Andy,

> PIC16F84 with 4MHz clock
> TMR0 driven from the external clock

I think you mean from the internal clock, i.e. T0CS = 0, right?

> the prescaler assigned to TMR0 and set to /256
> load TMR0 with 0x3D, so that TMR0 increments 195 time before wrapping
> This generates an interrupt from the timer wrap every 50ms or so. I want
> this timer to keep running, so as part of the interrupt handler, I reprime
> TMR0 with 0x3D to wait for another 50ms,

OK so far.

> but in doing so, I discovered, this
> turns off the prescaler.

No.  writing to TMR0 register does not change the prescaler
assignment.  It does _clear_ the prescaler register to 0, which
means that the clock cycles since rollover are discarded.  Exactly
how many cycles are "missed" depends on how many cycles your
interrupt routine uses.

> So my fix was also to reset the OPTION byte to
> reassign the prescaler each time I reprime TMR0.
> This works fine

But its not needed.  What was the symptom that led you to believe it
was needed?  Does the program work if you remove that re-set of
the OPTION register?

One other point- when you re-assign the prescalerto TMR0 by
writing OPTION, you should always CLRWDT first according to the
manual. There seems to be the possibility that a spurious WDT reset
will occur when the OPTION write flips the prescaler switches away
from the WDT.  Maybe that was what you were seeing?  A Reset will
set OPTION back to 0FFh, so the prescaler is back on the WDT.

Regards,
------------
Barry King, KA1NLH
Engineering Manager
NRG Systems "Measuring the Wind's Energy"
Hinesburg, Vermont, USA
www.nrgsystems.com