Maximum frequency count on 16F628/4MHz

[solara]

I should have mentioned this is an external asynchronous clock signal, which is being measured.

[Melanie]

Datasheet, Table 17-7, Parameters 45, 46 & 47. These values are independent of actual Processor Clock Speed.

[solara]

For an asynchronous external clock on a 16F628
45=30ns
46=30ns
47=60ns

This doesnt help me as there is no specified minimimum rise and fall times.
And from the figures above 45+46+rise+fall=47.

[solara]

Is this saying that the maximum external asynchronous clock frequency is 1/60ns =16 MHZ?
Sounds far too low.

[Melanie]

That's what it's telling you. If you don't believe it, take a signal generator and run a test. The PIC isn't exactly a speed demon.

[Darrel Taylor]

Hi solara,

You had specified Timer1, which is slower than Timer0.

If you use Timer0, you can measure up to 50Mhz, even with a 4Mhz osc.

Using the Prescaler, it actualy works like a 16-bit Counter. It's a little tricky to get the lower 8-bits. But it can be done.

Check out AN592 from Microchip
AN592: Frequency Counter Using PIC16C5x

HTH,
   Darrel

[solara]

Thanks Darrrel,
Thats what I wanted to hear.