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



On Nov 28, 7:47 am, "jetq88" <jetq5...@xxxxxxxxx> 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?

thanks

Philips (presumably NXP now) has some app notes on this subject.
Annoyingly devices that meet the I2C spec have their thresholds defined
as a fraction of the suply voltage - that means that you cannot
guarantee that a 3.3V device will talk to a 5V device. Some of the
buffers from NXP and others (e.g. LTC) can do the conversion although a
simple one is just to use an N-channel MOSFET in each of the two lines
with the gate connected to the 3.3V line with the drain and source
going to the two sides (doesn't matter which). You need a separate
pull-up for each side. This will give you bidirectional level
conversion.

SMB is a variant of I2C (System Management Bus, that Intel promotes).
The SMB defines a fixed threshold level so that the 3.3V and 5V devices
can communicate. There are some SMB devices around although I have
only seen them form Intel.

kevin

.