+}
+
+void _writeLeU32(Uint8List bytes, int offset, int value) {
+  final data = ByteData.sublistView(bytes);
+  data.setUint32(offset, value, Endian.little);
+}
+
 class _FakeFirmwareFilePicker implements FirmwareFilePicker {
   _FakeFirmwareFilePicker({
     required this.selection,

test/service/firmware_update_service_test.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:typed_data';
 
 import 'package:abawo_bt_app/model/shifter_types.dart';
 import 'package:abawo_bt_app/service/firmware_update_service.dart';
@@ -6,413 +7,469 @@ import 'package:anyhow/anyhow.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
-  group('FirmwareUpdateService', () {
+  group('FirmwareUpdateService bootloader flow', () {
-    test('completes happy path with START, data frames, and FINISH', () async {
+    test('completes happy path with START, offset data, FINISH, and verify',
-      final transport = _FakeFirmwareUpdateTransport();
+        () async {
+      final image = _validImage(130);
+      final transport = _FakeFirmwareUpdateTransport(totalBytes: image.length);
       final service = FirmwareUpdateService(
         transport: transport,
-        defaultWindowSize: 4,
+        defaultStatusTimeout: const Duration(milliseconds: 100),
-        defaultAckTimeout: const Duration(milliseconds: 100),
       );
 
-      final image = List<int>.generate(130, (index) => index & 0xFF);
       final result = await service.startUpdate(
         imageBytes: image,
         sessionId: 7,
       );
 
       expect(result.isOk(), isTrue);
-      expect(transport.controlWrites.length, 2);
+      expect(transport.steps, [
-      expect(
+        'isConnectedToBootloader',
-          transport.controlWrites.first.first, universalShifterDfuOpcodeStart);
+        'enterBootloader',
-      expect(transport.controlWrites.last, [universalShifterDfuOpcodeFinish]);
+        'waitForAppDisconnect',
-      expect(transport.dataWrites.length, greaterThanOrEqualTo(3));
+        'connectToBootloader',
-      expect(
+        'negotiateMtu',
-        transport.postFinishSteps,
+        'readStatus',
-        [
+        'waitForBootloaderDisconnect',
-          'waitForExpectedResetDisconnect',
         'reconnectForVerification',
         'verifyDeviceReachable',
-        ],
+      ]);
-      );
+      expect(
+          transport.controlWrites.first.first, universalShifterDfuOpcodeStart);
+      expect(
+          transport.controlWrites.last, [universalShifterDfuOpcodeFinish, 7]);
+      expect(transport.dataWrites, isNotEmpty);
+      expect(transport.dataWrites.first[0], 7);
+      expect(transport.dataWrites.first.sublist(1, 5), [0, 0, 0, 0]);
       expect(service.currentProgress.state, DfuUpdateState.completed);
       expect(service.currentProgress.sentBytes, image.length);
+      expect(service.currentProgress.expectedOffset, image.length);
 
       await service.dispose();
       await transport.dispose();
     });
 
-    test('rewinds to ack+1 and retransmits after ACK stall', () async {
+    test('starts directly when already connected to bootloader', () async {
-      final transport = _FakeFirmwareUpdateTransport(dropFirstSequence: 1);
+      final image = _validImage(80);
+      final transport = _FakeFirmwareUpdateTransport(
+        totalBytes: image.length,
+        alreadyInBootloader: true,
+      );
       final service = FirmwareUpdateService(
         transport: transport,
-        defaultWindowSize: 3,
+        defaultStatusTimeout: const Duration(milliseconds: 100),
-        defaultAckTimeout: const Duration(milliseconds: 100),
+      );
-        maxNoProgressRetries: 4,
+
+      final result = await service.startUpdate(
+        imageBytes: image,
+        sessionId: 8,
+      );
+
+      expect(result.isOk(), isTrue);
+      expect(transport.steps, [
+        'isConnectedToBootloader',
+        'negotiateMtu',
+        'readStatus',
+        'waitForBootloaderDisconnect',
+        'reconnectForVerification',
+        'verifyDeviceReachable',
+      ]);
+      expect(
+          transport.controlWrites.first.first, universalShifterDfuOpcodeStart);
+
+      await service.dispose();
+      await transport.dispose();
+    });
+
+    test('tolerates enter bootloader write error when app disconnects',
+        () async {
+      final image = _validImage(80);
+      final transport = _FakeFirmwareUpdateTransport(
+        totalBytes: image.length,
+        failEnterBootloader: true,
+      );
+      final service = FirmwareUpdateService(
+        transport: transport,
+        defaultStatusTimeout: const Duration(milliseconds: 100),
+      );
+
+      final result = await service.startUpdate(
+        imageBytes: image,
+        sessionId: 12,
+      );
+
+      expect(result.isOk(), isTrue);
+      expect(transport.steps, contains('waitForAppDisconnect'));
+      expect(service.currentProgress.state, DfuUpdateState.completed);
+
+      await service.dispose();
+      await transport.dispose();
+    });
+
+    test('backs off on queue-full status and resumes from GET_STATUS',
+        () async {
+      final image = _validImage(80);
+      final transport = _FakeFirmwareUpdateTransport(
+        totalBytes: image.length,
+        queueFullOnFirstData: true,
+      );
+      final service = FirmwareUpdateService(
+        transport: transport,
+        defaultStatusTimeout: const Duration(milliseconds: 100),
       );
 
-      final image = List<int>.generate(190, (index) => index & 0xFF);
       final result = await service.startUpdate(
         imageBytes: image,
         sessionId: 9,
       );
 
       expect(result.isOk(), isTrue);
-      expect(transport.dataWrites.length, greaterThan(4));
+      expect(
-      expect(transport.sequenceWriteCount(1), greaterThan(1));
+        transport.controlWrites
+            .where((write) => write.first == universalShifterDfuOpcodeGetStatus)
+            .length,
+        1,
+      );
+      expect(
+          transport.dataWriteOffsets.where((offset) => offset == 0).length, 2);
       expect(service.currentProgress.state, DfuUpdateState.completed);
 
       await service.dispose();
       await transport.dispose();
     });
 
-    test('fails after bounded retries when ACK progress times out', () async {
+    test('reconnects and resumes from status after transient data failure',
-      final transport = _FakeFirmwareUpdateTransport(suppressDataAcks: true);
+        () async {
+      final image = _validImage(130);
+      final transport = _FakeFirmwareUpdateTransport(
+        totalBytes: image.length,
+        failDataWriteAtOffsetOnce:
+            universalShifterBootloaderDfuMaxPayloadSizeBytes,
+      );
       final service = FirmwareUpdateService(
         transport: transport,
-        defaultWindowSize: 1,
+        defaultStatusTimeout: const Duration(milliseconds: 100),
-        defaultAckTimeout: const Duration(milliseconds: 40),
+        defaultBootloaderConnectTimeout: const Duration(milliseconds: 100),
-        maxNoProgressRetries: 2,
+      );
+
+      final result = await service.startUpdate(
+        imageBytes: image,
+        sessionId: 13,
+      );
+
+      expect(result.isOk(), isTrue);
+      expect(
+        transport.steps.where((step) => step == 'connectToBootloader').length,
+        2,
+      );
+      expect(
+        transport.controlWrites
+            .where((write) => write.first == universalShifterDfuOpcodeGetStatus)
+            .length,
+        1,
+      );
+      expect(
+        transport.dataWriteOffsets
+            .where(
+              (offset) =>
+                  offset == universalShifterBootloaderDfuMaxPayloadSizeBytes,
+            )
+            .length,
+        2,
+      );
+      expect(service.currentProgress.state, DfuUpdateState.completed);
+
+      await service.dispose();
+      await transport.dispose();
+    });
+
+    test('restarts START when reconnect status has no active session',
+        () async {
+      final image = _validImage(80);
+      final transport = _FakeFirmwareUpdateTransport(
+        totalBytes: image.length,
+        failDataWriteAtOffsetOnce:
+            universalShifterBootloaderDfuMaxPayloadSizeBytes,
+        resetSessionOnRecoveryStatus: true,
+      );
+      final service = FirmwareUpdateService(
+        transport: transport,
+        defaultStatusTimeout: const Duration(milliseconds: 100),
+        defaultBootloaderConnectTimeout: const Duration(milliseconds: 100),
+      );
+
+      final result = await service.startUpdate(
+        imageBytes: image,
+        sessionId: 14,
+      );
+
+      expect(result.isOk(), isTrue);
+      expect(
+        transport.controlWrites
+            .where((write) => write.first == universalShifterDfuOpcodeStart)
+            .length,
+        2,
+      );
+      expect(service.currentProgress.state, DfuUpdateState.completed);
+
+      await service.dispose();
+      await transport.dispose();
+    });
+
+    test('fails with bootloader status error on rejected START', () async {
+      final image = _validImage(40);
+      final transport = _FakeFirmwareUpdateTransport(
+        totalBytes: image.length,
+        startStatusCode: DfuBootloaderStatusCode.vectorError,
+      );
+      final service = FirmwareUpdateService(
+        transport: transport,
+        defaultStatusTimeout: const Duration(milliseconds: 100),
       );
 
-      final image = List<int>.generate(90, (index) => index & 0xFF);
       final result = await service.startUpdate(
         imageBytes: image,
         sessionId: 10,
       );
 
       expect(result.isErr(), isTrue);
-      expect(result.unwrapErr().toString(), contains('Upload stalled'));
+      expect(result.unwrapErr().toString(), contains('vector table error'));
-      expect(result.unwrapErr().toString(), contains('after 3 retries'));
-      expect(transport.controlWrites.last, [universalShifterDfuOpcodeAbort]);
-      expect(transport.sequenceWriteCount(0), 3);
       expect(service.currentProgress.state, DfuUpdateState.failed);
+      expect(
+          transport.controlWrites.last.first, universalShifterDfuOpcodeStart);
 
       await service.dispose();
       await transport.dispose();
     });
 
-    test('cancel sends ABORT and reports aborted state', () async {
+    test('cancel after START sends session-scoped ABORT', () async {
+      final image = _validImage(80);
       final firstFrameSent = Completer<void>();
       final transport = _FakeFirmwareUpdateTransport(
-        onDataWrite: (frame) {
+        totalBytes: image.length,
+        suppressFirstDataStatus: true,
+        onDataWrite: () {
           if (!firstFrameSent.isCompleted) {
             firstFrameSent.complete();
           }
         },
-        suppressDataAcks: true,
       );
       final service = FirmwareUpdateService(
         transport: transport,
-        defaultWindowSize: 1,
+        defaultStatusTimeout: const Duration(seconds: 1),
-        defaultAckTimeout: const Duration(milliseconds: 500),
       );
 
       final future = service.startUpdate(
-        imageBytes: List<int>.generate(90, (index) => index & 0xFF),
+        imageBytes: image,
         sessionId: 11,
       );
-
       await firstFrameSent.future.timeout(const Duration(seconds: 1));
       await service.cancelUpdate();
       final result = await future;
 
       expect(result.isErr(), isTrue);
       expect(result.unwrapErr().toString(), contains('canceled'));
-      expect(transport.controlWrites.last, [universalShifterDfuOpcodeAbort]);
+      expect(
+          transport.controlWrites.last, [universalShifterDfuOpcodeAbort, 11]);
       expect(service.currentProgress.state, DfuUpdateState.aborted);
 
       await service.dispose();
       await transport.dispose();
     });
-
-    test('fails when reconnect does not succeed after expected reset',
-        () async {
-      final transport = _FakeFirmwareUpdateTransport(
-        reconnectError: 'simulated reconnect timeout',
-      );
-      final service = FirmwareUpdateService(
-        transport: transport,
-        defaultWindowSize: 4,
-        defaultAckTimeout: const Duration(milliseconds: 100),
-      );
-
-      final image = List<int>.generate(130, (index) => index & 0xFF);
-      final result = await service.startUpdate(
-        imageBytes: image,
-        sessionId: 13,
-      );
-
-      expect(result.isErr(), isTrue);
-      expect(result.unwrapErr().toString(), contains('did not reconnect'));
-      expect(service.currentProgress.state, DfuUpdateState.failed);
-      expect(
-        transport.postFinishSteps,
-        [
-          'waitForExpectedResetDisconnect',
-          'reconnectForVerification',
-        ],
-      );
-
-      await service.dispose();
-      await transport.dispose();
-    });
-
-    test('fails when expected reset disconnect is not observed', () async {
-      final transport = _FakeFirmwareUpdateTransport(
-        resetDisconnectError: 'simulated missing disconnect',
-      );
-      final service = FirmwareUpdateService(
-        transport: transport,
-        defaultWindowSize: 4,
-        defaultAckTimeout: const Duration(milliseconds: 100),
-      );
-
-      final image = List<int>.generate(130, (index) => index & 0xFF);
-      final result = await service.startUpdate(
-        imageBytes: image,
-        sessionId: 15,
-      );
-
-      expect(result.isErr(), isTrue);
-      expect(
-        result.unwrapErr().toString(),
-        contains('expected post-FINISH reset disconnect'),
-      );
-      expect(service.currentProgress.state, DfuUpdateState.failed);
-      expect(
-        transport.postFinishSteps,
-        ['waitForExpectedResetDisconnect'],
-      );
-
-      await service.dispose();
-      await transport.dispose();
-    });
-
-    test('fails when post-update status verification read fails', () async {
-      final transport = _FakeFirmwareUpdateTransport(
-        verificationError: 'simulated status read failure',
-      );
-      final service = FirmwareUpdateService(
-        transport: transport,
-        defaultWindowSize: 4,
-        defaultAckTimeout: const Duration(milliseconds: 100),
-      );
-
-      final image = List<int>.generate(130, (index) => index & 0xFF);
-      final result = await service.startUpdate(
-        imageBytes: image,
-        sessionId: 14,
-      );
-
-      expect(result.isErr(), isTrue);
-      expect(
-        result.unwrapErr().toString(),
-        contains('post-update verification failed'),
-      );
-      expect(
-        result.unwrapErr().toString(),
-        contains('does not expose a version characteristic'),
-      );
-      expect(service.currentProgress.state, DfuUpdateState.failed);
-      expect(
-        transport.postFinishSteps,
-        [
-          'waitForExpectedResetDisconnect',
-          'reconnectForVerification',
-          'verifyDeviceReachable',
-        ],
-      );
-
-      await service.dispose();
-      await transport.dispose();
-    });
-
-    test('handles deterministic ACK sequence wrap-around across 0xFF->0x00',
-        () async {
-      const frameCount = 260;
-      final transport = _FakeFirmwareUpdateTransport();
-      final service = FirmwareUpdateService(
-        transport: transport,
-        defaultWindowSize: 16,
-        defaultAckTimeout: const Duration(milliseconds: 100),
-      );
-
-      final image = List<int>.generate(
-        frameCount * universalShifterDfuFramePayloadSizeBytes,
-        (index) => index & 0xFF,
-      );
-
-      final result = await service.startUpdate(
-        imageBytes: image,
-        sessionId: 16,
-      );
-
-      expect(result.isOk(), isTrue);
-
-      var ffToZeroTransitions = 0;
-      for (var i = 1; i < transport.ackNotifications.length; i++) {
-        if (transport.ackNotifications[i - 1] == 0xFF &&
-            transport.ackNotifications[i] == 0x00) {
-          ffToZeroTransitions += 1;
-        }
-      }
-
-      expect(ffToZeroTransitions, greaterThanOrEqualTo(2));
-      expect(service.currentProgress.lastAckedSequence, 0x03);
-      expect(service.currentProgress.sentBytes, image.length);
-      expect(service.currentProgress.state, DfuUpdateState.completed);
-
-      await service.dispose();
-      await transport.dispose();
-    });
   });
 }
 
 class _FakeFirmwareUpdateTransport implements FirmwareUpdateTransport {
   _FakeFirmwareUpdateTransport({
-    this.dropFirstSequence,
+    required this.totalBytes,
+    this.startStatusCode = DfuBootloaderStatusCode.ok,
+    this.alreadyInBootloader = false,
+    this.failEnterBootloader = false,
+    this.queueFullOnFirstData = false,
+    this.suppressFirstDataStatus = false,
+    this.failDataWriteAtOffsetOnce,
+    this.resetSessionOnRecoveryStatus = false,
     this.onDataWrite,
-    this.suppressDataAcks = false,
-    this.resetDisconnectError,
-    this.reconnectError,
-    this.verificationError,
   });
 
-  final int? dropFirstSequence;
+  final int totalBytes;
-  final void Function(List<int> frame)? onDataWrite;
+  final DfuBootloaderStatusCode startStatusCode;
-  final bool suppressDataAcks;
+  final bool alreadyInBootloader;
-  final String? resetDisconnectError;
+  final bool failEnterBootloader;
-  final String? reconnectError;
+  final bool queueFullOnFirstData;
-  final String? verificationError;
+  final bool suppressFirstDataStatus;
+  final int? failDataWriteAtOffsetOnce;
+  final bool resetSessionOnRecoveryStatus;
+  final void Function()? onDataWrite;
 
-  final StreamController<List<int>> _ackController =
+  final StreamController<List<int>> _statusController =
       StreamController<List<int>>.broadcast();
-
+  final List<String> steps = <String>[];
   final List<List<int>> controlWrites = <List<int>>[];
   final List<List<int>> dataWrites = <List<int>>[];
-  final List<int> ackNotifications = <int>[];
+  final List<int> dataWriteOffsets = <int>[];
-  final List<String> postFinishSteps = <String>[];
+
-  final Set<int> _droppedOnce = <int>{};
+  int _sessionId = 0;
-  int _lastAck = 0xFF;
+  int _expectedOffset = 0;
-  int _expectedSequence = 0;
+  int _connectCount = 0;
+  bool _sentDataFailure = false;
+  bool _sentQueueFull = false;
+  bool _suppressedDataStatus = false;
 
   @override
-  Future<Result<DfuPreflightResult>> runPreflight({
+  Future<Result<bool>> isConnectedToBootloader() async {
-    required int requestedMtu,
+    steps.add('isConnectedToBootloader');
-  }) async {
+    return Ok(alreadyInBootloader);
-    return Ok(
-      DfuPreflightResult.ready(
-        requestedMtu: requestedMtu,
-        negotiatedMtu: 128,
-      ),
-    );
   }
 
   @override
-  Stream<List<int>> subscribeToAck() => _ackController.stream;
+  Future<Result<void>> enterBootloader() async {
+    steps.add('enterBootloader');
+    if (failEnterBootloader) {
+      return bail('app disconnected before write response');
+    }
+    return Ok(null);
+  }
+
+  @override
+  Future<Result<void>> waitForAppDisconnect({required Duration timeout}) async {
+    steps.add('waitForAppDisconnect');
+    return Ok(null);
+  }
+
+  @override
+  Future<Result<void>> connectToBootloader({required Duration timeout}) async {
+    steps.add('connectToBootloader');
+    _connectCount += 1;
+    return Ok(null);
+  }
+
+  @override
+  Future<Result<int>> negotiateMtu({required int requestedMtu}) async {
+    steps.add('negotiateMtu');
+    return Ok(128);
+  }
+
+  @override
+  Stream<List<int>> subscribeToStatus() => _statusController.stream;
+
+  @override
+  Future<Result<List<int>>> readStatus() async {
+    steps.add('readStatus');
+    return Ok(_status(DfuBootloaderStatusCode.ok, 0, 0));
+  }
 
   @override
   Future<Result<void>> writeControl(List<int> payload) async {
     controlWrites.add(List<int>.from(payload, growable: false));
-
+    final opcode = payload.first;
-    final opcode = payload.isEmpty ? -1 : payload.first;
     if (opcode == universalShifterDfuOpcodeStart) {
-      _lastAck = 0xFF;
+      _sessionId = payload[17];
-      _expectedSequence = 0;
+      _expectedOffset = 0;
-      _scheduleAck(0xFF);
+      _scheduleStatus(startStatusCode, _sessionId, 0);
+    } else if (opcode == universalShifterDfuOpcodeGetStatus) {
+      if (resetSessionOnRecoveryStatus && _connectCount > 1) {
+        _sessionId = 0;
+        _expectedOffset = 0;
       }
-
+      _scheduleStatus(DfuBootloaderStatusCode.ok, _sessionId, _expectedOffset);
-    if (opcode == universalShifterDfuOpcodeAbort) {
+    } else if (opcode == universalShifterDfuOpcodeFinish) {
-      _lastAck = 0xFF;
+      _scheduleStatus(DfuBootloaderStatusCode.ok, payload[1], totalBytes);
-      _expectedSequence = 0;
+    } else if (opcode == universalShifterDfuOpcodeAbort) {
+      _scheduleStatus(DfuBootloaderStatusCode.ok, payload[1], 0);
     }
-
     return Ok(null);
   }
 
   @override
   Future<Result<void>> writeDataFrame(List<int> frame) async {
     dataWrites.add(List<int>.from(frame, growable: false));
-    onDataWrite?.call(frame);
+    onDataWrite?.call();
 
-    if (suppressDataAcks) {
+    final offset = _readLeU32(frame, 1);
+    dataWriteOffsets.add(offset);
+
+    if (failDataWriteAtOffsetOnce == offset && !_sentDataFailure) {
+      _sentDataFailure = true;
+      return bail('simulated BLE write failure');
+    }
+
+    if (queueFullOnFirstData && !_sentQueueFull) {
+      _sentQueueFull = true;
+      _scheduleStatus(
+          DfuBootloaderStatusCode.queueFull, _sessionId, _expectedOffset);
       return Ok(null);
     }
 
-    final sequence = frame.first;
+    if (suppressFirstDataStatus && !_suppressedDataStatus) {
-    final shouldDrop = dropFirstSequence != null &&
+      _suppressedDataStatus = true;
-        sequence == dropFirstSequence &&
-        !_droppedOnce.contains(sequence);
-
-    if (shouldDrop) {
-      _droppedOnce.add(sequence);
-      _scheduleAck(_lastAck);
       return Ok(null);
     }
 
-    if (sequence == _expectedSequence) {
+    final payloadLength =
-      _lastAck = sequence;
+        frame.length - universalShifterBootloaderDfuDataHeaderSizeBytes;
-      _expectedSequence = (_expectedSequence + 1) & 0xFF;
+    _expectedOffset = offset + payloadLength;
-    }
+    _scheduleStatus(DfuBootloaderStatusCode.ok, _sessionId, _expectedOffset);
-
-    _scheduleAck(_lastAck);
-
     return Ok(null);
   }
 
-  void _scheduleAck(int sequence) {
+  @override
-    final ack = sequence & 0xFF;
+  Future<Result<void>> waitForBootloaderDisconnect(
-    ackNotifications.add(ack);
+      {required Duration timeout}) async {
+    steps.add('waitForBootloaderDisconnect');
+    return Ok(null);
+  }
+
+  @override
+  Future<Result<void>> reconnectForVerification(
+      {required Duration timeout}) async {
+    steps.add('reconnectForVerification');
+    return Ok(null);
+  }
+
+  @override
+  Future<Result<void>> verifyDeviceReachable(
+      {required Duration timeout}) async {
+    steps.add('verifyDeviceReachable');
+    return Ok(null);
+  }
+
+  void _scheduleStatus(
+      DfuBootloaderStatusCode code, int sessionId, int offset) {
+    final status = _status(code, sessionId, offset);
     scheduleMicrotask(() {
-      _ackController.add([ack]);
+      _statusController.add(status);
     });
   }
 
-  @override
+  List<int> _status(DfuBootloaderStatusCode code, int sessionId, int offset) {
-  Future<Result<void>> waitForExpectedResetDisconnect({
+    return [
-    required Duration timeout,
+      code.value,
-  }) async {
+      sessionId & 0xFF,
-    postFinishSteps.add('waitForExpectedResetDisconnect');
+      offset & 0xFF,
-    if (resetDisconnectError != null) {
+      (offset >> 8) & 0xFF,
-      return bail(resetDisconnectError!);
+      (offset >> 16) & 0xFF,
-    }
+      (offset >> 24) & 0xFF,
-    return Ok(null);
+    ];
   }
 
-  @override
+  int _readLeU32(List<int> bytes, int offset) {
-  Future<Result<void>> reconnectForVerification({
+    final data = ByteData.sublistView(Uint8List.fromList(bytes));
-    required Duration timeout,
+    return data.getUint32(offset, Endian.little);
-  }) async {
-    postFinishSteps.add('reconnectForVerification');
-    if (reconnectError != null) {
-      return bail(reconnectError!);
-    }
-    return Ok(null);
-  }
-
-  @override
-  Future<Result<void>> verifyDeviceReachable({
-    required Duration timeout,
-  }) async {
-    postFinishSteps.add('verifyDeviceReachable');
-    if (verificationError != null) {
-      return bail(verificationError!);
-    }
-    return Ok(null);
-  }
-
-  int sequenceWriteCount(int sequence) {
-    var count = 0;
-    for (final frame in dataWrites) {
-      if (frame.first == sequence) {
-        count += 1;
-      }
-    }
-    return count;
   }
 
   Future<void> dispose() async {
-    await _ackController.close();
+    await _statusController.close();
   }
 }
+
+List<int> _validImage(int length) {
+  final image = Uint8List(length);
+  final data = ByteData.sublistView(image);
+  data.setUint32(0, 0x20001000, Endian.little);
+  data.setUint32(4, 0x00030009, Endian.little);
+  for (var index = 8; index < image.length; index++) {
+    image[index] = index & 0xFF;
+  }
+  return image;
+}