1 Reply Latest reply on Mar 16, 2016 7:36 AM by pavankumar.vibhute

    I2C with 3.3V pull-ups and PSoC3 5.0V operation

    rdonahoo1_1583441

      The component documentation has a note about reliable logic transitions on the I2C bus.

         

      I2C_v3_50.pdf states: 

         

      "Note The default threshold voltages for the Pins component is CMOS. When the I2C lines are pulled up to 3.3 V and the PSoC is running on 5.0 V, the CMOS threshold levels may not guarantee reliable logic transitions. In this case, the SCL and SDA pin component thresholds should be set to TTL."

         

      The options I see for the pins I have assigned for SDA and SCL don't show a TTL option to select for the input pin threshold.  I can select from the following values for input threshold in the drop-down menu:

         
            
      • CMOS
      •     
      • CMOS or LVTTL
      •     
      • LVTTL
      •     
      • Vddio
      •     
      • 0.5xVref
      •     
      • Vref
      •    
         

      I'm using PSoC Creator  3.3 CP2 (3.3.0.7343) released 03/01/2016 and have 4.99k pull-up resistors connected to 3.3V on the SDA and SCL lines.  Which threshold should I select to guarantee reliable logic transitions?

         

      Randy

         

      P.S.  I2C bus speed is set to 400kbps.  Seems that these pull-ups are too large based on what's shown in the component documentation.