Update system clock.

src/main.c

@@ -16,7 +16,9 @@ static void tick_delay(uint32_t msec) {
 }
 
 int main(int argc, const char *argv[]) {
-    SysTick_Config(4000); /* 1kHz */
+    SystemCoreClockUpdate();
+
+    SysTick_Config(SystemCoreClock / 1000); /* 1kHz */
 
     libch57x_gpio_config_t led_cfg = {
         .bank     = LED_PORT,