4 Replies Latest reply on Apr 23, 2019 5:38 PM by VisonZ_71

    I2C Design question about level shifting

    rupi_283656

      I am adding a level shifter (LTC4309) to connect to the slave I2C in the PSoC.  The slave is connected to SIO pins that are 5 volt compatible, and the connecting device uses a 5 volt I2c bus.  But since the LTC4309 can also be used as a level shifter, I am wondering if it would be better to level shift down to 3 volts, or to stay at 5 volts applied to the SIO pins?  Can anyone say for sure if one method would be preferred, and why?

       

      Thanks, Russ

       

        • 1. Re: I2C Design question about level shifting
          JoMe_264151

          I2C pins need to be open drain using pull-ups which usually is not maintained by a simple level shifter. Have a look into the pioneer board schematics how it is solved for different supply voltages. I would suggest you to stay with SIO at 5V.

           

          Bob

          • 2. Re: I2C Design question about level shifting
            VisonZ_71

            Since P5LP supports SIO feature, i don't think you need a level shifter.

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            • 3. Re: I2C Design question about level shifting
              rupi_283656

              Thanks Bob and VisonZ.  Bob I see I did not explain the use of the LTC4309 correctly.  Although it can be used as a level shifter, it also provides bus isolation and several other features.  it is designed specifically for I2C usage.  Thus my usage is to provide hot swap capability to protect the PSoC processor from possible voltage spikes when users connect or disconnect the bus.  My question is whether also using it as a level shifter is desirable.  In the one case the SIO inputs receive 5 volt pulses and their internal comparators (essentially) convert them to 3 volts internally, and in the other case I apply 3 volts directly,  I am not sure if one method is better than the other.

               

              VisonZ, I was interested when you showed the tip with a Hot Swap option.  But that option is not available for the Version 2.20 pins that I have.

               

              So my question is unanswered.  I will go without the level shifting if I do not get a more definitive answer here.

               

              Thanks, Russ

              • 4. Re: I2C Design question about level shifting
                VisonZ_71

                Russ,

                 

                Hot SWAP option still exist in Version 2.20 pins.

                Hot swap capability allows the voltage present on the pin to rise above the pin's VDDIO voltage, up to 6.0 V, enough to use. So I suggest you to stay with SIO at 5V too.

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