Single Pull up/down resistor for multiple Pins

[EEcarl]

I have always been told not to use one pull up/down resistor for more than one pin. Whether it is used for unused Input pins or multiple Enable pins. However I am starting to see it in some schematics.

Can anyone tell me why this shoudl not be done other than for ATE simplification?

 

[dspDad]

There are two reasons for pullup/pulldown resistors.

1) If a chip has i/o on the same pin, you can exceed its current output rating if it is tied to ground and it is inadvertently enabled as an output. With most modern chips this is a non-issue, but 20 years ago it was a good way to destroy one.
2) During prototyping and early production, or during test, it may be useful to force the state of the input either with cut-and-jump mods or with a test fixture.

If neither of these apply, then hard-wire the input to ground or vcc.

If either apply, then use a separate resistor per pin.

Check that the chip does not already have a built-in pullup or pulldown. An external resistor may not hurt, but it won't add any benefit.



Tieing multiple inputs together and then pulling them up or down together seems like a confused operation with all the disadvantages and none of the advantages.

DspDad

 

[nbucska]

DSPDAD:
Third reason: Some logic families e.g. regular or S- TTL-s
specify min. 1K0hm Pu-R for unused inputs to prevent input
damage due to spike on the VCC. Furthermore they specify
that you may use one R for up to 25 inputs. LS,ALS,F don't
need R.

The forth reason is to increase to current sourcing
capability. TTL-s usually can sink much more than source.

The fifth reason : to use open collector/drain/emitter outputs as wired ORs or ANDs.

The sixth reason : ECL IC-s seldom contain the necessary
pull-down resistors.

The seventh reason: CMOS inputs are open circuits -- can
charge up to an undefined -- even damaging -- voltage.
The PU/PD R prevent this.

Offhand I don't know more reasons -- perhaps the others
can contribute some, too.






<nbucska@pcperipherals.com>

 

[buzzp]

Also, you may want to force an input to default to some value (high or low). I always use a pull up or down on inputs. The resistor will actual help with transients and such that may occur on the DC bus or the ground bus. If I were going to tie them directly to the DC bus, you must verify that this is not going to damage the port (current limiting) as someone has pointed out.
The only disadvantage I see with using one resistor for multiple inputs is the pull-up or pull-down will not be as 'stiff' because the resistance will be divided amongst the pins. The other is it will add some confusion, as pointed out.

 

[markrac]

Besides the ATE reason, here's another: if your circuit isn't PERFECT, you MAY want to USE one or more input or output to fix a bug or add functionality; it's a lot easier to just remove a pull-up, and use the pad for a mod wire, than to try to isolate one pin by surgical means! Another trick I use is to make sure that EVERY net in a circuit has a test point (preferably with a via) for the same reason (try soldering a mod wire to a BGA pad!); it also keeps the ATE guys happy.