Talking between CMOS and TTL.

Thread Starter

hp1729

Joined Nov 23, 2015
2,304
Communicating between CMOS and TTL can be a small problem, especially when there is a voltage difference. Attached are a few examples. Does anybody have ideas to add? Maybe something on interfacing to 3.3 V or lower systems? I haven't spent much time with newer things yet.
 

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dl324

Joined Mar 30, 2015
9,605
In general, you need to consider input thresholds/loading and output swing and drive current. You also need to make sure CMOS input or output diodes don't conduct sufficient current to cause latch up and/or electromigration.
 

Thread Starter

hp1729

Joined Nov 23, 2015
2,304
In general, you need to consider input thresholds/loading and output swing and drive current. You also need to make sure CMOS input or output diodes don't conduct sufficient current to cause latch up and/or electromigration.
Good words, of course.
 

ScottWang

Joined Aug 23, 2012
6,860
The R1 of circuit of the right top should be change to 330Ω, otherwise it can't say it used for any TTL, it only can be used with 74LSxx, because the low level input voltage will be too high.

Do you mean TTL interfacing to 3.3V CMOS or lower voltage and 3.3V CMOS or lower voltage interfacing to TTL?
 

nwvlab

Joined Feb 3, 2016
4
Hi there!

There are a lot of ways to interface TTL and CMOS or CMOS and CMOS with different voltages. It depends on your speed, cost, area and power consuption requirements.

Here are some (except those you already mentioned):

Connecting a 3-3V CMOS ouput to TTL input:
If your CMOS can sink/source enough current, it should be fine, direct connection (beware of possible latch-up problems, as the TTL input is pulled high).

Connecting a 3.3V CMOS output to 5-V CMOS input or connecting a TTL output to 5-V CMOS input:
1) Use an HCT for the input.
2) Use a level translator IC between the two gates.
3) Use a BTJ/MOSFET with the base/gate connected to 3.3V (through a resistor if using BJT), the emitter/source connected to the CMOS output, and the collector/drain (pulled-up @ 5V, through a resistor) to the 5-V CMOS input.
4) If you can accept an inverted signal, use a BJT/MOSFET with emitter/source to ground, base/gate connected to the input (with a resistor if using BJT) and collector/collector pulled high @5V.

Connecting a 5V CMOS to 3.3V CMOS
1) Use a resistor divider.
2) Use a diode (better if schottky), with cathode toward the input, and a pull-up resistor.
3) Use a level translator IC.

cheers!
 
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