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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

  • 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.

Reply

  • 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.

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