At 10:55 PM 5/3/2005 -0700, you wrote:
>Denny & Byron,
>
>Thanks very much for the response.  I understand that I am probably
>setting myself up with a handicap by using the 12f675, but that is
>what I have so I'm trying it out first.

Okay, by going for one interrupt per cycle, I think it can be made
to work.


isr:movwf       w_temp                          ; save context
    swapf        STATUS,W
    bcf          STATUS,RP0
    bsf          GPIO,0                          ; bring pin high
    movwf        status_temp
    incf counter,F                       ; used by background program for 
timing
    bcf          INTCON,2                        ; clear timer interrupt flag
    nop
    nop
    nop
    nop
    nop
    nop
    movlw        .250
    bsf          GPIO,0                          ; bring pin low
    movwf        TMR0
    swapf        status_temp,W                   ; restore context
    movwf        STATUS
    swapf        w_temp,F
    swapf        w_temp,W
    retfie

In simulation, it gives about 37.7kHz at 44% or so duty cycle with a 4MHz 
clock.
(remember to 'calibrate' the internal oscillator in your code)
The background routine runs like a 60kHz or so PIC, so very slow,
but not insane. If you eliminate some of the nops above you'll
get faster background operation but less duty cycle. If you're driving
the LED directly you might want to swap bsf and bcf and sink current,
because the PIC is better at sinking current.

The 'counter' byte rolls over every 6.78ms or so, and can be used
to control your slower timing in the background routine. You would
detect the rollover by looking at the msb of counter. That would
allow several hundred instructions to execute in each background
loop before you'd miss a rollover. A single byte in the background
routine could time up to 1.7 seconds or so.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com




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