Must be Steve's day off.

Hi Squib,

Well it depends on the internal oscillator type, and how you're using the interrupts.

The typical general purpose crystal will have a tolerance of +/- 50 PPM (Parts per million) Although there are other crystals with tighter tolerances, some down to less than 1 PPM. But they come at a higher cost.

If you're using one of the older 4mhz internal oscillators, depending on temperature and proper calibration numbers, they can vary up to +/-10% (100,000 PPM). A HUGE difference.

The newer 8mhz oscillators will vary a modest +/- 2.5% over the temperature range, but that's still 25,000 PPM.

If the interrupts are being used for "Long Term" timing like a clock, a difference of 1 PPM translates to an error of +/- 31.5 seconds per year. At 50 PPM it's 26.3 minutes/yr

But with the 8mhz INT_OSC, you're looking at a whopping 9.1 Days/Yr.. And with the old 4mhz osc you're off by +/-36.5 days. Yikes!

Of course these are "Worst case" numbers with maximum temperature extremes, actual results may vary.

hth,