4 Replies Latest reply on Mar 16, 2020 6:42 AM by user_4564791

    I2C with 3.3V pull ups

    user_4564791

      Hello guys,

       

      I need to communicate with a PSoC as master (working with 5V) to a device via I2C. The device only allows 3.3V. Therefor I pull the I2C lines with external resistors (4.7k) to 3.3V. The I2C Pins are set to "open drain drive low" and the input treshold is set to "LVTTL".

      The measured voltages don't show the expected 3.3V Levels but rather 4V...

      Does anyone know how I can decrease the voltage? I think I applied all the things mentioned in the I2C datasheet.

       

      Thanks in advance

      Kurt

        • 1. Re: I2C with 3.3V pull ups
          LePo_1062026

          Kurt,

           

          I did not see anything wrong with your configuration of SCL or SDA pins.

           

          If you're measuring 4V on SCL or SDA, check your bias source to the external 4.7K resistors.   You bias may not be 3.3V as you were hoping.

           

          A Note on your note:  The reference to "TTL" is the input thresholds used in the PSoC to determine logic levels.  A "true" open-drain configuration should not pull the voltage high.  It requires a pullup resistor elsewhere on that signal.

           

          Len

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          • 2. Re: I2C with 3.3V pull ups
            user_4564791

            Hi Len,

             

            thank you for your prompt reply. When I change the SDA Pin from 12.1 to 12.5 and the SCL Pin from 12.0 to 12.4 it is working...

            Any idea why? When I measure the voltage on the 12.0 and 12.1 Pin I measure ~4.7V and not the expected 0V.

            My problem is that I plug the PSoC to a PCB and therefor need the Pins 12.01 and 12.1 for the I2C Interface.

             

            Thanks in advance

            Kurt

            • 3. Re: I2C with 3.3V pull ups
              VasanthR_91

              Hi,

               

              I programmed your project on a CY8CKIT-059 and checked the SCL pin after giving a pull-up to 3.3V. I could measure 3.3V. Is there anything I am missing ?

               

              Best Regards,
              Vasanth

              1 of 1 people found this helpful
              • 4. Re: I2C with 3.3V pull ups
                user_4564791

                Hi,

                 

                thanks for checking my project. I guess it is because I am using the KitProg. In the electric schematics you can see that there are some on Board Pull-ups. I guess this is why Pin 12.0 and 12.1 aren't working but the other Pins from Port 12 are working...

                When I desolder R9 and R7 I get the 3.3V as expected!

                Thanks for your help.

                 

                Kurt