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From: rpw3@amdcad.AMD.COM (Rob Warnock)
Newsgroups: sci.electronics
Subject: Re: TTL Questions
Message-ID: <17725@amdcad.AMD.COM>
Date: Tue, 28-Jul-87 04:02:54 EDT
Article-I.D.: amdcad.17725
Posted: Tue Jul 28 04:02:54 1987
Date-Received: Wed, 29-Jul-87 04:32:39 EDT
References: <1395@crash.CTS.COM> <1008@speech1.cs.cmu.edu> <294@uvicctr.UUCP>
Reply-To: rpw3@amdcad.UUCP (Rob Warnock)
Distribution: na
Organization: [Consultant] San Mateo, CA
Lines: 54

In article <294@uvicctr.UUCP> collinge@uvicctr.UUCP (Doug Collinge) writes:
+---------------
| I read a neato book once by a guy who actually went out and tested things
| like this,...  He said that open TTL inputs WILL NOT
| GLITCH even under the most extreme conditions.  Remember, before you flame,
| he actually made circuits and tried it out...
+---------------

Then he (whoever this anonymous person was) was very, very lucky, and was
(in my opinion) grossly negligent in recommending such behavior to others!

Yes, on a *small* breadboard on your bench in a quiet (electrically) lab,
you can get away with it. I have. I do, still. But I (see signature below
for a real name you can quote) have also personally been burned very badly
(some 25 years ago) by that sort of thing, in production products. I had a
manager giving me the same sort of stuff, "You don't need to pull those guys
up! Pullups are expensive! The design time to wire them is expensive!", and
I let him intimidate me into saving a few bucks. Of course, when the product
failed in the field, it was my fault! (And those things are *hard* to diagnose!)

In a high-speed, high-power system (and if you think TTL is not high power,
calculate the total instantaneous peak power when you pull all 32 lines of
a data bus from TTL high to low within a few nanoseconds, when the bus has
several hundred pF of load per pin -- we're talking *amps*!), unterminated
inputs *do* pick up trash and cause glitches, *especially* inputs such as
preset/clear on a flip-flop. (Yep! They fail only on "some" data patterns...)

*DO NOT* leave inputs floating on commercial products. (What you do in
your home lab is another thing.) You may find yourself with a product
liability suit otherwise (or a criminal charge, if the circuit fails in
such a way as to harm humans, as in an elevator or an airplane).

For the last 25 years, I have preferred to be safe & sure. As a quick & dirty
rule of thumb, put a 1000 ohm to +5 and tie no more than 10-15 inputs to it.
Use "SIP" packages of several 1k pullups to cheaply & safely terminate all
sucj inputs. Don't use long wires on your pullups (they can pick up noise),
use more pullups instead (closer to the packages being pulled up).

Incidentally, if you are careful to never tie the set and preset inputs
of a flip-flop to the same pullup, you can also win some side-effects in
testability (when using "bed-of-nails" testers), since you can set/clear
groups of flops by pulling down the pullup.  (In fact, a testability engineer
I once worked with wouldn't even let us tie constant-zero inputs to ground,
but made us use a low-ohm resistor, so that the testers could override them
for fault isolation. But that's another story...)


Rob Warnock
Systems Architecture Consultant

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