Rough 'n Ready Audio Frequency extraction with a PIC

[HankMcSpank]

HankMcSpank ,I would personally count the time between interrupts instead of the number of interrupts in 1/2 second. That way you can increase the low end resolution you are looking for. Even a 1000 hz clock will give you .1% resolution at 100 hz....

Dave Purola,
N8NTA

Dave you're right on the money there - I'm now getting much better stability with your suggested method, here's what I'm seeing onscreen for Timer1 counts between comparator interrupts (this was for approx 100Hz)...

Timer1 Count= 5007
Timer1 Count= 5005
Timer1 Count= 5007
Timer1 Count= 5004
Timer1 Count= 5007
Timer1 Count= 5004
Timer1 Count= 5007
Timer1 Count= 5005
Timer1 Count= 5007
Timer1 Count= 5004
Timer1 Count= 5007
Timer1 Count= 5005
Timer1 Count= 5007
Timer1 Count= 5004
Timer1 Count= 5007
Timer1 Count= 5004
Timer1 Count= 5007
Timer1 Count= 5004


...so my jitter is now something like 0.001% - that couls be down to my sigen not being stable even - it has been known to drift about a bit)

I've got a bit of a mental block with the last remaining bit though (converting to frequency in hertz! - which for all I don't actually need the result in Hz - as I'll just be comparing on the timer count alone to establish whether of not the frequency is stable - it'd be a nice to have!!)

So, what I'm seeing with my timer counts...

A 'Timer1 count' of 5000 equals 100Hz
A 'Timer1 count' of 500 = 1000Hz
Therefore the timer clock cycle = 2us (0.000002s)

What formula do I use to convert my Timer1 count into Hertz? (it's the floating point aspect that messing me up - else I'd just use this simple formula....

1 / ('timer1 count' * 0.000002)

If I add to the left side of the equation (to take the right hand side out of 'decimals' territory)...it cranks up into the millions - tilt!!!!

So how should I approach this Hertz conversion?