Add pico_get_unique_board_id_string API (#281)

* Add pico_get_unique_board_id_string API

Add a new API to pico_unique which will turn the unique ID into a
canonical text string.

Use this API to update the USB device serial number in stdio_usb.

Supercedes #280

* Clean up -Wconversion=error issues

* Address review comments, fix api typing

Use cleaner binary-to-hex conversion.  Update the length parameter to
use uint per the SDK standard .

src/rp2_common/pico_stdio_usb/CMakeLists.txt

@@ -13,6 +13,7 @@ if (TARGET tinyusb_device_unmarked)
         tinyusb_device_unmarked
         pico_stdio
         pico_time
+        pico_unique_id
     )
 
     target_compile_definitions(pico_stdio_usb INTERFACE

src/rp2_common/pico_stdio_usb/stdio_usb_descriptors.c

@@ -30,6 +30,7 @@
 
 #include "tusb.h"
 #include "pico/stdio_usb/reset_interface.h"
+#include "pico/unique_id.h"
 
 #define USBD_VID (0x2E8A) // Raspberry Pi
 #define USBD_PID (0x000a) // Raspberry Pi Pico SDK CDC
@@ -98,10 +99,12 @@ static const uint8_t usbd_desc_cfg[USBD_DESC_LEN] = {
 #endif
 };
 
+static char usbd_serial_str[PICO_UNIQUE_BOARD_ID_SIZE_BYTES * 2 + 1];
+
 static const char *const usbd_desc_str[] = {
     [USBD_STR_MANUF] = "Raspberry Pi",
     [USBD_STR_PRODUCT] = "Pico",
-    [USBD_STR_SERIAL] = "000000000000", // TODO
+    [USBD_STR_SERIAL] = usbd_serial_str,
     [USBD_STR_CDC] = "Board CDC",
 #if PICO_STDIO_USB_ENABLE_RESET_VIA_VENDOR_INTERFACE
     [USBD_STR_RPI_RESET] = "Reset",
@@ -121,6 +124,11 @@ const uint16_t *tud_descriptor_string_cb(uint8_t index, uint16_t langid) {
     #define DESC_STR_MAX (20)
     static uint16_t desc_str[DESC_STR_MAX];
 
+    // Assign the SN using the unique flash id
+    if (!usbd_serial_str[0]) {
+        pico_get_unique_board_id_string(usbd_serial_str, sizeof(usbd_serial_str));
+    }
+
     uint8_t len;
     if (index == 0) {
         desc_str[1] = 0x0409; // supported language is English

src/rp2_common/pico_unique_id/include/pico/unique_id.h

@@ -57,6 +57,21 @@ typedef struct {
  */
 void pico_get_unique_board_id(pico_unique_board_id_t *id_out);
 
+/*! \brief Get unique ID in string format
+ *  \ingroup pico_unique_id
+ *
+ * Get the unique 64-bit device identifier which was retrieved from the
+ * external NOR flash device at boot, formatted as an ASCII hex string.
+ * Will always 0-terminate.
+ *
+ * On PICO_NO_FLASH builds the unique identifier is set to all 0xEE.
+ *
+ * \param id_out a pointer to a char buffer of size len, to which the identifier will be written
+ * \param len the size of id_out. For full serial, len >= 2 * PICO_UNIQUE_BOARD_ID_SIZE_BYTES + 1
+ */
+void pico_get_unique_board_id_string(char *id_out, uint len);
+
+
 #ifdef __cplusplus
 }
 #endif

src/rp2_common/pico_unique_id/unique_id.c

@@ -26,3 +26,14 @@ static void __attribute__((constructor)) _retrieve_unique_id_on_boot(void) {
 void pico_get_unique_board_id(pico_unique_board_id_t *id_out) {
     *id_out = retrieved_id;
 }
+
+void pico_get_unique_board_id_string(char *id_out, uint len) {
+    assert(len > 0);
+    size_t i;
+    // Generate hex one nibble at a time
+    for (i = 0; (i < len - 1) && (i < PICO_UNIQUE_BOARD_ID_SIZE_BYTES * 2); i++) {
+        int nibble = (retrieved_id.id[i/2] >> (4 - 4 * (i&1))) & 0xf;
+        id_out[i] = (char)(nibble < 10 ? nibble + '0' : nibble + 'A' - 10);
+    }
+    id_out[i] = 0;
+}