improve sleep_us for low values

global.c

@@ -101,10 +101,12 @@ void led_toggle(void)
 
 void sleep_us(uint32_t duration)
 {
-	if (0 == duration) {
+	if (duration <= 5) { // less than the setup time
+		for (volatile uint32_t nop = 0; nop < 5 * duration; nop++); // busy loop, approximate, hand tuned for 72 MHz system clock
 		return;
 	}
 
+	duration -= 5; // subtract setup time
 	systick_counter_disable(); // disable SysTick to reconfigure it
 	if (!systick_set_frequency(1000000, rcc_ahb_frequency)) { // set SysTick frequency to microseconds
 		 while (true); // unhandled error