Re: connect I2C devices with diferent voltages, 3.3v and 5v



jetq88 wrote:

hi,

Anyone out there has experience to connect I2C devices with diferent
voltages, 3.3v and 5v respectively? I'm working on a project with
MCU(3.3v) to control three devices, one is 3.3v, other two are 5v, I
have trouble to get it work. my question are:
1. can SCL and SDA be connected directly among those devices?
2. should pull up resistors be connected to 3.3v or 5v?
3. do I have to use voltage translator between 3.3v and 5v?
4. should I use series resistors on SCL and SDA, should they be close
to 5v devices or 3.3v devices?


First, refer to the manual. Very well could be that the 5V I2C device is specified for the operation with 3.3V levels.

Otherwise you will need the translation of the levels from 3.3V to 5V. If you don't have to read from the 5V devices, and if I2C is running slow enough to guarantee that there is no wait states, this is done fairly straightforward. The translator is unidirectional.

It gets complicated if there is a bidirectional transfer. Then you will have to implement the I2C as bit banging using the separate pins for SDA_IN, SDA_OUT, SCL_IN, SCL_OUT.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com
.

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