Hiya, gang.

I'm trying to measure the period of a pulse that varies from 5mS through
30mS with at better than 10 uS resolution.  The target processor is running
at 4 MHz (1uS instruction cycles).  My problem is that I need a timeout
function, so that a missing signal doesn't become a fatal condition.

The easiest way seemed to call for a 16 bit counter.  I immediately thought
of Andrew Warren's fast 5 cycle counter, but I can't think of an easy way to
detect overflow without increasing the loop time drastically.

so... I came up with the following:

;measure HI period

    clrf        HiPerLO
    clrf        HiPerHI
    movlw       1

MeasHiPerLoop                       ;8 cycles per count
    addwf       HiPerLO,F           ;add puts carry in known state
    skpnc                           ;lower byte roll over?
      addwf     HiPerHI,F
    skpnc                           ;upper byte roll over?
      goto      TimeOut
    btfsc       PIN                 ;pin still HI?
      goto      MeasHiPerLoop

I get 8 cycles for the loop.  I can also limit my timeout period by using
bit tests on the high byte if necessary (replace the 2nd 'skpnc' with a
btfsc  HiPerHI, n).  But I REALLY like Andy's 5 cycle version and am hoping
that someone can come up with an idea on how to do better than what I've
come up with.

Just a reminder: here is Andy's 5 cycle timing loop:

Andrew Warren wrote:
        ; 5 cycles per count

         CHECK_PULSE:

             INCFSZ   pulse_width_lo
             DECF     pulse_width_hi

             BTFSC    PULSE_PORT,PULSE_BIT
             GOTO     CHECK_PULSE

         DONE:

             MOVF     pulse_width_lo,W
             ADDWF    pulse_width_hi


My other thought was to use Andy's version, then use the watchdog timer to
limit how long I would wait for the pulse to end.  But I need the prescaler
for my main timekeeping routines - and I couldn't reliably measure pulses
that would approach the full time that the counters would be capable of.

Ideas, suggestions?

Thanks!

dwayne