diff --git a/ports/atmel-samd/mpconfigport.h b/ports/atmel-samd/mpconfigport.h
index 73686e1445..6fb969e47b 100644
--- a/ports/atmel-samd/mpconfigport.h
+++ b/ports/atmel-samd/mpconfigport.h
@@ -173,6 +173,30 @@
 #define BOARD_HAS_CRYSTAL (0)
 #endif
 
+#ifndef BOARD_XOSC_FREQ_HZ
+// 0 Indicates XOSC is not used.
+  #define BOARD_XOSC_FREQ_HZ (0)
+#else
+// For now, only allow external clock sources that divide cleanly into
+// the system clock frequency of 120 MHz.
+  #if (120000000 % BOARD_XOSC_FREQ_HZ) != 0
+    #error "BOARD_XOSC_FREQ_HZ must be an integer factor of 120 MHz"
+  #endif
+#endif
+
+#if BOARD_XOSC_FREQ_HZ != 0
+// External clock sources are currently not implemented for SAMD21 chips.
+  #ifdef SAMD21
+    #error "BOARD_XOSC_FREQ_HZ is non-zero but external clock sources are not yet supported for SAMD21 chips"
+  #endif
+  #ifndef BOARD_XOSC_IS_CRYSTAL
+    #error "BOARD_XOSC_IS_CRYSTAL must be defined to 0 or 1 if BOARD_XOSC_FREQ_HZ is not 0"
+  #endif
+#else
+// It doesn't matter what the value is in this case.
+  #define BOARD_XOSC_IS_CRYSTAL (0)
+#endif
+
 // if CALIBRATE_CRYSTALLESS is requested, make room for storing
 // calibration data generated from external USB.
 #ifndef CIRCUITPY_INTERNAL_CONFIG_SIZE
diff --git a/ports/atmel-samd/peripherals b/ports/atmel-samd/peripherals
index baca4c0843..82e514b6e0 160000
--- a/ports/atmel-samd/peripherals
+++ b/ports/atmel-samd/peripherals
@@ -1 +1 @@
-Subproject commit baca4c084334aa8625f525a4032d66a397199ea6
+Subproject commit 82e514b6e0d1a2b09dc73be9973663b6b837a817
diff --git a/ports/atmel-samd/supervisor/port.c b/ports/atmel-samd/supervisor/port.c
index 8cba5c61e0..8ae2d3740e 100644
--- a/ports/atmel-samd/supervisor/port.c
+++ b/ports/atmel-samd/supervisor/port.c
@@ -353,10 +353,10 @@ safe_mode_t port_init(void) {
     if (strcmp((char *)CIRCUITPY_INTERNAL_CONFIG_START_ADDR, "CIRCUITPYTHON1") == 0) {
         fine = ((uint16_t *)CIRCUITPY_INTERNAL_CONFIG_START_ADDR)[8];
     }
-    clock_init(BOARD_HAS_CRYSTAL, fine);
+    clock_init(BOARD_HAS_CRYSTAL, BOARD_XOSC_FREQ_HZ, BOARD_XOSC_IS_CRYSTAL, fine);
     #else
     // Use a default fine value
-    clock_init(BOARD_HAS_CRYSTAL, DEFAULT_DFLL48M_FINE_CALIBRATION);
+    clock_init(BOARD_HAS_CRYSTAL, BOARD_XOSC_FREQ_HZ, BOARD_XOSC_IS_CRYSTAL, DEFAULT_DFLL48M_FINE_CALIBRATION);
     #endif
 
     rtc_init();