AVR-Chat

Ouch. PIC 16F parts do a read-modify-write on the IO port that a pin is 
on when you use the bsf/bcf instructions, so if you do a bsf PORTA,7 
you'll read all of port A, set bit 7 and then write PORTA back out. 
This means that you could change any of the pins on PORTA depending upon 
what was read on those lines.  A far safer way to do this is to keep a 
"shadow" register for a port, do the bsf/bcf on that shadow register and 
when completed write the whole register out to the port.  This will 
guarantee that your bsf/bcf operations don't munge up other bits on the 
port.  Thank goodness the 18F, 30/33/24F parts DON'T have this side effect.

DLC

OWEN-A wrote:
> Using  bsf  and bcf   PORT?,n
> Thanking You,
> Owen.
> 
> 
>>I know this is perhaps a bit off topic but I have several boards with
>>PIC16F877 processors driving an HC595 shift register and the output
>>pins from the pic driving the HC595 are virtually all glitches.
>>
>>It is hard to pick up a high or low as such on the logic analyser
>>other outputs seem ok, has anyone any idea what may be causing this.
>>
>>And yes I do not like pic's I normally use AVR'S.
> 
> 
> Perhaps you are having read/modify/write problems, are you using the bit 
> manipulation instructions?
> 
> Leon 
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Dennis Clark    TTT Enterprises
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