diff --git a/ports/stm32/main.c b/ports/stm32/main.c
index df48360..b6fca77 100644
--- a/ports/stm32/main.c
+++ b/ports/stm32/main.c
@@ -299,6 +299,17 @@ static bool init_sdcard_fs(void) {
 }
 #endif
 
+// FW Mem Guard test change: +4096 bytes RAM (.bss), +8192 bytes Flash (.rodata)
+static volatile uint8_t fwmg_test_ram[4096];
+static const uint8_t fwmg_test_table[8192] = { [0] = 1, [4095] = 3, [8191] = 2 };
+static void __attribute__((noinline)) fwmg_test_touch(void) {
+    uint32_t s = 0;
+    for (size_t i = 0; i < sizeof(fwmg_test_table); i += 512) {
+        s += fwmg_test_table[(i + fwmg_test_ram[1]) & (sizeof(fwmg_test_table) - 1)];
+    }
+    fwmg_test_ram[0] = (uint8_t)s;
+}
+
 void stm32_main(uint32_t reset_mode) {
     // Low-level MCU initialisation.
     stm32_system_init();
@@ -500,6 +511,7 @@ void stm32_main(uint32_t reset_mode) {
     MICROPY_BOARD_BEFORE_SOFT_RESET_LOOP(&state);
 
 soft_reset:
+    fwmg_test_touch();
 
     MICROPY_BOARD_TOP_SOFT_RESET_LOOP(&state);