diff --git a/libraries/AP_HAL_ChibiOS/hwdef/scripts/chibios_hwdef.py b/libraries/AP_HAL_ChibiOS/hwdef/scripts/chibios_hwdef.py
index 92aac71b8530d..dbad4d8f33ba6 100644
--- a/libraries/AP_HAL_ChibiOS/hwdef/scripts/chibios_hwdef.py
+++ b/libraries/AP_HAL_ChibiOS/hwdef/scripts/chibios_hwdef.py
@@ -768,6 +768,8 @@ def write_mcu_config(f):
         'FLASH_RESERVE_START_KB', default=16, type=int)
     f.write('\n// location of loaded firmware\n')
     f.write('#define FLASH_LOAD_ADDRESS 0x%08x\n' % (0x08000000 + flash_reserve_start*1024))
+    f.write('#define EXTERNAL_PROG_FLASH_MB %u\n' % get_config('EXTERNAL_PROG_FLASH_MB', default=0, type=int))
+    env_vars['EXTERNAL_PROG_FLASH_MB'] = get_config('EXTERNAL_PROG_FLASH_MB', default=0, type=int)
     if args.bootloader:
         f.write('#define FLASH_BOOTLOADER_LOAD_KB %u\n' % get_config('FLASH_BOOTLOADER_LOAD_KB', type=int))
         f.write('#define FLASH_RESERVE_END_KB %u\n' % get_config('FLASH_RESERVE_END_KB', default=0, type=int))
@@ -870,10 +872,23 @@ def write_mcu_config(f):
 #define HAL_NO_UARTDRIVER
 #define HAL_NO_PRINTF
 #define HAL_NO_CCM
+#define HAL_USE_I2C FALSE
+#define HAL_USE_PWM FALSE
+#define CH_DBG_ENABLE_STACK_CHECK FALSE
+#define CH_CFG_USE_DYNAMIC FALSE
+// avoid timer and RCIN threads to save memory
+#define HAL_NO_TIMER_THREAD
+#define HAL_NO_RCOUT_THREAD
+#define HAL_NO_RCIN_THREAD
+#define HAL_NO_SHARED_DMA FALSE
+#define HAL_NO_ROMFS_SUPPORT TRUE
+#define HAL_NO_FLASH_SUPPORT TRUE
+''')
+        if not env_vars['EXTERNAL_PROG_FLASH_MB']:
+            f.write('''
 #define CH_CFG_USE_TM FALSE
 #define CH_CFG_USE_REGISTRY FALSE
 #define CH_CFG_USE_WAITEXIT FALSE
-#define CH_CFG_USE_DYNAMIC FALSE
 #define CH_CFG_USE_MEMPOOLS FALSE
 #define CH_DBG_FILL_THREADS FALSE
 #define CH_CFG_USE_SEMAPHORES FALSE
@@ -882,13 +897,6 @@ def write_mcu_config(f):
 #define CH_CFG_USE_EVENTS FALSE
 #define CH_CFG_USE_EVENTS_TIMEOUT FALSE
 #define CH_CFG_USE_MEMCORE FALSE
-#define HAL_USE_I2C FALSE
-#define HAL_USE_PWM FALSE
-#define CH_DBG_ENABLE_STACK_CHECK FALSE
-// avoid timer and RCIN threads to save memory
-#define HAL_NO_TIMER_THREAD
-#define HAL_NO_RCOUT_THREAD
-#define HAL_NO_RCIN_THREAD
 ''')
     if env_vars.get('ROMFS_UNCOMPRESSED', False):
         f.write('#define HAL_ROMFS_UNCOMPRESSED\n')