Re: Problem with 8255 PIO design


"Udo Giacomozzi" <jampy@xxxxxx> schreef in bericht
news:1188231039.260934.97650@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi,

we wanted to use the 8255 for a relatively simple design. It seemed
that the 8255 has three independent 8bit I/O ports so we used one port
for a (slow) bi-directional bus and the other ports for other
purposes. We already have the prototype but the design is invalid
because of this one little sentence in the whole data sheet:

"Any port programmed as an output port is initialized to all zeros
when the control word is written."

So, when we want to toggle the *direction* of port A (the one used for
the bidirectional bus), all *outputs* of ports B and C will be reset
as well! This makes the 8255 completely useless for our purposes (we
need the other outputs to stay unchanged).

Any can propose a workaround for this? Since we want to avoid adding
much additional logic: Is there a similar near-24 bit I/O chip that
would be useful for our needs?

The 8255 currently is controlled via the ISA bus of a PC/104 board.
Note we can't use an additional PC/104 doughter board because of lack
of space (and costs).

Thanks for any hint,
Udo


It has been quite some time I used a 8255 but I remember a port could be set
to bidirectional mode. Once you'd done so, there was no need to write to the
control register for changing input to output. Guess you have to study the
datasheet a little more carefull. Details are pretty well explained in it.

petrus bitbyter


.

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