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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

Parents

  • 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.

Reply

  • 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.

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