diff --git a/ports/stm32/boardctrl.h b/ports/stm32/boardctrl.h
index 551be3453..851922f41 100644
--- a/ports/stm32/boardctrl.h
+++ b/ports/stm32/boardctrl.h
@@ -60,6 +60,12 @@
 #define MICROPY_BOARD_END_SOFT_RESET boardctrl_end_soft_reset
 #endif
 
+// Called when USBD CDC data is available.
+// Default function defined in usbd_cdc_interface.h.
+#ifndef MICROPY_BOARD_USBD_CDC_RX_EVENT
+#define MICROPY_BOARD_USBD_CDC_RX_EVENT usbd_cdc_rx_event_callback
+#endif
+
 // Constants to return from boardctrl_run_boot_py, boardctrl_run_main_py.
 enum {
     BOARDCTRL_CONTINUE,
diff --git a/ports/stm32/usbd_cdc_interface.c b/ports/stm32/usbd_cdc_interface.c
index 61a7c8248..b369524ca 100644
--- a/ports/stm32/usbd_cdc_interface.c
+++ b/ports/stm32/usbd_cdc_interface.c
@@ -37,6 +37,7 @@
 #include <stdbool.h>
 #include <stdint.h>
 
+#include "boardctrl.h"
 #include "usbd_cdc_msc_hid.h"
 #include "usbd_cdc_interface.h"
 #include "pendsv.h"
@@ -308,7 +309,7 @@ int8_t usbd_cdc_receive(usbd_cdc_state_t *cdc_in, size_t len) {
         }
     }
 
-    usbd_cdc_rx_event_callback(cdc);
+    MICROPY_BOARD_USBD_CDC_RX_EVENT(cdc);
 
     if ((cdc->flow & USBD_CDC_FLOWCONTROL_RTS) && (usbd_cdc_rx_buffer_full(cdc))) {
         cdc->rx_buf_full = true;