Hi,

I am working with a RTC from Philips (PCF8593) but did work in the past with
Maxim ones, and they always face you with the accuracy problem, and how to
finetune the setup to get the max accuracy.

Given the basic accuracy of the device is directly related to the crystal
accuracy, having 20-50ppm typically is more than 5 minutes / year, but that
is what you can get.

Datasheet suggest to use a variable cap to finetune frequency to get better
accuracy, but I do not really understand the added value of that when
comparing with digital compensation from the PIC. I mean, I plan to have it
running for some time, then assuming temperature being almost constant (+-6=
=BA
C) the difference I would get with an accurate cronometer, could be let's
say RTC goes fast by 1 min, 27 sec in a month for instance, that number can
be written on EEPROM and then used by the PIC to reprogram the RTC
regularly, say once a week, to automatically compensate. I guess this is
much better than playing with variable caps that are so sensible than even
touching them will change the oscillator frequency. Or I am missing
something? This is not even suggested in the datasheet, and I wonder why.

-- =

Ariel Rocholl
Madrid, Spain
-- =

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