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
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist