diff --git a/ports/stm32f3/boards.h b/ports/stm32f3/boards.h
index 25880e1..34253fb 100644
--- a/ports/stm32f3/boards.h
+++ b/ports/stm32f3/boards.h
@@ -35,7 +35,9 @@
 #include "board.h"
 
 // Flash Start Address of Application
+#ifndef BOARD_FLASH_APP_START
 #define BOARD_FLASH_APP_START   0x08004000
+#endif
 #define BOARD_RAM_START 0x20000000
 #define BOARD_RAM_SIZE 0x9FFF
 
diff --git a/ports/stm32f4/boards.h b/ports/stm32f4/boards.h
index 59b5f46..fd53b75 100644
--- a/ports/stm32f4/boards.h
+++ b/ports/stm32f4/boards.h
@@ -35,7 +35,9 @@
 #include "board.h"
 
 // Flash Start Address of Application
+#ifndef BOARD_FLASH_APP_START
 #define BOARD_FLASH_APP_START   0x08010000
+#endif
 
 // Double Reset tap to enter DFU
 #define TINYUF2_DFU_DOUBLE_TAP  1