> > >> 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 anothe=
r pin
> > > in port B in the interrupt routine, a bit banged serial output for ex=
ample,
> > > you have to make sure that the interrupt routine isn't executed betwe=
en when
> > > you read the content of the port to the shadow register until you wri=
te it
> > > back. Otherwise the change made by the interrupt routine will be undo=
ne when
> > > you write back the shadow register.
> > =

> > Good programming practice would dictate that the interrupt routine also =

> > uses the shadow, so there would be no problem.
> > =

> =

> Yes, there still could be problems if the interrupt is just after IORWF =

> SHADOWB,W. Then changes done in the interrupt routine will be undone when=
 the
> non interrupt code does MOVWF PORTB. The key is, appart from using a shad=
ow
> register, is to never read back from the port to the shadow register.
> =


Also remember to not store any intermediate results in another register (ev=
en =

W) and then write it back to the shadow register. Only use atomic, non =

interruptable methods (single instructions or disable interrupts) when =

manipulating the bits in the (global) shadow register.

/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

-- =

http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist