FreeLists / si-list / [SI-LIST] Re: Regarding Pull down

[pwelling@xxxxxxxxxxxxxx]

While on the subject of Pull-up and Pull-Down resistors...

I have seen in many designs the error of using a single pull-up for many
devices used across real estate of a board. I've seen a single pull-up for
20 inputs on a board. The inputs that it sourced covered 25 square inches of
board area. It saved a few cents of parts but was a susceptibility risk.
Since it was not a critical trace, it made a long trace on the top layer of
the board. Perfect for an antenna for high impedance CMOS devices. 

I guess it could have been worse by placed it near the edge of the board
front panel so it could have been an ESD risk too!

I always recommend that pull-ups and pull-downs be located "geographically".
It is much better to use more pull-ups located near the devices that require
them than to stretch a trace all over the board to save a nickel. If you
have to use one pull-up for a large circuit area, add decoupling as required
to keep it a pull-up over the large area.

Another thing I have seen is using a pull-up pack (resistor array) used for
multiple pull-ups on a board. I have seen VERY long traces from one side of
the board to the other as a pull-up was used from the common package for a
function across the board. As the long (non- critical trace) was stretched
across the board, it was run very close to some data bus traces and
crosstalk occurred to the higher impedance pull-up trace. Then it wasn't a
pull-up trace anymore. It became a noisy trace (data bus related noise - not
an easy problem to track down) and it degraded noise margins on the board.
It is okay to use pull-up and pull-down packs geographically. It is better
to spend the extra nickel and place a pull-up resistor near the area that
will be using it.

Food for thought.

Philip Ross Wellington
Mgr. Signal Integrity & EMI
L-3 Communications CSW

-----Original Message-----
From: Todd Westerhoff [mailto:twesterh@xxxxxxxxx]
Sent: Tuesday, January 07, 2003 7:21 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Regarding Pull down



It's also an in-circuit test issue.  If you ground an input, you can't test
it (and any associated functions of the device it enables).  If you tie the
input to ground/power using a resistor and have a test point accessible,
it's possible for a board tester to exercise the pin and the device logic
associated with it.

Todd.

Todd Westerhoff
High Speed Design Specialist
Cisco Systems
1414 Massachusetts Ave - Boxboro, MA - 01719
email:twesterh@xxxxxxxxx
ph: 978-936-2149
============================================

"When did the choices get so hard, with so much more at stake?
 Life gets mighty precious when there's less of it to waste"

- Bonnie Raitt, "Nick of Time"


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Ingraham, Andrew
Sent: Tuesday, January 07, 2003 8:21 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Regarding Pull down


>         I wanna ask the question that in normal applications when
> inputs are
> not needed for test reasons, is it OK to tie the un-used inputs of TTL
> or
> NAND devices directly to Ground or one must always need to use the
> pull-down
> resistors.

If you never ever need to pull an input pin high, I think there is no
reason why you cannot directly ground it.

If it is a bidirectional pin, you should make sure it can't spend any
time with the output enabled (say, when powering up or down).

With some true TTL families, and unused inputs that you wanted to appear
High, there was a reason for using a resistor and not tying them
directly to VCC.  I think it was related to the absolute maximum input
voltages.  If I remember correctly, the supply voltage on the VCC pin
was allowed to reach +5.5V (or more?), but input pins could not go above
+5.25V.  Using a resistor on such pins, as instructed in the databooks,
limited the input pin current should a transient on the supply voltage
exceed +5.25V.  But this does not apply to inputs that you want
grounded.

Regards,
Andy




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