Re: OT - TTL Circuit: Pullup vs Supply

On Wed, 06 Sep 2006 18:10:18 -0600, Jess Askey <jess@askey.org> wrote:

>can anyone explain what any pro's or con's there might be on tying
>static input pins on a TTL device 'high' through a pull-up resistor vs
>straight to the +5V supply? Im working on laying out a ASIC replacement
>PCB and I had the board designed with pull-up resistors but tracing them
>is becoming a PITA so I thought about just tying them to the +5V rail as
>an alternative.

Here's what I found:

<www.zonn.com/misc/DesigningWithTTL(AN-363).pdf>

Apparently a common failure for TTL is an input short, if you look at the design
of TTL you can kinda see why:

<http://www.siliconfareast.com/ttl.htm> - which also says to use a pull-up
resistor, but doesn't explain why.

It looks like there is nothing but silicon from input to ground, if it were to
"fuse" it'd be a dead short. This would most likely be true even if it where the
output that overloaded in such a way as to cause a failure, since it all shares
the same silicon substrate.

So the resistor keeps a failed TTL device from becoming an explosion (or taking
out the power supply, etc.).

The inputs are also apparently susceptible to VCC surges, another reason the
resistor is recommended since it limits the current during a surge.

Taken together, if a VCC surge where to cause a TTL failure, and a common
failure mode causes a short to ground, then you can see why the pull-up resistor
is highly recommended.

But the good news is, you can use one 1K resistor for up to 10 inputs.

-Zonn

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Received on Thu Sep 7 03:17:19 2006