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[Reply #15 on:]

I turn my egg-timer over.

[Reply #16 on:]

The answer is "Yes, it can very easily be done", with the rider "...but why would you want to?".

Two riders.  Resetting millis can cause serious problems that are difficult to debug and impossible to fix (except by letting millis run free).

[Reply #17 on:]

Resetting millis()? That is very cool, but you don't need it, as other people said. It may also cause bad effects to some functions. Most people offset millis() when they want to execute some statements at intervals or time how long an action takes, etc. That is the same as boiling an egg for 10 minutes; if you start at 11:00 a.m., you would check back at 11:10 a.m., right?

You don't reset your watch to midnight every time you want to boil an egg.

I have devoloped a crude and somewhat flexible library class for that purpose: http://arduino.cc/forum/index.php/topic,107716.msg813967.html#msg813967. There is also http://arduino.cc/forum/index.php/topic,93959.0.html and many other examples.

Maybe add:
      TCNT0 = 0;

Add it where?

elapsed = finished - start

[Reply #18 on:]

Maybe add:
      TCNT0 = 0;

Add it where?

elapsed = finished - start

Code:

void millis_reset(void) {
  cli(); // Disable interrupts
  timer0_millis = timer0_fract = 0;
  TCNT0 = 0;
  sei(); // Re-enable interrupts
}

[Reply #19 on:]

@JimEli (et al): You failed to reset the internal state of the clock divisor.

[Reply #20 on:]

I appear to have stirred a bit of a hornets nest, to which I apologise.

The reason that I wanted to be able to reset millis() is that I the circuit is running long term, it needs to have an awareness of time but not of "when" it is, just that time has happened. A bit like a stop watch, it is able to tell you the duration of an event, but not the time it started and finished.

You don't reset your watch to midnight every time you want to boil an egg.

No, but you do reset an egg timer every time you want to boil an egg. There is a difference between telling the time and recording what time has passed. A watch is good for telling you when you are in relation to our passage round the Sun (or the Earth's rotation for a 24 hour clock), and ensuring everyone is synced up. A timer is more about how much time has passed since it was arbitrarily started/reset, the actual time it was started and stopped is irrelevant. To continue that analogy; you don't wait for a stopwatch to roll over to zero before starting the next 100 meter race.

That is the sort of functionality I feel the millis() should be providing. If I want a clock I'll connect up to an IC that gives me time and date (and to a greater accuracy then millis() can provide), all millis() needs to do is provide a reasonable sense of time, not the time. When you only need to worry about small durations of time (minutes at best) having to program in extra to deal with roll-overs is just a PITA, when all I really want is to reset my stopwatch.

[Reply #21 on:]

, all millis() needs to do is provide a reasonable sense of time, not the time

But that is exactly what it does do.

[Reply #22 on:]

When you only need to worry about small durations of time (minutes at best) having to program in extra to deal with roll-overs is just a PITA, when all I really want is to reset my stopwatch.

First entry...
http://lmgtfy.com/?q=arduino+stopwatch+library

[Reply #23 on:]

When you only need to worry about small durations of time (minutes at best) having to program in extra to deal with roll-overs is just a PITA, when all I really want is to reset my stopwatch.

You're making handling rollover out to be a big deal, when it isn't.

This code handles the millis() roll over:

Code:

if ((long)(millis() - waitMillis) >= 0) {
    // do whatever...
    waitMillis += 1000;  // Increment the timer.
}

That is all you need to do.  This is the method I use in all my projects, most recently a Binary Clock, which has been running for a couple of months with no problem.

For detailed explanation on why this works, read this article:
http://arduino.cc/playground/Code/TimingRollover

[Reply #24 on:]

@JimEli (et al): You failed to reset the internal state of the clock divisor.

maybe add some more...

Code:

void millis_reset(void) {
  cli(); // Disable interrupts
  timer0_millis = timer0_fract = 0;
  CLKPR = B10000000;
  CLKPR = 0;
  TCNT0 = 0;
  sei(); // Re-enable interrupts
}

[Reply #25 on:]

maybe add some more...

Congratulations.  There's just one thing left.

And, before you waste any more time trying to figure out what's missing, bear in mind that resetting millis wreaks havoc with some libraries; something you will not be able to overcome.  Resetting millis is unnecessary and can cause serious difficult to debug problems.