Mixed voltage I2C

[towlerg]

Using a 5v PIC talking to a 3.3v device via I2C. Is it safe to simply tie I2C pullup resistors to 3.3v rail or should I use full on bi-directional logic level convertor?

George

[solutions]

Depends

What's the mystery "device" (don't answer if it blows your MI5 cover)?

[PICker]

Several answers:
http://www.microchip.com/forums/m440988.aspx
http://www.edaboard.com/thread171379.html
https://www.ccsinfo.com/forum/viewtopic.php?p=43813
Have a look also to this document:
http://ww1.microchip.com/downloads/en/DeviceDoc/chapter%208.pdf

[towlerg]

@solutions

What's the mystery "device" (don't answer if it blows your MI5 cover)?

Sorry, wasn't trying to be mysterious, just didn't want to include meaningless details. In this particular case, it's just 'cause I'm curious.


@PICKer

Thanks for those. I had in fact seen them and loads more like them. The CCS one is just plain wrong

Selection of any I2C mode with the SSPEN bit set,
forces the SCL and SDA pins to be open drain,

I'm gonna see what my friend Proteus has to say.

George

[Checksum8]

You can use this bi-directional level shifter from Sparkfun. It's basically from the NXP app.
Schematic link on this page
https://www.sparkfun.com/products/12009

[Signal]

Using a 5v PIC talking to a 3.3v device via I2C. Is it safe to simply tie I2C pullup resistors to 3.3v rail or should I use full on bi-directional logic level convertor?

Typical value for minimum input high voltage for I2C pin of 5V PICs is 0.7*Vdd =~ 3.5V @5Vdd. Even with reduced Vdd=4.4 V and threshold about 3.1V the more "upper space" is needed due to RC slope.
But simple answer - I2C module is intended to work with pull-ups to Vdd supplying this module so converter is needed.

You can use this bi-directional level shifter from Sparkfun. It's basically from the NXP app.

http://www.nxp.com/documents/application_note/AN10441.pdf
I doubt that this circuit is an NXP invention though.
In case of missing n-MOSFETs close at hand they can be replaced by NPN transistors with additional serial resistor in base.
Specialized level converters for I2C are more speedy and could have signal conditioners built-in. So there is no universal suggestion what to use.

[towlerg]

@Checksum8
I know I can use level level convertors, my question is, do I have to?



@Signal
Looks like we have a winner. I didn't realize that a PIC needed such a high level to "see" a one.

The standard cuircuit using mosfets seems to have pullups on both low voltage in/out and high voltage in/out. So the pullup I2C resistor should be increased to allow for the two resistor in parallel. Best I can tell the sparkfun/ebay units use 10k, so my I2C pullup will be 8.2k?

George

[Signal]

Looks like we have a winner. I didn't realize that a PIC needed such a high level to "see" a one.

Did not know that it was a competition. Feel free to read a datasheet for chosen IC - you will find many interesting things there

[towlerg]

@Signal

Sorry, didn't mean to offend. Simply meant that yours was a definitive answer.

George