WBR, Thanks so much.  What you state makes complete sense to me.  I should be able to fix the routine to account for the current lower limit problem based on your help.  I'm still not sure why the result varies approximately a quarter to a half hertz though.  
Thanks again!
Fran
----------------------------------------
> Date: Sat, 6 Mar 2010 16:15:37 +1300
> From: joecolquitt@clear.net.nz
> Subject: Re: [PIC]:How to measure low frequencies with PIC
> To: piclist@mit.edu
>
>> doesn't work below about 1200hz
>
> Fran, here's why (as I see it)
>
> PIC processing speed = 5MHz
> Timer1 can record 65,535 before overflowing
> 16 waves to count
>
> 700Hz = 7,143 cycles @ 5MHz => * 16 = 114,286. Too many for
> your measurement routine. The result is Timer1 = 48750 and TMR1IF
> set (a total of 65536 + 48750). Your s/w can add a 'TMR1IF set' test
>
> Re your observation that ~ 1200Hz is the lower limit
>
> 65,535 / 16 = 4,095.9 cycles per wave = 1221Hz
>
> Using a 2:1 pre-scaler that will extend the lower limit to 610Hz
>
> Your 1/25th Hz precision is still well within spec with a pre-scaler. At
> 4kHz 1/25th of a wave is 10 cycles. A 2:1 pre-scaler reduces this to 5
> cycles for just one wave measurement. At 512 waves the reduction /
> error is negligible
>
> wbr
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