Synth-DIY Yahoo! Groups Archives

logic levels..

2005-09-12 by Anurag Chugh

Hi people...

Since we were talking of logic levels... i thought this would be a nice anecdote to add on..

I wrote to one of the authors of the digital design books we use as text in college..

on why 5v was choosen as logic one...

this is what he has to say..

PS please read the linked essay

Anurag

----- Original Message -----

From: Anurag Chugh

To: John F. Wakerly

Sent: Thursday, January 06, 2005 4:49 AM

Subject: Re: Why +5V?
Dear Sir,

I am a student of electronics engineering currently in third year of my four year course. Although I am done with Digital Design Course and should have asked this question then but it never struck me then! Just have been taking things as they were told to me. But now that I am trying my luck by constructing all sorts of circuits, (microcontroller, digital, analog et al) I find my self asking this Question : Why 5 Volts? why not any other level for logic 1.

I tried your book but i couldnt find the answer. Maybe I didnt search well but still I suppose there is a bit of history there if there is no reason behind it.. after all if not 5Volt then what else could it be 6 or 7 ?

It had to be something and i sure would like to know if there was any reason for it!. I used your book during the courses called Digital Design I and Digital Design II. I have never found any other author write so passionately and in so much completeness as yourself and so I thank you very much for making my DD courses real enlightening and enriching experience.

Looking forward to your reply

Thanking you

Yours Faithfully

Anurag Chugh

Third Year (Electronics Engineering)

Vivekananda Education Society's Institute of Technology (Affiliated to Mumbai University),

Chembur, Mumbai (Bombay) - 400071

India

PS:

Our course didnt require us to know how the gates were actually implemented, so i conveniently bunked Digital Circuits chapter when studying thoroughly for exams but I had an overview of it and I couldnt find the answer to my question there. I wonder if many other students have asked you the same question before?

From: John F. Wakerly

To: Anurag Chugh

Sent: Thursday, January 06, 2005 4:49 AM

Subject: Re: Why +5V?

This was the voltage chosen for the original TTL family. I'm not sure of the exact reason. It's possible that some previous logic families used 5V, but then there would still be the question for that family. It was undoubtedly a tradeoff between power consumption, reliable switching operation, and noise margins.
Or perhaps it's more like the reason that railroad tracks are separated by the width that they are (see www.naciente.com/essay94.htm).

John

Re: logic levels..

2005-09-12 by Dave VanHorn

Well, that was an interesting reply.

5V is NOT the high level for TTL. Never was.
It's not the high level for CMOS either.

5V is the VCC level for TTL though.

Each logic family determines it's low and high thresholds.
TTL levels are well defined here: 
http://media.iet.hist.no/lab3ee/avgrensa/74xx.htm

Then there's the more modern LSTTL
http://media.iet.hist.no/lab3ee/avgrensa/74lsxx.htm

Good comparison here:
http://www.interfacebus.com/voltage_threshold.html


The why of it, is that this is how the inputs of the gates are 
designed, and the difference between the high and low threshold gains 
you noise immunity as you increase it, but it costs you power, since 
you spend more in resistive dissipation, and charging/discharging 
gate capacitances.

Re: [AVR-Chat] Re: logic levels..

2005-09-13 by Dave Hylands

> Good comparison here:
> http://www.interfacebus.com/voltage_threshold.html

And that's only for 5v devices. Lots of new stuff runs on 3.3v, 2.7v,
1.8v, 1.5v, etc, and the logic low/high threasholds all move around
depending on your voltage.

-- 
Dave Hylands
Vancouver, BC, Canada
http://www.DaveHylands.com/

Re: logic levels..

2005-09-25 by Graham Davies

--- In AVR-Chat@yahoogroups.com, "Anurag Chugh" <lithiumhead@m...> 
wrote:

> I wrote to one of the authors of
> the digital design books we use
> as text in college on why 5v was
> choosen as logic one...

5 volts was not "chosen as logic one". You're confusing logic levels 
with the supply voltage of the logic circuits.

> this is what he has to say.. 
> PS please read the linked essay

> Or perhaps it's more like the reason
> that railroad tracks are separated
> by the width that they are
> (see www.naciente.com/essay94.htm).

This is pretty pathetic. If you read the essay, for heavens sake also 
read up on the railroad boom in the United States and the wide variety 
of gauges that were originally used for the different lines. The essay 
may be funny, but it does not accurately represent the facts.

Your author clearly doesn't know the answer to your question. If you're 
really interested, you should research RTL and DTL, which were wiped 
out by TTL and also ECL which was around at the time and still lives 
on. That shouldn't be to hard for a college student. First, get it 
clear in your mind whether you are interested in the supply voltage or 
logic levels.

Here's another gross simplification of reality: those that can do, 
those that can't teach, and those that can't teach write the text books.

Graham.