Gavin Jackson Wrote:

> Hi there
>
> I would just like some opinions on this. Does this
> solve the read-modify-write problem.
>
> All PIC pins have external pull-ups.  When the program
> starts the instruction CLRF PORTX is issued to write a
> logic '0' to the data latch.  If you want a logic '1' on the
> port you simply make it an input, by writing a '1' in the
> TRISX register corresponding to the bit position.
> If you want a logic '0' you make the port pin an output and as
> the data latch already contains a '0' you know that the pin
> will pull the external pull-up to ground.
>
> You no longer read and modify the PORT, but rather the
> TRIS register for that port.
>
> Any thoughts on this method?
>
> Regards
>
> Gavin


Sounds good to me.  This is exactly the method I used to implement an IIC
bus on a 16C74 at www3.mistral.co.uk/brian.clewer/  .  The macro's SDAUP
SCLUP etc are in the general.equ file.

Regards,
Brian.