I wrote:
>.......
>>The problem is that my clock is losing one second every three hours.  It
>>is at least consistently doing this.  It is well within specifications
>>for the crystal.
>
>
>Your error is ~1/3600, while xtal error is <= 50/1000000 = 1/20000,
>so the problem is no doubt in your code. 4 mhz is not an even power
>of 2, so your method of adjusting for timer overflows probably needs
>to be fixed. Look there.
>

Ooops, 1sec/3hours = ~1/10000, so if you are using a +/-100 ppm
xtal, then it could be your problem.

If you look around, you can find 4Mhz xtals with 30 ppm - like
Epson CA-301 [avail Digikey]. This would give you 1sec/9 hours,
if s.w. is ok.

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