On Monday, July 9, 2012, Peter wrote:

> I have some code I wrote a long time ago and reactivated now. It has a
> weird
> RBPU problem. Description:
>
> The code is for 16F84A (no discussion on this please), it sets PORTB RB0
> and RB1
> as inputs, turns on _RBPU, and writes xxxxxx11 into PORTB. This is to tur=
n
> on
> pullups and allow 2 tact switches to be read. This works, but later, whil=
e
> the
> program runs, the xxxxxx11 becomes xxxxxx00 somehow. I am sure I am missi=
ng
> something but none of the simulators I used duplicate the problem.
>
>
I don't think the 84a has a separate output latch. So writing pins on PORTB
that are set to tristate has no effect.  What is the voltage on RB0 and RB1
when PORTB reads 0 for those pins?




> So, assuming that:
> - PORTB is always in tris mode 00000011
> - OPTION _RBPU is set to 0 and never changed
> - all writes to PORTB write 11 into RB0 RB1
>
> WHAT can change the RB0 RB1 to 0s and turn off the pullups?
>
> The accesses to PORTB are of the following types:
>
> 1.  ...
>   iorwf Fbflags,w ; temp register which has 11 in its lower bits, always,
> checked
>   movwf PORTB
>
> 2. movf Fbflags,w ; as above
>    iorwf PORTB,f
>
> 3. movf PORTB,w ; pure read
>    andlw SW_MASK ...
>
> 4. bsf PORTB,SOMEBIT ; other than 0 or 1
>    nop
>    bcf PORTB,SOMEBIT
>
> Which one of these is suspect of resetting the PORTB output bits RB0 and
> RB1, do
> you think? Also, none of the sims I have reproduce the PORTB written as
> xxxxxx11
> correctly when its 0,1 pins are inputs. There should be a display for the
> write
> latch and another for the read latch, I think?
>
> tia,
>
> -- Peter
>
>
>
>
> --
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--=20
Regards,
Mark
markrages@gmail
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