Connecting 5V AVR to 3V logic

Go To Last Post
7 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I have seen several products in production that use a 5V AVR with 3v ethernet chips. Although these devices are 5V tolerant, I'm concerned about reading data . Typically these 5v tolerant devices have a TTL level output, that is 2.4V min for a logic high (attainable from a 3.3V supply). However the data I/O of an avr running at 5V is not TTL level but CMOS levels. which requires a min voltage of 0.7 x VCC (3.5V) for a logic high. This would suggest a need for level translators. However these products use a design that connect directly between a 5v avr & 3.3v logic.
Am I being overly conserned about this, it obviously works, but how about over a range of temperature/voltage And doesn't CMOS devices consume more power when a signal is in the mid range.
Anybody have any thoughts on this?

Colin Stanley

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

from my expiriante you will not need any "level" translator for AVR MCU ans 3V device!
regards

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I interfaced a AVR to a 3V3 SD-Cards. It's output what open collector so i could use a pull-up to 5V to get AVR levels. For other projects i used a 74LVC24x for level translation. It was always working.

Hava a look at my web page -> http://www.tobiscorner.at.tf

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Just today I was looking at what it would take to hook the XPORT (3.3V) to a 5V Mega128. It's possible that it might work, but I wouldn't want to try it. Obviously, there is no problem with the logic 0 levels, but there are two issues with a logic 1. The AVR specs a minimum VIH of 0.6VCC or 3V while the XPORT puts out a minimum VOH of 2.4V. The real kicker is that the XPORT specs a maximum VIH high input of VCC (3.3V) while the AVR will put out a minimum VOH of 4V. As suggested, best to go with the TI level translator parts (SN74LVC...). They are available from Digikey - $0.50 for a 1 bit version in qty 1.

Dave

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

You should be able to use a 100 ohm resistor in series if you are worried on each line.

You can use the 74LVC or AHC type technology to level-shift as well. They will either step-up or step-down a level. They will always output whatever VCC's level is.

Regards

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Quote:

Just today I was looking at what it would take to hook the XPORT (3.3V) to a 5V Mega128. It's possible that it might work, but I wouldn't want to try it. Obviously, there is no problem with the logic 0 levels, but there are two issues with a logic 1. The AVR specs a minimum VIH of 0.6VCC or 3V while the XPORT puts out a minimum VOH of 2.4V. The real kicker is that the XPORT specs a maximum VIH high input of VCC (3.3V) while the AVR will put out a minimum VOH of 4V. As suggested, best to go with the TI level translator parts (SN74LVC...).

As far as I know the Xport is a CMOS device so the outputs should go to 3.3V. (Correct me if Im wrong) Therefore you can connect the RX of the AVR UART directly to the Xport TX. Obviously the you can't connect the 5V TX from the AVR UART to the 3.3V Xport RX but you can simply step it down with a voltage divider. Try using R values of 470R and 680R

Dave

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I found the following article while searching for an ultra-simple 5V-3.3V interface. Maybe it's of use within this discussion...

http://www.reed-electronics.com/ednmag/index.asp?layout=article&stt=000&articleid=CA193193&pubdate=2/7/2002&spacedesc=designideas

-- Thilo

Einstein was right: "Two things are unlimited: the universe and the human stupidity. But i'm not quite sure about the former..."