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ISO1050: unexpected output on RXD

Mark Nickerson
Prodigy 20 points
Part Number: ISO1050

Tool/software:

Hello, I made a quick video to demonstrate the issue- the output initially looks normal (0 to 3.3V square wave) for a very brief period after powering on, but then switches to a 2V to 3.3V square wave:

https://www.youtube.com/shorts/wjrvwyceRhU

RXD is connected by the PCB to the input pin of an ESP32.  Here is the line of code setting up that pin:

#define ESP32_CAN_RX_PIN GPIO_NUM_16
I also notice that when I hold the Reset button of the ESP32, the full 0 to 3.3V signal is present.
I wonder if this type of truncated bottom half is what's expected for Undervoltage Lockout?  I do see a section on that in the datasheet, but haven't noticed any issues with my power.   Another idea was this:
The TXD pin should be pulled up through a resistor to VCC1 to force a recessive input level if the microprocessor output to the pin floats
The current circuit neglects to do this, but could TXD affect RXD?
Thank you for any advice!
  • +1
    Guru 290610 points

    Apparently, some other device is pulling the line high. If the only other device connected to the line is the ESP32, then the GPIO configuration must be wrong. Can you step through the GPIO initialization with a debugger to see when the pin is configured as an output?

  • +1 in reply to Clemens Ladisch
    TI__Mastermind 23195 points

    Hello Mark, 

    Thank you for reaching out and with good details. Please follow the debug steps Clemens recommended and confirm that the ESP32 isn't configured as an output and driving the RXD pin. The output of the RXD pin should only be a 0V to 3.3V (or VCC1) waveform.  

    Below are my responses to your other questions for completeness.

    • what's expected for Undervoltage Lockout?
      • The RXD should output be tri-stated while side 1 is unpowered or powering (8.3.3.3 Undervoltage Lockout and Fail-Safe).
    • The TXD pin should be pulled up through a resistor to VCC1 to force a recessive input level if the microprocessor output to the pin floats. The current circuit neglects to do this, but could TXD affect RXD?
      • A floating TXD pin could cause a bit error but not the truncated waveform.
      • However, a pull-up resistor at TXD is a good recommendation if the MCU pin could float. (8.3.3.4 Floating Pins).

    Best,
    Andrew

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    I have confirmed that the issue has disappeared with a different ESP32- good chance that the original was somehow damaged.  Thanks for your insight.

    https://imgur.com/8wAMtQ9

  • 0 in reply to Mark Nickerson
    TI__Mastermind 23195 points

    Hi Mark, thank you for letting us know the issue is resolved. I will close this thread. Let us know if you have any more questions. 

    Best,
    Andrew