N STEENKAMP [M.ING E&E] wrote:
>
> > Depending upon CPU speed and power requirements, you may be able to get
> > by without an external RTC chip.  There are three approaches you may
> > wish to consider:
> >
> > [1] Use any PIC with a normal-speed oscillator and keep it powered; since
> >     the PIC will use about 1-4mA, you will probably want to have it wall
> >     supplied.
>
> What kind of accuracy can one expect from a PIC with a X MHz crystal?
> If you assume that the crystal is off by 25ppm, you would be off by
> 0.000025 seconds per second.  This would mean gaining (or losing)
> 0.000025*3600*24=2.16 seconds in a 24 hour period.  This does not
> seem very accurate.
>
> >
> > [2] Use any PIC with a 32768Hz clock.  This will drop power consumption
>
> Is the 32768KHz crystals used for RTC's more accurate than the 'normal'
> crystals?  Or do you get the same kind of accuracy from them?
>
> Just wondering...
>
> Niki
You use correction factors:

Keep a count...

Every X counts = 1s
Every 60s = 1min - adjust count...
etc...

Did it with an 84 with 4mHz clock. Had to do corrections every
two seconds and on the hour and on the day. Accuracy is a couple
of seconds a year.

Nic van der Walt