Hi Steve,

I think the situation is a bit worse. First of all you need 4 instruction
cycles, not 4 instructions... GOTO takes 2...
Secondly, 455kHz means you need to toggle the port bit at every 2nd cycle,
not every 4th.

If a little glitch is acceptable you may can try something like this:

LOOP:
  BSF   GPIO,outbit
  BTFSS    GPIO,inrxbit
  BCF   GPIO,outbit
  BTFSS    GPIO,inrxbit
  BSF   GPIO,outbit
  BTFSS    GPIO,inrxbit
  BCF   GPIO,outbit
  BTFSS    GPIO,inrxbit
...
... doing this for a while makes a continuous signal
...
  BSF   GPIO,outbit
  BTFSS    GPIO,inrxbit
  BCF   GPIO,outbit
  BTFSS    GPIO,inrxbit
  GOTO   LOOP ; glitch occures!

  GOTO   MAIN

But I think you should go for a mid-range device instead where you have
interrupts and PWM module as Dario suggested...

Tamas



On Wed, Jul 29, 2009 at 5:56 PM, Steve Smith <xygax@blueyonder.co.uk> wrote:

> Guys:
> During a program I have to periodically make a burst of 455Khz pulses and
> stop when a pin changes state....
>
> Using an pic 12F510 I can get an 8Mhz internal osc that gives me about 4
> instructions to manufacture this burst and sense the end. It goes something
> like this
>
>                Code                            cycles (500ns )
>        Bsf     GPIO,outbit                     ;0              6
>        btfss   GPIO,inrxbit            ;1
>        goto    main                            ;
>        bcf     GPIO,outbit                     ;3
>        goto    $-4                             ;4
>
>
> best I can do is 5... Anybody got any better ideas I am trying to get a one
> chip solution in the smallest possible space and it needs a comparator
>
> TIA
>
> STeve
>
>
>
>
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