Confirm no systick_isr() interruption

This saves a cycle - and checks both values to avoid _isr change of either value based on code/execution

teensy4/delay.c

@@ -62,6 +62,17 @@ void delay(uint32_t msec)
 	// TODO...
 }
 
+uint32_t micros(void)
+{
+	uint32_t ccdelta, usec, smc, scc;
+	do {
+		smc = systick_millis_count;
+		scc = systick_cycle_count;
+	} while ( smc != systick_millis_count || scc != systick_cycle_count ); // repeat if systick_isr
+	ccdelta = ARM_DWT_CYCCNT - scc;
+	usec = 1000*smc + (ccdelta/(F_CPU_ACTUAL/1000000));
+	return usec;
+}
 uint32_t micros(void)
 {
 	uint32_t ccdelta, usec, smc;