Denny Esterline wrote:

>>>I'm sure I can find a way around this, but it seems like the kind of thing 
>>>that must have been done before. Maybe I don't have to reinvent the wheel :-)
>>>
>>>I have a sensor hooked to an input capture, the data I care about is 
>>>frequency. (DSPIC 30F4012, freq<3kHz if it matters) The input capture 
>>>trigers an interrupt every 16 pulses and I read a timer value. This works, 
>>>but it's not actualy measuring frequency, it measures period which is 
>>>1/frequency. The result is an exponential function when I wanted a linear 
>>>one. How have other people dealt with this?
>> 
>> frequency = 1/period. If you want it refreshed with every 16 pulses,
>> you will have more than 5 msec to do the calculation, which should be
>> plenty on regular PIC let alone a dsPIC.
> 
> Like I said, I'm sure I can find a way around this, the math to do
> 1/period is certainly one way. Or a lookup table, or changing it to
> count pulses over a timer interrupt. All of which have various
> drawbacks, I was realy just polling for other peoples ideas.

Depending on your measuring range, resolution and measuring interval
requirements, you can set a fixed timer interrupt and read the pulse count
between the interrupts -- which is proportional to the frequency. When the
frequency to be measured is high compared to the measuring frequency, that
works. But this doesn't sound to be your case. 

Gerhard

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