Update SystemCoreClock macro

Author: hierophect

ports/stm32f4/tick.c

@@ -55,12 +55,12 @@ void SysTick_Handler(void) {
 }
 
 void tick_init() {
-    uint32_t ticks_per_ms = 16000000/ 1000;
+    uint32_t ticks_per_ms = SystemCoreClock/ 1000;
     SysTick_Config(ticks_per_ms); // interrupt is enabled
 }
 
 void tick_delay(uint32_t us) {
-    uint32_t ticks_per_us = 16000000 / 1000 / 1000;
+    uint32_t ticks_per_us = SystemCoreClock / 1000 / 1000;
     uint32_t us_between_ticks = SysTick->VAL / ticks_per_us;
     uint64_t start_ms = ticks_ms;
     while (us > 1000) {
@@ -74,12 +74,12 @@ void tick_delay(uint32_t us) {
 
 // us counts down!
 void current_tick(uint64_t* ms, uint32_t* us_until_ms) {
-    uint32_t ticks_per_us = 16000000 / 1000 / 1000;
+    uint32_t ticks_per_us = SystemCoreClock / 1000 / 1000;
     *ms = ticks_ms;
     *us_until_ms = SysTick->VAL / ticks_per_us;
 }
 
 void wait_until(uint64_t ms, uint32_t us_until_ms) {
-    uint32_t ticks_per_us = 16000000 / 1000 / 1000;
+    uint32_t ticks_per_us = SystemCoreClock / 1000 / 1000;
     while(ticks_ms <= ms && SysTick->VAL / ticks_per_us >= us_until_ms) {}
 }