> Can the Timer1 Module a 16F628 (@4Mhz) be used as a software 16bit
counter.

Depends what you mean.  The clock source is either derived from the
instruction clock or an external pin.

> I have scanned the last 30000 emails and looked at PICLIST.com  but all
the
> discussion seems to be using it driven via hardware.

That was 30,001 emails ago.

> I was experimenting with MPLAB and I got TMR1L to rollover to FF when I
> decremented TMR1H from FF to 00 but cant repeat the results. I tried all
> other combinations but they both appear to behave has 2 separate 8 bit
files
> when using decf or incf

Right.  They only act as one 16 bit register when incremented by the
hardware, and sometimes in conjunction with the CCP module.  I think you can
set the timer 1 source to RB6, set up RB6 as an output, then toggle RB6 from
software, although I haven't tried this.

> I thought perhaps MPLAB does not simulate the TMR1 faithfully.

It usually does a pretty good job of these things.

> What I am trying to do is see how many 26uS units there are in a ext
pulse.
> Using 8 bits for counter the max pulse width is 6.6mS. 16bit gives 1690mS.
>
> I got TMR0 to generate 26uS periods but the overheads involved in context
> saving and bank switching did leave much time for anything else.  It did
> work but not enough time to see if an 8bit counter rolled over and then
inc
> another counter.
>
> If Timer1 behaved as a 16bit software counter it would save much in
> overheads and be a great solution.  Anyone used it this way.

It sounds like you are trying to use timer 1 backwards.  Let the hardware
increment it at a steady rate, and take a snapshot of it at the begginning
and end of the period you want to measure.  The CCP module in capture mode
can help do this very accurately.  This is the basis for many frequency
measurement type applications.


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