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PICList Thread
'4 74LS245's (was EERPOM (sic) Problem)'
1997\04\27@025836 by Mike Ghormley

David Nicholls wrote:

> ... in my circuit I multiplex 32 I/O lines into Port B using an arrangement of
> 4 74LS245's and a 3 to 8  decoder chip (74ls138).  However I can only seem to
manage to get 3 of the 4 chips in without things starting to act eraticly.

First of all, 54/74LS chips have an input unit load of 20uA (high) and 400uA
(low).  Four inputs per port B pin would be 1.6mA tops.  Not exactly a tractor
pull for the PIC.

Here is what I would do:

1) Be sure to have swapped out the fourth `245 with the first or second one so
that we know it isn't a flakey chip.

2) Ask what are you doing with pins 4,5, and 6 on the `138?  Are you sure that
they are either tied to the appropriate logic level or being driven correctly?
floating input on these control pins can cause erratic behavior.  Also how about
deglitching caps near the Vcc and GND pins?

3) Straighten pin 19 of the fourth `245 so that it sticks straight out of the
package instead of down into the socket (I call it "winging the pin"), insert
chip in the socket (with the power OFF of course) and tie the winged pin high.
When you run the board, do you still have the problem?  My guess is that you

4) If the problem goes away, take all the code that accesses the fourth `245
remove the chip (if the board won't run with it in) and place a logic probe on
19 (or its socket pin).  If you are getting pulses then your decoding circuitry
has a timing problem or crosstalk.

Hope some of this helps.


When the way of the Tao is forgotten, kindness and ethics must be taught.
Men must learn to pretend to be wise and good.  --  Lao Tzu

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