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UCD3138128: I2C communication fail

Kevin Hong
Prodigy 170 points
Part Number: UCD3138128

hi All,

 

           My customer uses I2C function ( pin19 & pin20 ) on UCD3138128.

           Customer has some chips on hand, only one chip has weird I2C waveform as right side. ( left side is normal wave form )

           Customer uses 10Kohm pull-up resistor and 22pF filter for all chips. ( see below circuit )

           Customer can't find any recommend pull-up resistor, and can't find any electrical characteristics about input capacitance of pin19  & pin 20 ( I2C_CLK & I2C_DATA ) on spec.

           Customer want to know exactly the electrical characteristics of I2C function on UCD3138128, please reply these questions from design team, many thanks.

 

           The questions as below :

           1. What is the recommend pull-up resistor range (1k-10k?) for UCD3138128 I2C function?

           2. What is input capacitance spec of pin19 & pin 20 ( I2C_CLK & I2C_DATA ) ?

           3. Does the chip with weird waveform has bigger input capacitance than other chips on pin19 & pin20, it caused I2C communication failed?

  • 0
    TI__Genius 17090 points
    Some UCD devices do have slower rise times than others when they release the PMBus lines. We don't have a general recommendation for pull up resistors, because it is also a function of the overall bus capacitance. For example, if you put protection diodes on the pins like we do in our EVMs, that will increase the capacitance a lot. They're not really necessary on the EVMs, although if you have a big static charge on the board before you plug in the PMBus adapter, they might be useful. On a finished system, with a permanent connection, they're pretty useless.

    The lower limit on the resistor is set by the limit of 4 mA for drive capability on the pin. Many customer have done well with 750 or 1K ohm resistors.
  • 0 in reply to Ian Bower
    Prodigy 170 points

    Hi Ian,

    Thanks for your answer.

    One more question :

    Customer did 50 set, only one set has weird waveform ( as I showed upper ) , does it means this chip has different parameter on I2C function?

    Should we take this chip back, and do more test on it?

  • 0 in reply to Kevin Hong
    TI__Mastermind 18370 points
    Not all chips will have the same rise time.
    If you use lower values of pull-up resistors, you will be protected against worst case.
    No need to take the chip back and run more tests.

    Regards,
  • 0 in reply to Yitzhak Bolurian
    TI__Mastermind 18370 points
    If the above addresses your question, please set the status of this post to resolved.
    Regards,