Or if you are using the 18F series use the LAT registers which keep an
image of what you latched out to the parallel port.

John Dammeyer


Automation Artisans Inc.
http://www.autoartisans.com
Ph. 1 250 544 4950


> -----Original Message-----
> From: piclist-bounces@mit.edu 
> [mailto:piclist-bounces@mit.edu] On Behalf Of Jinx
> Sent: Thursday, March 15, 2007 3:30 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [PIC] First shot errors
> 
> 
>  > rlf     PORTB           ; move to RB1
> 
> Rikard, it's not recommended to use content-modifying instructions
> (incf, decf, rlf, rrf etc) directly on a port. In your case 
> it will probably
> work because you're just "moving" an LED. The issue is read-modify-
> write. The contents of the port are read, modifed, and then written
> back to the port. The problem is that hardware, commonly if any
> capacitance is present, may not react as fast as the s/w
> 
> http://www.piclist.com/techref/readmodwrite.htm
> 
> What would be better for you to do is use a shadow register. This
> will be internal RAM and therefore can be modified correctly as fast
> as the s/w. When the change has been made, it's written to the port
> 
> Like this -
> 
> movf portb,w ; read the port
> movwf b_shadow ; copy into shadow reister
> rlf b_shadow,w ; modify, result in W
> movwf portb ; write modified value back to port
> 
> As for the delays, I use these routines when all that's needed are
> approximate delays. They're easily modified for exact delays, but
> with IntRC these are usually good enough
> 
> ;================================================
> ;        Delays @ 4MHz Fosc
> ;================================================
> 
> s2       movlw   .20             ;2 second delay
>          movwf   del4
> dec2     call    ms100           ;call 100ms 20 times
>          decfsz  del4
>          goto    dec2
>          return
> 
> ms100    movlw   .156            ;100ms delay
>          movwf   del3
>          movlw   .5
>          movwf   del1
>          nop
>          nop
>          nop
>          nop
>          nop
> next1ms  call    ms1
>          nop
>          incfsz  del3
>          goto    next1ms
>          return
> 
> ms1      movlw   .10             ;1ms delay
>          movwf   del1
> lp1      nop
>          incfsz  del1
>          goto    lp1
>          nop
>          nop
>          return
> 
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