This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TCA9803: step voltage in SDAB/SCLB falling edge

Part Number: TCA9803
Other Parts Discussed in Thread: TCA9517, TCA9801, TCA9800

Hi, team,

I have one question about TCA9803 may need your help.

Customer's application is like below. Master is NXP LS1046A CPU, Slave is EEPROM.

In testing, we found there is a 700~800mV/100ns voltage step in SDAB and SCLB falling edge as shown below. Is it caused by the device internal current source?

Thanks.

Johnny

  • Hi Johnny,

    This is strange. My initial thoughts is it could be the Master's FET not turning on fully for the initial 100ns. If this partially on impedance ended up being 200-Ohms, the current source could drive this voltage.

    How often is this occurring? Do all of your boards exhibit this behavior or just a few?
    To help narrow down the possibilities, you could try replacing the TCA9803 with a TCA9517 (with the B side towards the Master so the SVO won't interfere) and add pullup resistors to the B side. If the step still appears, its more likely a result of the current source.

    Regards,
    Eric
  • The step is not narrowd when  i  changed the end up resister to 200 ohms ,but after I have replaced the TCA9803 with a PCA9617 ,there is not step on the falling edge.The waveform is below.

    PCA9617/SCLB/falling edge

    PCA9617/SCLB/rising edge

  • Hi Johnny,

    This response looks correct for the PCA9617. The static voltage offset (SVO) of 500mV can be seen clearly.

    Regarding the step on the TCA9803, this may be caused by a pull-up resistor on the B side of the device or another buffer that may have a SVO. Are there any pull-up resistors or other devices (maybe a switch with pull-up resistors on the far side) on this bus?

    Regards,
    Eric
  • Hi Eric

    1.PORTB has no pull-up resistor

    2.We tried the TCA9800/TCA9801 two devices again. In contrast to the waveform, the steps seem to be caused by different current source sizes of PORTB.The edge of  TCA9800/TCA9801 is monotonous

    TCA9800/SCLB/falling edge

    TCA9801/SCLB/falling edge

    3.However, the SCL&SDA rising edge of the TCA9800 device still has a small step, which helps me see why.

  • Hello user4179467 ,

    I want to confirm, did you check to see if your master has an internal pull up resistor?

    One way you can test this if you are unsure is to place the PCA9617 back on the board and try to talk but do NOT populate the B side pull up resistor. When you pull low on SDA/SCL if the signal gets pulled back high after pulling low then the device has an internal pull up. If the device remains low then the device does not have an internal pull up.

    -Bobby
  • Hi Bobby

    I am sure the master has not an internal pull up resistor.I remove the pull-up resistor of SCL/SDA to make the pin float, then send the data, and the signal is detected to be unchanged, and remains low.

  • Hello user4179467 ,

    I believe we have taken this discussion offline via Email and have sent back a message asking about the test set up and have not yet heard back. (this was a few weeks ago)

    Thanks,
    -Bobby