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MAX3232: RX level differs from TX level - IC with internal pull up?

Marcus Rohm
Prodigy 10 points
Part Number: MAX3232

Hello,

i´m working on a simple ESP8266 and MAX3232CPWR design.

I´m experiencing following issue at the input side of the MAX3232 IC:

- when the ESP Micro is connected to the MAX, RX signal voltage level is ~ 1.5V (below the treshhold of the IC), TX pin works fine signal level is here at the expected 3.3V

- ESP not connected to the MAX has a perfect output of the signals

Behaviour looks like some voltage divider / pull up inside the ic?

Could someone point me towards the exact issue?

Below are details for this issue.

Thanks in advance!

Following actions have been done:

- changed MAX chip to avoid defect part -- no change

-checked resistance between RX and TX  and each line to GND and the sourunding components. No observations found.

Osci shot of signal overview, both have the same V/div setting.

detailed view on the RX signal, measured signal amplitude is 1.5V.

ESP output without the MAX3232 Connected

Routing

Schematic

  • 0
    TI__Guru 60392 points

    Marcus,

    Thanks for bringing this question to E2E and for sharing all of this information up front, it helps tremendously in our debug efforts. Can you share more information about the ESP micro pin connected the MAX3232 RX pin? Is there any kind of integrated pull-up or pull-down resistance?

    I've notified an expert about this thread. Thanks for your patience.

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    TI__Genius 15345 points

    Hi Marcus,

    The MAX3232 has an internal pull down resistor of 5K on the RIN1 (Pin 13) and RIN2 (Pin 8) pins. I will wait to hear your answer to Eric's question.

    Best,

    Chris

  • +1
    Prodigy 10 points

    Hello,

    thanks for reaching back on this topic.


    First i would like to share my latest findings / observations:

    - above shown scopes showes the response of a serial signal which was transmitted via the internal USB-Serial transmitter. (Send via USB to the attached ESP + MAx and measured between ESP and MAX). I was constantly confused as i have not seen this signal amplitude drop while using a break out board, attached the same way.

    Anyhow i figgured out that this won´t be any acutal problem for my usage. The RS232 side of the MAX communicates fine, i can send data to my ESP and get a propper response.

    Hope this is understandable to read.. Not my first language ;)

    Still wondering why de behaviour with exact "schematic" differs between the breakout board (may this one has a cheap chinese chip version?) and the MAX3232 in my PCB.

    If you still want to deep-dive into this topic the information regarding the pin, as of the Datasheet (ESP8266) are below:

    TXD0 - GPIO1

    RXD0 - GPIO3

    No information found in the datasheet regards pull-up or pull-down.

    Thanks and have a great day!

    Marcus

  • 0 in reply to Marcus Rohm
    TI__Guru 60392 points

    Marcus,

    Thanks for the update and all of this information. Please let us know if you have any other questions.

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    Prodigy 10 points

    Hello Eric,

    so far not.

    Thanks for supporting me!


    Have a good day!

    Marcus