After looking at the last message that Scott sent me, I think that I have a
solution that fits my purposes.  It will accumulate up to 65,535 counts
without error and times out after 65,791 counts.  The count in the region
between those two numbers is held at 65,535 (ffff).

;measure puse width on a 12c508 with 5 uSec resolution.
;Times out after about 327 mSec.

        clrf    c_low
        clrf    c_high

s1:
        BTFSC   IOPORT,IOBIT    ;5 cycles
         goto   went_high

        INCFSZ  c_low,F
         goto   s1

        clrwdt                  ;

        BTFSC   IOPORT,IOBIT    ;5 cycles
         goto   went_high

        INCFSZ  c_high,F
         goto   s1

;note: each time HI byte increments, LO byte to misses a count
;final count == LoByte + HiByte

time_out:
;do time out stuff here

went_high                       ;fix up the mod 257 counter

        movfw       c_high      ;
        addwf       c_low,F
        skpnc                   ;no carry - leave Hi byte alone
          incfsz    c_high,F    ;carry - does Hi byte contain ff?
            goto    $+2         ;either no carry or did not wrap
        movwf       c_low       ;wrapped past ffff - hold at ff
        movwf       c_high      ;ditto

The logic is simple - if the high byte contained ff and a carry was
generated when the 2 bytes were added together (which would wrap the high
byte to 00), the value in W (which could only be ff) is written to both
bytes.  The choice of $+2 for the junp is deliberate - both paths take the
same # of cycles.


My thanks to all who contributed ideas and suggestions.  Scott - Thank you
again for taking time out of your very busy day.

dwayne