diff --git a/ports/stm32f4/common-hal/busio/UART.c b/ports/stm32f4/common-hal/busio/UART.c
index c4ab237cd1..5ddfa6a4c7 100644
--- a/ports/stm32f4/common-hal/busio/UART.c
+++ b/ports/stm32f4/common-hal/busio/UART.c
@@ -250,10 +250,10 @@ size_t common_hal_busio_uart_read(busio_uart_obj_t *self, uint8_t *data, size_t
     }
 
     size_t rx_bytes = 0;
-    uint64_t start_ticks = ticks_ms;
+    uint64_t start_ticks = supervisor_ticks_ms64();
 
     // Wait for all bytes received or timeout, same as nrf
-    while ( (ringbuf_count(&self->rbuf) < len) && (ticks_ms - start_ticks < self->timeout_ms) ) {
+    while ( (ringbuf_count(&self->rbuf) < len) && (supervisor_ticks_ms64() - start_ticks < self->timeout_ms) ) {
         RUN_BACKGROUND_TASKS;
         //restart if it failed in the callback
         if (errflag != HAL_OK) {
diff --git a/ports/stm32f4/common-hal/microcontroller/__init__.c b/ports/stm32f4/common-hal/microcontroller/__init__.c
index c7493a4d60..ffab5d1e50 100644
--- a/ports/stm32f4/common-hal/microcontroller/__init__.c
+++ b/ports/stm32f4/common-hal/microcontroller/__init__.c
@@ -48,9 +48,9 @@ STATIC uint32_t get_us(void) {
     uint32_t ticks_per_us = HAL_RCC_GetSysClockFreq()/1000000;
     uint32_t micros, sys_cycles;
     do {
-        micros = ticks_ms;
+        micros = supervisor_ticks_ms32();
         sys_cycles = SysTick->VAL; //counts backwards
-    } while (micros != ticks_ms); //try again if ticks_ms rolled over
+    } while (micros != supervisor_ticks_ms32()); //try again if ticks_ms rolled over
     return (micros * 1000) + (ticks_per_us * 1000 - sys_cycles) / ticks_per_us;
 }