Arduino millis() jumps by 2 every 43 milliseconds

I’ve been doing some software-modulated infrared light detection work, and for the signal to be sent correctly I need millisecond-accurate timings.  I realize professionals do this sort of thing with interrupts, but I hate debugging interrupt code, and I’m always afraid there’s some rare timing glitch that will kill the entire project at the worst possible moment.

I was using Arduino’s millis() function as my time base, and I kept getting weird screwy timing every 43 milliseconds.  I assumed it was my code, which does a bunch of other stuff like serial communication, but the glitch didn’t change regardless of what I did.  Turns out, the Arduino Uno’s oscillator runs at a power of two rate, 1.024 milliseconds per overflow, so every 1/0.024 = 41.666 milliseconds, it’s a full millisecond off.  Arduino wiring.c fixes this by keeping track of the fractional milliseconds, and adds a sort of “leap millisecond” every 43 milliseconds to keep things in sync.  This results in millis() instantly jumping up by more than one.

Here’s an example Arduino Uno sketch that demonstrates these timing jumps, and shows two fixes.

/* Demonstrate timing gaps in the millis() function */
void setup(void) {
 Serial.begin(57600); // to report the horror
}

extern "C" volatile unsigned long timer0_overflow_count; // from wiring.

unsigned long last=0; // last value of millis()
void loop(void) {
 unsigned long next;
 do { // watch the timer until it changes
   next=millis(); // jumps by 2 every 43 ms
   //Two possible fixes:
   // next=micros()/1000; // slow, but works
   // cli(); next=timer0_overflow_count; sei(); // faster, but 1024 microseconds/tick
 } while (next==last);

 int delta=next-last;
 if (delta!=1) {
   Serial.print(delta);
   Serial.print(" jump at ");
   Serial.println(next);
 }

 if (last%10000==0) {
   Serial.print("Still running at ");
   Serial.println(last);
 }
 last=next;
}

On my Arduino Uno, the millis() version of this reports regular timing glitches:

Still running at 0
2 jump at 43
2 jump at 86
2 jump at 128
2 jump at 171
2 jump at 214
2 jump at 256
2 jump at 299
2 jump at 342
...

The easy way to avoid these timing jumps is never to use millis() at all!  It works to just use micros()/1000, although looking at wiring.c’s implementation of delay(), you need to be a bit careful about when micros() overflows every hour.  You can also directly read the raw timer0_overflow_count as shown above, although the actual speed this increments depends on the Arduino clock rate, and you need to do cli/sei around the read to avoid jumps of +-256, as the interrupt function updates the two bytes of that value.  Both fixes never have leap milliseconds, and result in dependable timing.