Just wanted to say thanks Steve.  Your approach did the trick nicely.

I did one thing differently.  When the pin goes low indicating we want to
read the TMR0 and prescaler values, I switch the TMR0 to increment from the
PA4/T0CKI pin instead of the internal CLKOUT clock.  This allows me to clock
the PA4/T0CKI pin manually to determine the count remaining in the prescaler
without having to have a tight counting/polling loop.

Thanks again,
--Anil

At 12:00 PM 2/12/97 EST, you wrote:
>> From: Anil Patel <anil.patel@AUTODESK.COM>
>> [cut]
>> If anyone has a way of measuring pulse widths accurately, please let me know.
>
>The '84 does have a 16-bit counter, i.e. the prescaler (which is not
>directly accessible) and TMR0.  The trick is to somehow find out
>the contents of the prescaler at a particular point.  Although I have
>not tested the following idea, it should work.  Polling is used
>rather than interrupts because:
>        . interrupt latency is relatively tricky to deal with
>        . since this is a servo application, there's plenty of
>          time between pulses to accomplish something else.
>
>Set the prescaler to 1:256 so that TMR0 increments every 128us.  Now
>poll the input until it goes high.  Clear TMR0 (which also clears the
>prescaler).  Poll input until it goes low.  The combination of
>prescaler/TMR0 holds the low and high byte of the count (units of
>0.5us).  Save the high byte (TMR0), then poll TMR0 until it changes --
>you can just check its LSB.  While polling, an auxiliary counter is
>incremented to get an estimate of the remaining count left in the
>prescaler.  If the complete polling loop is N instruction cycles, then
>the aux count is multiplied by N to get the prescaler remainder, then
>subtracted from 256 to estimate the prescaler count at the time the
>input went low -- this becomes the low byte of the overall timer
>count.
>
>In reality, you will need a few other corrections to account for
>differing code path delays etc., however you should be able to get a
>resolution of about 3us.
>
>Regards,
>SJH
>Canberra, Australia
>