Go Down

Topic: 3.3v logic levels interface with standard TTL/CMOS logic - How to do it... (Read 2 times) previous topic - next topic

simonbond

Dear Forum,

Hi, I am fairly new to the Arduino Due.

I note that the Due uses 3.3v levels. In that case, how would one interface to the standard 5v TTL levels, or the even higher voltages of CMOS? Say for instance octal data latches, Digital to Analogue Converters etc.

I thought there may be some suitable voltage translator ICs, or could be achieved with some resistors & discrete devices maybe?

Any help or advice would be greatly appreciated.

Thanks,

Simon B.
-----------------------------------------------
Simon Bond MEng, BSc(Hons), IEng, MRAeS
-----------------------------------------------

CrossRoads

This chip looks like it could work well
http://www.adafruit.com/products/735#Technical_Details
Power from 3.3V, output on 5V side should go high enough to be seen as a 1.

Other variations in pin count, packages in the same family:
http://www.digikey.com/product-search/en/integrated-circuits-ics/logic-translators/2556437?k=74lvc
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

CrossRoads

Interesting - digikey search for 74lvc245 brings up the thru hole part.
More general search for 74lvc does not show any thru hole parts.

"74lvc through hole" works tho.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

promacjoe

This problem has been around since the early days of logic circuits.
When it comes to CMOS chips, There is no easy answer.
The original CMOS chips Ranged in voltage from 15V  and in some cases up to 30V.
I do not recommend using the older chips. However, current CMOS chips will run at 15V or 5V.
I have never tried to run true CMOS at 3.3V. If you're referring to the 74H series which is a CMOS -based " TTL " series, They normally run at 5V, check your data sheet. Some of these chips will run fine at 3.3 V.  Even if you run them at 5V, Going from 3.3 V to 5 V should not give you a problem. However, you will need a voltage divider when going in the opposite direction. A level shifter is always recommended.

All 7400 series TTL logic runs at 5V and maybe less, depending on the series. It used to be pretty easy to keep up with the 74xx series and the voltage that they would run at.  Now there are two many 74xx series to keep up with. Check the data sheet for specific specifications.

Note: some TTL chips put out a small current from the input side. The output of the preceding chip either pulls it low or forces it to go high. This voltage can cause erratic output if the input is not tied to either a positive or negative voltage. If the gate is not in use, always tie the input so it produces a LOW output. For this reason a logic shifter is always recommended when going from one voltage to another.

Hope this helps.
Joe.




promacjoe

Note: the 74LVC245 Is a "Octal bus transceiver with direction pin with 5-volt tolerant inputs/outputs 3-State".
It is not specifically a logic shifter for individual bits. It will Shift the logic for a entire bite. Which is not recommended For a microcontroller, because any bit maybe set for output or input. If any two outputs are connected together it could cause problems or damage the microcontroller..Unless you are intending to shift one entire bite at a time, and all Bits are known to be either input or output, Do not use it.
Also note: This chip needs to be powered by 3.3 votes only.

Hope this helps.
Joe.

Go Up