James wrote:
>
> I am programming an interrupt service for 16F84A which provides a time base in units of seconds.  My project is to make . . . yet another clock, but this "time" using a 4.000 MHZ xtal, which is not the most natural choice, but offers some
> challenge, maybe more than I thought.  Have not yet produced accurate time :-(   and don't know why.  Off by about 1 to 2 percent.

Instead of adjusting the TMR0 value, which can cause large long term
effects, try adjusting another value further down the time data chain.

For example, if on TMR0 overflow, you increment some registers, and when
these registers equal a set value, the seconds register increments and
so forth.

Try adjusting the comparison value for these registers, so that the
value is lower if the time is slow, or raise the value if the time is
fast.

You will find that the clock will drift one way or the other depending
on temperature, so using the mains as a frequency source is much more
accurate.


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Best regards

Tony

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