At 11:01 PM 12/12/01 -0600, Bala Chandar wrote:

>I like the suggestion by Tim McDonough of using 4021 that will give 8
>inputs. (74HC165 can also be used, I think, as it looks a functional
>equivalent of 4021.) Since I already have two pins working as common Clock
>and Data outputs, I can use them for 4021 as Clock and Load outputs. This
>means, by using only one pin to receive the serial data, I can get 8
>additional inputs.

You can avoid using that extra pin by using the data pin used to talk to
the output SRs as an input pin.  Just add a single 10K resistor in series
with the data out pin of the 4021 and feed the serial data pin on the
PIC.  Set the pin as input, read the 4021, set the pin as output, write the
output SRs, assert strobe.

I do this in most of my products: 3 pins to talk to all my SR based inputs
and outputs.  Add 1 more pin if I am also talking to my watchdog chip of
choice (Xicor X25043).

dwayne



Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 17 years of Engineering Innovation (1984 - 2001)

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