> You should be safe with BSF and MOVWF though. Particularly if the
> port contents are read into a buffer or shadow register, modified, and
> then written back with MOVWF
> =

> eg
> =

> MOVF PORTB,W
> MOVWF SHADOWB
> MOVLW B'100'
> IORWF SHADOWB,W
> MOVWF PORTB
> =


One thing to watch out for here is interrupts. If you write to another pin =
in =

port B in the interrupt routine, a bit banged serial output for example, yo=
u =

have to make sure that the interrupt routine isn't executed between when yo=
u =

read the content of the port to the shadow register until you write it back=
. =

Otherwise the change made by the interrupt routine will be undone when you =

write back the shadow register.

One way to solve this is to never read the the value from the port to the =

shadow register but keep the shadow register as a global variable and only =

manipulate it with atomic operations (single instructions that can't be =

interrupted or where an ongoing change in non interrupt code can't be undon=
e by =

interrupt code or vice versa) like IORWF, BSF, BCF and so on. Then just wri=
te =

the entire shadow register to the port when you have changed it. This would =

have the same effect as using the LAT registers in higher end pics.

Note that using the LAT register handles classic rmw issues with ouput port =

pins but it doesn't automatically solve problems where main code and interr=
upt =

code uses the same port (or any register) unless changing of individual bit=
s is =

done with care (changed in what I call atomic, uninterruptable steps).

Another way to put it is that interrupt routines can introduce rmw problems =

where you normally wouldn't see it (or think about it). And this goes for a=
ny =

shared register, not just IO ports.

/Ruben
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
Ruben J=F6nsson
AB Liros Electronic
Box 9124, 200 39 Malm=F6, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D

-- =

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