Go Down

Topic: CMOS vs TTL (Read 5923 times) previous topic - next topic

funkyguy4000

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?
Accelerate to 88 miles per hour.

CrossRoads

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.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

funkyguy4000

By functionality, you mean if you want high speed or low power consumption, chow how much faster is TTL over CMOS?
Accelerate to 88 miles per hour.

JoeN


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
I have only come here seeking knowledge. Things they would not teach me of in college.

CrossRoads

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.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

funkyguy4000

Interesting, so is TTL being zoned out?
Accelerate to 88 miles per hour.

Grumpy_Mike

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.

funkyguy4000

Interesting
What are some of the other more common factors?  Just like a list that I could research on  my own.
Accelerate to 88 miles per hour.

James C4S

I would suggest starting here:
https://www.google.com/search?q=difference+between+TTL+and+CMOS

Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

retrolefty


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

CrossRoads

Here's a good chart showing how the different logic families can play with each other.

http://www.interfacebus.com/voltage_threshold.html
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

CrossRoads

Also this is a good read.
http://www.allaboutcircuits.com/vol_4/chpt_3/10.html
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

funkyguy4000

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?
Accelerate to 88 miles per hour.

CrossRoads

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.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

funkyguy4000

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
Accelerate to 88 miles per hour.

Go Up