diff --git a/src/boards/include/boards/adafruit_feather_rp2040.h b/src/boards/include/boards/adafruit_feather_rp2040.h
index a27f9a3..5049f6b 100644
--- a/src/boards/include/boards/adafruit_feather_rp2040.h
+++ b/src/boards/include/boards/adafruit_feather_rp2040.h
@@ -15,6 +15,11 @@
 // For board detection
 #define ADAFRUIT_FEATHER_RP2040
 
+// On some samples, the xosc can take longer to stabilize than is usual
+#ifndef PICO_XOSC_STARTUP_DELAY_MULTIPLIER
+#define PICO_XOSC_STARTUP_DELAY_MULTIPLIER 64
+#endif
+
 //------------- UART -------------//
 #ifndef PICO_DEFAULT_UART
 #define PICO_DEFAULT_UART 0
diff --git a/src/boards/include/boards/adafruit_itsybitsy_rp2040.h b/src/boards/include/boards/adafruit_itsybitsy_rp2040.h
index 3d3004e..26cde1d 100644
--- a/src/boards/include/boards/adafruit_itsybitsy_rp2040.h
+++ b/src/boards/include/boards/adafruit_itsybitsy_rp2040.h
@@ -15,6 +15,11 @@
 // For board detection
 #define ADAFRUIT_ITSYBITSY_RP2040
 
+// On some samples, the xosc can take longer to stabilize than is usual
+#ifndef PICO_XOSC_STARTUP_DELAY_MULTIPLIER
+#define PICO_XOSC_STARTUP_DELAY_MULTIPLIER 64
+#endif
+
 //------------- UART -------------//
 #ifndef PICO_DEFAULT_UART
 #define PICO_DEFAULT_UART 0
diff --git a/src/boards/include/boards/adafruit_qtpy_rp2040.h b/src/boards/include/boards/adafruit_qtpy_rp2040.h
index e3384fb..c273ddc 100644
--- a/src/boards/include/boards/adafruit_qtpy_rp2040.h
+++ b/src/boards/include/boards/adafruit_qtpy_rp2040.h
@@ -15,6 +15,11 @@
 // For board detection
 #define ADAFRUIT_QTPY_RP2040
 
+// On some samples, the xosc can take longer to stabilize than is usual
+#ifndef PICO_XOSC_STARTUP_DELAY_MULTIPLIER
+#define PICO_XOSC_STARTUP_DELAY_MULTIPLIER 64
+#endif
+
 //------------- UART -------------//
 #ifndef PICO_DEFAULT_UART
 #define PICO_DEFAULT_UART 1
diff --git a/src/rp2_common/hardware_xosc/include/hardware/xosc.h b/src/rp2_common/hardware_xosc/include/hardware/xosc.h
index 0aa0842..a5e33b0 100644
--- a/src/rp2_common/hardware_xosc/include/hardware/xosc.h
+++ b/src/rp2_common/hardware_xosc/include/hardware/xosc.h
@@ -10,6 +10,15 @@
 #include "pico.h"
 #include "hardware/structs/xosc.h"
 
+
+// Allow lengthening startup delay to accommodate slow-starting oscillators
+
+// PICO_CONFIG: PICO_XOSC_STARTUP_DELAY_MULTIPLIER, Multiplier to lengthen xosc startup delay to accommodate slow-starting oscillators, type=int, min=1, default=1, group=hardware_xosc
+#ifndef PICO_XOSC_STARTUP_DELAY_MULTIPLIER
+#define PICO_XOSC_STARTUP_DELAY_MULTIPLIER 1
+#endif
+
+
 #ifdef __cplusplus
 extern "C" {
 #endif
diff --git a/src/rp2_common/hardware_xosc/xosc.c b/src/rp2_common/hardware_xosc/xosc.c
index 977f0bd..9a95e37 100644
--- a/src/rp2_common/hardware_xosc/xosc.c
+++ b/src/rp2_common/hardware_xosc/xosc.c
@@ -11,16 +11,25 @@
 
 #include "hardware/platform_defs.h"
 #include "hardware/regs/xosc.h"
-#include "hardware/structs/xosc.h"
+#include "hardware/xosc.h"
+
+#if XOSC_MHZ < 1 || XOSC_MHZ > 15
+#error XOSC_MHZ must be in the range 1-15
+#endif
+
+#define STARTUP_DELAY (((((XOSC_MHZ * MHZ) / 1000) + 128) / 256) * PICO_XOSC_STARTUP_DELAY_MULTIPLIER)
+
+// The DELAY field in xosc_hw->startup is 14 bits wide.
+#if STARTUP_DELAY >= (1 << 13)
+#error PICO_XOSC_STARTUP_DELAY_MULTIPLIER is too large: XOSC STARTUP.DELAY must be < 8192
+#endif
 
 void xosc_init(void) {
-    // Assumes 1-15 MHz input
-    assert(XOSC_MHZ <= 15);
+    // Assumes 1-15 MHz input, checked above.
     xosc_hw->ctrl = XOSC_CTRL_FREQ_RANGE_VALUE_1_15MHZ;
 
     // Set xosc startup delay
-    uint32_t startup_delay = (((12 * MHZ) / 1000) + 128) / 256;
-    xosc_hw->startup = startup_delay;
+    xosc_hw->startup = STARTUP_DELAY;
 
     // Set the enable bit now that we have set freq range and startup delay
     hw_set_bits(&xosc_hw->ctrl, XOSC_CTRL_ENABLE_VALUE_ENABLE << XOSC_CTRL_ENABLE_LSB);
@@ -43,4 +52,4 @@ void xosc_dormant(void) {
     xosc_hw->dormant = XOSC_DORMANT_VALUE_DORMANT;
     // Wait for it to become stable once woken up
     while(!(xosc_hw->status & XOSC_STATUS_STABLE_BITS));
-}
\ No newline at end of file
+}