how exactly does the pic capture frequencies of up to 50 MHz with only a
4 MHz ocillator?  what is the mechanism?

jon

Regulus Berdin wrote:
>
> Peter Gee wrote:
> >
> > What would happen in the schematic shown if the input voltage when negative
> > with respect to the gnd of the PIC ?
> The protections diodes may protect it from damage but excessive current
> may destroy it ultimately.
>
> > my guess is that it wouldnt be good for the PIC.
> > in a similar project ( from memory), they use a coil, capacitor, signal
> > diodes, and a transistor to filter and switch the incoming frequency to make
> > it nicer for the PIC to count.
> The posted circuit is incomplete, for a more sensitive frequency
> counter, it needs a pre-amplifier and convert its output to TTL level.
>
> > i built the project, and it seemed to give accurate results in comparison to
> > a bench frequency counter at a frequency of 40Mhz using a 10Mhz 16cf84.  (i
> > used an fm RC transmitter to calibrate it) -- i don't have a frequency
> > generator higher than that so i can't test its high end response.
> The PIC doesn't need to be 10MHz, the input is captured in the RTCC
> using it's prescaler which is rated at 50MHz. The pre-scaler cannot be
> read directly, that is why RB0 is connected at RA4 to self clock the
> pre-scaler to get it's value.  The 470 ohm resistor is to prevent
> contention from the input and RB0.
>
> Reggie