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Is level shifting really needed for I2C?

fustini

Howdy, I've written a blog post on interfacing the 5V DS1307 RTC via I2C, and I'd like to verify whether a logic level converter (e.g. level shifter) is really needed.   I became aware that it may not be needed after reading this comment in LED Clock using Raspberry Pi blog post:

Gordon Henderson said...

Not quite sure why you're using a level convertor here... I2C is driven by open collector outputs with the master providing the pull-ups - which the Pi does via on-board 1.8K ohm resistors... The display should never drive the bus back to the Pi, so there really shouldn't be a need...

 

-Gordon

August 21, 2012 3:51 PM

Simon responded:

Simon Monk said...

@Gordon. I think you are right I don't think it is necessary. I had a level converter anyway and did the wiring before finding out about the protocol. I thought their may be some 'acks' going on.

 

No, the display should never be driving the bus.

August 22, 2012 12:40 AM

and then later:

Simon Monk said...

@Gordon, I just checked that the display doesn't do any nasty pulling up of the bus to 5V, and it doesn't. So I took the level converter out and it works just fine without it.

August 24, 2012 1:06 AM

This left me wondering if this should be the case for all I2C modules.  Anyone have thoughts on this?

 

Thanks,

Drew

  • It depends upon the devices you hang on the iic bus. The bus origin is television units. The bus was used to connect different chips to the microcontroller. In those days, everything was 5V. The bus normally has pullup resistors to the 5V power rail. Every device can pull the SDA to GND. Only the master should control the SCL and pull that to GND.

     

    The rpi SOC gpio lines have clamping diodes to 3V3 and GND to protect the input. This means that the voltage on such a line should not be higher than 3V3 +0.7 = 4V. So, if the bus has pullup resistors to 5V, you will have a current flow from 5V to 3V3 and the voltage on the bus will be only 4V. Most devices will probably still work fine with only 4V on their IIC lines. The ViH min should be somewhere in the datasheets.

     

    So, if the device isn't having any pullups, you can safely connect it to the Pi, but you need to check if the Pi 3V3 is enough voltage to be seen by the device as a logical high.

     

    If the device is having pullups to the 5V, I would place a 3V3 zener between the IIC lines and GND to limit the voltage to 3V3. Again, you need to check if 3V3 is enough voltage for the device.

  • Since I2C drivers are open drain, you should be able to connect them without worrying about damaging anything.  However, correct functionality depends on the individual I2C device.  According to the DS1307 data sheet, the DS1307 has TTL input thresholds: 0.8V for Vil and 2.2V for Vih.  So the DS1307 should work fine with either a 3.3V or 5.0V pull-up.  OTOH, the I2C standard only requires a fixed Vih to be 3.0V, so pulling up to 3.3V would not leave a safe margin.  So it depends on the specific I2C device and how it implements the standard.

     

    Supplementing Luc Cool's comments, watch out for rogue pull-ups.  RasPi's built-in 3.3V pull-ups should be the only ones on the bus.  5V pull-ups will result in unnecessary DC currents and any parallel pull-ups make it harder for I2C devices to pull down SCL and SDA.  For example, the DS1307 can only sink 5 mA, so at 3.3V the total pull-up must not be less than 660 Ohms.  The 1.8K value used by RasPi is a good choice.

  • in reply to johnbeetem

    Thanks for the advice, Luc and John. 

     

    Coincidently, Adafruit just posted their own DS1307 RTC tutorial for the Raspberry Pi and Ladyada warns:

     

    When building the kit, leave out the 2.2K ohm resistors - by leaving them out, we force the RTC to communicate at 3.3V instead of 5V, which is better for the Pi!