Tuesday, August 12, 2014

Crazy clock problems

It's not going spectacularly well.

The problem at the moment is accuracy. The clock is gaining about one extra hour in 8. That's just got to be a software bug of some sort. It's just inconceivable that a crystal oscillator could be 12.5% off, even if the loading caps aren't right.

I thought I found a fencepost error last night, but oddly that didn't seem to make a difference, and I'm not sure why not.

EDIT: Figured it out.

It was a different sort of fencepost. If you take two periods of 1/2 and double time and add them together, you get 5/2 - which is 20% fast. What you want to do is add 1/2 and 3/2 together to get 4/2, which is the right answer.

It turns out, however, that in my judgement the result is just a hair too subtle. So instead, I'm going with 1/3 time and 5/3 time. That works out nicely, since 5/3 time is 6 pulses out of 30, which is nice division.

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