I2C communication between 3.3V uC and 5V uC

Question

Hello all,

I'm currently working on a project which has a few different microcontroller boards to form a network. I'm planning to use I2C to communicate with each other. But two of my development boards are working on 3.3V range and other boards( arduinos ) are working at 5V range. As I want to use same I2C bus to communicate with them all, what kind of conversion circuit I have to use to interface my 3.3V development boards to I2C bus?

Thanks ahead,

vish

Former Member · Accepted Answer

The correct way to do it is given here http://www.nxp.com/documents/application_note/AN10441.pdf

There are also specific devices designed to do it like http://www.ti.com/product/pca9306

What John alludes to above is that you have to check the spec of the device. If the spec says 0.7 * Vcc for a 5V part, then follow the spec and use proper level translators. While it's true that the device may well use 2.0V today, you don't know if that'll be true tomorrow.. and I feel that it would be irresponsible to design something that you know is operating outside the manufacturers defined spec, or to encourage others to do so. If the spec for the 5V part says Vih=2.0V then it's perfectly ok to do it.

johnbeetem · Answer

I2C devices have open-drain drivers, which only pull the SCL and SDA lines low to 0V. Each I2C line should have a single pull-up resistor: make sure they pull up to 3.3V and not 5V. Also make sure that each 5V part has a reasonable Vih (Input voltage high) threshold. Many 5V chips use TTL Vih which is 2.0V, which works. Others may have Vih = 0.7 x 5.0 = 3.5V, which means you'll have to do something else.

If you want to make sure 3.3V I2C inputs don't go too high, clamp SCL and SDA to 3.3V with Schottky diodes. That will limit them to 3.5V or so. They should not be needed if the SCL and SDA pull-up resistors are connected to 3.3V, but it doesn't hurt and may protect you from killing a board accidentally.

Wikipedia has a good article on I2C: http://en.wikipedia.org/wiki/I2C

I2C communication between 3.3V uC and 5V uC

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