Connect I2C devices with different voltage(3.3v & 5v)

[jetq88]

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

 

[DJ Delorie]

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:

The gumstix/robostix combination does this. There are I2C level
converters ranging from simple to complex, or you can use a simple
N-channel enhancement MOSFET on each line. Google for "i2c level
shift".

1. can SCL and SDA be connected directly among those devices?

No, unless the 3.3v devices are all "5v tolerant" and the 5v devices
are all "TTL tolerant".

2. should pull up resistors be connected to 3.3v or 5v?

Both, one on each side of the converter.

3. do I have to use voltage translator between 3.3v and 5v?

Most likely.

4. should I use series resistors on SCL and SDA, should they be close
to 5v devices or 3.3v devices?

Each branch should be considered separately.

 

[Frank-Christian Kruegel]

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:

read this!
http://wwwasic.kip.uni-heidelberg.de/lhcb/Publications/external/AN97055.pdf

Mit freundlichen Grüßen

Frank-Christian Krügel

 

[Andrew Holme]

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

There's a really simple way to do it using only two MOSFETs. Refer to the
paragraph 18.1 on page 43 of the specification:
http://www.nxp.com/acrobat_download/literature/9398/39340011.pdf