diff --git a/ports/stm32/mphalport.c b/ports/stm32/mphalport.c
index 2025388413..9961c3d998 100644
--- a/ports/stm32/mphalport.c
+++ b/ports/stm32/mphalport.c
@@ -114,6 +114,12 @@ void mp_hal_pin_config(mp_hal_pin_obj_t pin_obj, uint32_t mode, uint32_t pull, u
     GPIO_TypeDef *gpio = pin_obj->gpio;
     uint32_t pin = pin_obj->pin;
     mp_hal_gpio_clock_enable(gpio);
+    if (mode == MP_HAL_PIN_MODE_ALT || mode == MP_HAL_PIN_MODE_ALT_OPEN_DRAIN) {
+        // To avoid any I/O glitches, make sure a valid alternate function is set in
+        // AFR first before switching the pin mode. When switching from AF to INPUT or
+        // OUTPUT, the AF in AFR will remain valid up until the pin mode is switched.
+        gpio->AFR[pin >> 3] = (gpio->AFR[pin >> 3] & ~(15 << (4 * (pin & 7)))) | (alt << (4 * (pin & 7)));
+    }
     gpio->MODER = (gpio->MODER & ~(3 << (2 * pin))) | ((mode & 3) << (2 * pin));
     #if defined(GPIO_ASCR_ASC0)
     // The L4 has a special analog switch to connect the GPIO to the ADC
@@ -124,7 +130,6 @@ void mp_hal_pin_config(mp_hal_pin_obj_t pin_obj, uint32_t mode, uint32_t pull, u
     #endif
     gpio->OSPEEDR = (gpio->OSPEEDR & ~(3 << (2 * pin))) | (2 << (2 * pin)); // full speed
     gpio->PUPDR = (gpio->PUPDR & ~(3 << (2 * pin))) | (pull << (2 * pin));
-    gpio->AFR[pin >> 3] = (gpio->AFR[pin >> 3] & ~(15 << (4 * (pin & 7)))) | (alt << (4 * (pin & 7)));
 }
 
 bool mp_hal_pin_config_alt(mp_hal_pin_obj_t pin, uint32_t mode, uint32_t pull, uint8_t fn, uint8_t unit) {