> I need to measure a continuous frequency between 0 - 250 cycles/MINUTE,
> and transmit it via RS-232 at 2400 baud in a message containing 6 bytes
> once 5 seconds.
>
> The PIC I plan to use is the low power 84 variant, running at 32 KHz,
> if possible.

I'd suggest doing the following:

[1] Use the following variables:
        CurrTime        : 16-bit
        LastEvent       : 16-bit
        LastEvLatch     : 16-bit
        PrevEvent       : 16-bit
        NextOutput      : 16-bit

        NumCounts       : 8-bit
        NumCtLatch      : 8-bit
        PrevCounts      : 8-bit

[2] Every 1/256 second (note: the time from the start bit to the beginning
    of the stop bit is just under 1/256 sec at 2400 baud) do the following:

        CurrTime++;
        if (EventHappened)
        {
                LastEvent=CurrTime;
                NumCounts++;
        }
        if (--NextOutput)
        {
                NextOutput = 1560; /* 256*5 */
                LastEvLatch = LastEvent;
                NumCtLatch = NumCounts;
        }

[3] To initialize, set NextOutput to 1560 and everything else to zero.

[4] For the main loop, run this:


..gotta go--will finish later.