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Using Arduino => General Electronics => Topic started by: funkyguy4000 on Sep 04, 2012, 03:13 pm

Title: CMOS vs TTL
Post by: funkyguy4000 on Sep 04, 2012, 03:13 pm
Hello,

So my brother asked me a question a few days ago.  What is the difference between TTL and CMOS?  Well I was sorta able to answer, although my knowledge of those is only theory and not actual application specific.  So what are the pros and cons between the two?  Like, why would one choose CMOS over TTL, and vise versa?
Title: Re: CMOS vs TTL
Post by: CrossRoads on Sep 04, 2012, 03:26 pm
Difference is in the type of transistors used, MOSFETs vs BJTs.

Used to be that CMOS was low power, TTL was high speed.

Used to be that CMOS was more susceptible to electrostatic damage, TTL less so.

CMOS less power hungry than TTL, with CMOS power needs based on switching speeds, with TTL power usage more steady state.

Nowadays, CMOS used for a lot more stuff, using one vs the other more a matter of what functionality you want.
Title: Re: CMOS vs TTL
Post by: funkyguy4000 on Sep 04, 2012, 04:46 pm
By functionality, you mean if you want high speed or low power consumption, chow how much faster is TTL over CMOS?
Title: Re: CMOS vs TTL
Post by: JoeN on Sep 04, 2012, 04:56 pm

By functionality, you mean if you want high speed or low power consumption, chow how much faster is TTL over CMOS?


Anymore, you will not know unless you read the datasheet for the specific part.  It's no longer true that CMOS is always slower than TTL because there are so many newer packages that were never made into TTL and that run at really high speeds.  Read the datasheets for the parts in question.  Propogation delays are usually on the top half of the first page, you won't have to go far into it.

There are also some voltage differences and CMOS can be run at higher  voltages.  In fact, some packages run faster at higher voltages.  Of course if you are running your system at 10V-15V to take advantage it better be all TTL or have level converters in it.  Not all CMOS chips can be run at these voltages, check the datasheet.

More information:

http://www.allaboutcircuits.com/vol_4/chpt_3/10.html

More information still:

http://www.kpsec.freeuk.com/components/74series.htm
http://www.kpsec.freeuk.com/components/cmos.htm
Title: Re: CMOS vs TTL
Post by: CrossRoads on Sep 04, 2012, 05:03 pm
http://www.ti.com/lit/ds/symlink/sn74ls595.pdf
http://www.ti.com/lit/ds/symlink/sn74hc595.pdf

An example - look at the tables for Tpd from SRCK to Qa-h.
LS595, max is 18/25nS for High & low outputs
HC595, max is 40nS.

With 16 MHz clock of 62.5nS the slower part works.
With 32MHZ clock of 31.25nS, the slower part would not.

So choice of part depends on situation.
Title: Re: CMOS vs TTL
Post by: funkyguy4000 on Sep 04, 2012, 05:45 pm
Interesting, so is TTL being zoned out?
Title: Re: CMOS vs TTL
Post by: Grumpy_Mike on Sep 04, 2012, 06:10 pm
Quote
so is TTL being zoned out?

No, it is not only about speed but drive capacity. In general you can get more current out of TTL, there are TTL devices that do not have the equivalent CMOS parts so functionality is an issue as well.
The fastest TTL will still go faster than the fastest CMOS. It is just that the slowest CMOS is not as slow as it used to be.
Title: Re: CMOS vs TTL
Post by: funkyguy4000 on Sep 04, 2012, 07:52 pm
Interesting
What are some of the other more common factors?  Just like a list that I could research on  my own.
Title: Re: CMOS vs TTL
Post by: cmiyc on Sep 04, 2012, 07:58 pm
I would suggest starting here:
https://www.google.com/search?q=difference+between+TTL+and+CMOS

Title: Re: CMOS vs TTL
Post by: retrolefty on Sep 04, 2012, 07:59 pm

Interesting
What are some of the other more common factors?  Just like a list that I could research on  my own.


Several factors help one decide which IC family type were best for any one specific application, power consumption per function, input noise immunity, Vcc operating voltage range, etc.

TTL logic using npn/pnp transistors were 'king of the hill' for many decades for most applications, but gradually CMOS logic using mosfet transistors improved where they are the dominate tech used these days.

Lefty
Title: Re: CMOS vs TTL
Post by: CrossRoads on Sep 04, 2012, 09:02 pm
Here's a good chart showing how the different logic families can play with each other.

http://www.interfacebus.com/voltage_threshold.html
Title: Re: CMOS vs TTL
Post by: CrossRoads on Sep 04, 2012, 09:08 pm
Also this is a good read.
http://www.allaboutcircuits.com/vol_4/chpt_3/10.html
Title: Re: CMOS vs TTL
Post by: funkyguy4000 on Sep 04, 2012, 09:56 pm
Oh that is a great read! 

The chart of the logic voltage thresholds is quite intriguing.  Especially the LowV one.  Is that what the devices that take in 3.3v logic are or is that something totally different and unrelated?
Title: Re: CMOS vs TTL
Post by: CrossRoads on Sep 04, 2012, 10:09 pm
For 3.3V devices, the levels are typically a factor of the supply voltage.

For ATMega328s:
With Vcc = 3.3V:
Low Input must be <= 0.3 x Vcc, = 0.99V
High input must be >= 0.6 x Vcc = 1.98V

1V to 1.97V: input is not defined, may be seen as high or low.

With Vcc = 3V:
Low output will not higher than 0.6V with 10mA load
High output wil not be less than 2.3V with 10mA load

Numbers are from Section 29 of the '328 datasheet.
Title: Re: CMOS vs TTL
Post by: funkyguy4000 on Sep 04, 2012, 10:25 pm
Okay so its pretty much unrelated. 
Thank you so much everybody!

If you remember anything or have anything more, feel free to throw it in here. 
Otherwise, thank you
Title: Re: CMOS vs TTL
Post by: MarkT on Sep 05, 2012, 12:32 am
Of course CMOS scales to much faster clock rates with smaller processes and lower voltages - a 1.8V CMOS logic family would be interesting.

I have an old Sci Am special on microelectronics, published some time in the 80's - they forecast that I2L (integrated injection logic) would take over from MOSFET technology.  Never happened!

The other feature of TTL that ought to be mentioned is that the static power dissipation of gates is many orders of magnitude higher than CMOS (factor of millions I think) - this means TTL cannot be used for VLSI at all since the quiescent power dissipation would be measured in kW and MW.  A chip with 50 TTL gates on it is feasible, with a million gates: totally impossible.
Title: Re: CMOS vs TTL
Post by: funkyguy4000 on Sep 05, 2012, 01:37 am
Aren't FTDI chips TTL?  Such as the one and the arduino Duemilanove?
Title: Re: CMOS vs TTL
Post by: JoeN on Sep 05, 2012, 01:45 am

Aren't FTDI chips TTL?  Such as the one and the arduino Duemilanove?


You mean this part?

http://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT232R.pdf

Though it interfaces CMOS to TTL, I can't see it being TTL.  Maybe BiCMOS?  The datasheet does not what technology the chip is made from.
Title: Re: CMOS vs TTL
Post by: retrolefty on Sep 05, 2012, 02:53 am

Aren't FTDI chips TTL?  Such as the one and the arduino Duemilanove?


I think you would be hard pressed to find any mass produced IC developed in the last 10 years (perhaps even longer) that is bipolar (as pure TTL is based on) tech, rather then the various CMOS derived tech. Once they got the speed and output drive to be competitive with bipolar TTL, the other overwhelming features of CMOS made it the mainstream tech to use in IC development. I think newer bipolar IC designs are probably only used in few low volume specialized niche applications, it at all?
Title: Re: CMOS vs TTL
Post by: davidhbrown on Dec 21, 2019, 02:41 pm
I would suggest starting here:
https://www.google.com/search?q=difference+between+TTL+and+CMOS
Couldn't resist the thread necromancy to say that's almost exactly how I found _this_ page, 7+ years later :-)  (excepting I included the term 'Arduino'). Thank you all (belatedly) for your insights.
Title: Re: CMOS vs TTL
Post by: Paul__B on Dec 21, 2019, 10:51 pm
I think it is fair to say that now, seven years later, TTL (74LS) is completely obsolete.  Even for service replacement (just use a 74HCT part).  :smiley-roll-sweat:
Title: Re: CMOS vs TTL
Post by: raschemmel on Dec 22, 2019, 08:33 am
As a follow up you might want to look up Don Lancaster's TTL COOKBOOK & CMOS COOKBOOK.

Most of these guys either have a tattered old copy or had one at one time and maybe gave it to their son
as a hand me down.. ;D


At least one of them will add:
Quote
or threw it in the trash !
Title: Re: CMOS vs TTL
Post by: Smajdalf on Dec 22, 2019, 10:00 am
TTL (74LS) is completely obsolete.  Even for service replacement (just use a 74HCT part).
AFAIK unconnected TTL inputs are held safely HIGH. Unconnected CMOS input float even in HCT family. When you do the replacement in a circuit which rely on this feature...
Title: Re: CMOS vs TTL
Post by: Grumpy_Mike on Dec 22, 2019, 10:40 am
Quote
AFAIK unconnected TTL inputs are held safely HIGH.
Well they float high, I wouldn't say it was safe because they are prone to EMI pickup in this mode. That is it is not a recommended operating configuration.

Quote
Ah, this is obviously some strange usage of the word 'safe' that I wasn't previously aware of.
Douglas Adams, Arthur Dent in "The Hitchhikers Guide to the Galaxy"
Title: Re: CMOS vs TTL
Post by: Paul__B on Dec 22, 2019, 11:45 am
As a follow up you might want to look up Don Lancaster's TTL COOKBOOK & CMOS COOKBOOK.

Most of these guys either have a tattered old copy or had one at one time and maybe gave it to their son
as a hand me down.. ;D
Look it up you say?

Not actually necessary (https://www.tinaja.com/ebooks/cmoscb.pdf)!
Title: Re: CMOS vs TTL
Post by: Smajdalf on Dec 22, 2019, 01:53 pm
Well they float high, I wouldn't say it was safe because they are prone to EMI pickup in this mode.
Unconnected pin has quite low capability for EMI pickup. I made a very loosely related "experiment". In my project I had standalone ATMega with reset pin unconnected. No reset was caused in about half a year. After adding a programming connector (about 10 cm of wire) there was a reset about monthly.

I would guess a floating TTL input is HIGH at least 99.9% of time. Any LOW period will be very short. I can imagine plenty of situations where this is no issue (i.e. Enable of a 7 segment LED display driver). OTOH HCT input may float to LOW for considerable time period.

But you are right, "safe" was not the best word to use there.