diff --git a/ports/atmel-samd/Makefile b/ports/atmel-samd/Makefile
index 888729c7b7f3d..9582eff212d60 100644
--- a/ports/atmel-samd/Makefile
+++ b/ports/atmel-samd/Makefile
@@ -107,7 +107,7 @@ else
   endif
 endif
 
-CFLAGS += $(INC) -Wall -Werror -std=gnu11 -nostdlib -fshort-enums $(BASE_CFLAGS) $(CFLAGS_MOD) $(COPT) -Werror=missing-prototypes
+CFLAGS += $(INC) -Wall -Werror -std=gnu11 -nostdlib -fshort-enums $(BASE_CFLAGS) $(CFLAGS_MOD) $(COPT) -Werror=missing-prototypes -Werror=old-style-definition
 
 ifeq ($(CHIP_FAMILY), samd21)
 CFLAGS += \
diff --git a/ports/atmel-samd/audio_dma.c b/ports/atmel-samd/audio_dma.c
index d5f841f371dd9..e25b74d9a2893 100644
--- a/ports/atmel-samd/audio_dma.c
+++ b/ports/atmel-samd/audio_dma.c
@@ -30,7 +30,7 @@ static audio_dma_t *audio_dma_state[AUDIO_DMA_CHANNEL_COUNT];
 // This cannot be in audio_dma_state because it's volatile.
 static volatile bool audio_dma_pending[AUDIO_DMA_CHANNEL_COUNT];
 
-uint8_t find_sync_event_channel_raise() {
+uint8_t find_sync_event_channel_raise(void) {
     uint8_t event_channel = find_sync_event_channel();
     if (event_channel >= EVSYS_SYNCH_NUM) {
         mp_raise_RuntimeError(MP_ERROR_TEXT("All sync event channels in use"));
diff --git a/ports/atmel-samd/common-hal/_pew/PewPew.c b/ports/atmel-samd/common-hal/_pew/PewPew.c
index 84ff7bc16c734..c3f9330faaef9 100644
--- a/ports/atmel-samd/common-hal/_pew/PewPew.c
+++ b/ports/atmel-samd/common-hal/_pew/PewPew.c
@@ -38,7 +38,7 @@ void pewpew_interrupt_handler(uint8_t index) {
     tc->COUNT16.INTFLAG.reg = TC_INTFLAG_MC0;
 }
 
-void pew_init() {
+void pew_init(void) {
     pew_obj_t *pew = MP_STATE_VM(pew_singleton);
 
     common_hal_digitalio_digitalinout_switch_to_input(pew->buttons, PULL_UP);
@@ -105,6 +105,6 @@ void pew_reset(void) {
     MP_STATE_VM(pew_singleton) = NULL;
 }
 
-uint16_t pew_get_ticks() {
+uint16_t pew_get_ticks(void) {
     return pewpew_ticks;
 }
diff --git a/ports/atmel-samd/common-hal/analogio/AnalogIn.c b/ports/atmel-samd/common-hal/analogio/AnalogIn.c
index c633cf44b678c..801ab18f7fd6d 100644
--- a/ports/atmel-samd/common-hal/analogio/AnalogIn.c
+++ b/ports/atmel-samd/common-hal/analogio/AnalogIn.c
@@ -64,7 +64,7 @@ void common_hal_analogio_analogin_deinit(analogio_analogin_obj_t *self) {
     self->pin = NULL;
 }
 
-void analogin_reset() {
+void analogin_reset(void) {
 }
 
 uint16_t common_hal_analogio_analogin_get_value(analogio_analogin_obj_t *self) {
diff --git a/ports/atmel-samd/common-hal/touchio/TouchIn.c b/ports/atmel-samd/common-hal/touchio/TouchIn.c
index dc31bfe634be5..822d614a60298 100644
--- a/ports/atmel-samd/common-hal/touchio/TouchIn.c
+++ b/ports/atmel-samd/common-hal/touchio/TouchIn.c
@@ -91,7 +91,7 @@ void common_hal_touchio_touchin_deinit(touchio_touchin_obj_t *self) {
     self->config.pin = NO_PIN;
 }
 
-void touchin_reset() {
+void touchin_reset(void) {
     Ptc *ptc = ((Ptc *)PTC);
     if (ptc->CTRLA.bit.ENABLE == 1) {
         ptc->CTRLA.bit.ENABLE = 0;