Sabananthan Paramananthan wrote:
>
> Hi,
>
> I've had lot more trouble with PORTA of PIC16C73A/JW. I used RA4:RA0 of
> PORTA as test out puts and RA5 as a jumper pin which is pull-up to 5V via
> resistor. When I set 1 of the bits or clear one of the bits (bsf, bcf) all
> other bits which were set are now cleared!  I couldn't figure out why. Then
> I read through the part which says every write to i/o port is read modify
> write operation. However, this will affect only if I use the pins as both
> inputs and outputs but not when they are fixed as either input or output.

Not true, the problem you have noticed is also if the port pin are all
outputs and you don't change direction. You are correct to say that
write to port is actually read/modify/write but the read part gets the
value _directly_ form the output pins. So if your program drives bit 3
high but the corresponding pin is tied to ground the read operation gets
a '0' for that bit. When the PIC writes back the modifyed value to the
port the bit 3 is written as '0'.

> The solution was to use a temporary variable to and set or clear bits in it
> and write the whole register back to port and use the register as a copy of
> port of outputs. Problem with with this method is instead of one
> instruction, we need three instruction for every single bsf, bcf operation
> on a port.
>
>      bsf          PORTA,3
>
> becomes:
>
>      bsf         TEMPA,3
>      movf      TEMPA,W
>      movwf    PORTA

Yes, this is a solution if you have this kind of problem.

Ciao
  Marco

----
Marco DI LEO                  email: m.dileo@sistinf.it
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