If you can't find a canned oscillator of 25Hz, you could look for one tha=
t=20
divides a bit easier (like 1.0Mhz?).  Ideally, you should divide in your=20
16F870 code, by setting the appropriate prescalers for Timer1, and adjust=
ing=20
as necessary, etc.

As an alternative, you could use a small PIC like a 12F629 (8-pin) and cr=
eate=20
the divider in that to produce a small stand-alone 25Hz signal.  Feed tha=
t=20
output to the Timer1 input of the 16F870.  Simpler I think, but it requir=
es=20
another PIC, but a small/low-cost one.

Cheers,
-Neil.



On Wednesday 27 August 2003 11:49, Hulatt, Jon scribbled:
> >> - if you are in 50 hz country: divide the mains frequency by two
>
> it's an automotive project unfortunately
>
> >> - what on earth would you need it for, why no generate it inside the
>
> PIC?
>
> much simpler programming i think. i need to adc at 25Hz precisely. i'm
> using 16F870 @ 20Mhz, so,
>  a 5Mhz instruction clock. So, even using Timer1 (16 bit), with the max=
imum
> prescaler, i still
> have to handle 9 full rollover interupts and then time an odd number of
> clocks on the last time.
> it's very doable, but i'd hoped to be able to use an external timer at
> 25Hz.
>
> >> Is there another clock output available that you can use?
>
> no, sic
>
>
> thanks chaps
>
> PS. Thanks wouter, my Wisp628 arrived today. Unfortunately I have to go=
 out
> tonight so no fun til tomorrow!

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