Improved accuracy of common_hal_mcu_delay_us of STM port.

SysTick Current Value Register must be cleared before enabling.

Author: EmergReanimator

ports/stm/common-hal/microcontroller/__init__.c

@@ -41,13 +41,14 @@
 #include "supervisor/shared/safe_mode.h"
 
 void common_hal_mcu_delay_us(uint32_t delay) {
-    uint32_t ticks_per_us = HAL_RCC_GetSysClockFreq() / 1000000;
+    uint32_t ticks_per_us = HAL_RCC_GetSysClockFreq() / 1000000UL;
     delay *= ticks_per_us;
+    SysTick->VAL = 0UL;
     SysTick->LOAD = delay;
     SysTick->CTRL = SysTick_CTRL_CLKSOURCE_Msk | SysTick_CTRL_ENABLE_Msk;
     while ((SysTick->CTRL & SysTick_CTRL_COUNTFLAG_Msk) == 0) {
     }
-    SysTick->CTRL = 0;
+    SysTick->CTRL = 0UL;
 }
 
 volatile uint32_t nesting_count = 0;