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MAX3232: MAX3232 output waveform issue

Poh Kok Keng
Genius 5112 points
Part Number: MAX3232

Dear Sir/Madam,

I am facing issue with MAX3232CPWR, when running at baud rate 115200, i can see garbage start showing on the UART terminal output. But when running lower speed like 57600, 9600, not issue found. some of the board is working correctly. 

Please refer to the attached picture here. the waveform captured is when UART is power up and no sending any data at RX/TX line from PC via USB2UART converter. Can you advice if the pulse output look normal? and should I change the capacitor value, will it help?  

6114.UART issue.docx

 

best regards,

kpk

 

  • 0
    TI__Mastermind 25705 points
    Poh Kok,
    Sorry to hear your problem. When you run with problem, can you check V+ pin waveform? What are external cap value and rating you use? Another thing to check is the TX line loading. We recommend smaller than 1000pF cap. Sorry I don't have immediate solution for you. Please share more information, I will continue to work with you to debug the issue.
    Regards,
    Hao
  • 0 in reply to Hao L
    Genius 5112 points

    hi Hao,

    The external cap is all 0.1uF following the norminal value as described in recommended circuit in the datasheet.

    Best Regards,

    kpk

  • 0 in reply to Poh Kok Keng
    TI__Mastermind 25705 points

    Poh Kok,

    The cap should be OK if you use 3.3V supply. Have you been able to check V+ pin? I attach a system diagram from my understanding. Can you please give a little more information? Thanks!

  • 0 in reply to Hao L
    Genius 5112 points

    hi,

    we are following the recommmended circuit as suggest in datasheet with all 0.1uF capacitor populated. (capacitor part no: GRM188R71H104KA93D) Multilayer Ceramic Capacitors MLCC - SMD/SMT 0603 0.1uF 50volts X7R 10%

    Below is the V+ output for not good part, measured voltage is 5.4Vdc


     

    For Good part, measured voltage at V+ is 5.8Vdc


     

    One observation is the UART message is output from the rasberry-pi CM3 module. When setting to lower baud rate like 57600. UART can output the message correctly. However, when setting baud rate at 115200, UART start to show garbage. 


     

    Best Regards,

    Kpk

  • 0 in reply to Poh Kok Keng
    TI__Mastermind 25705 points

    Poh Kok,

    Sorry my attachment somehow got lost in last post. I repost it here. Can you check if it describes your system?

    Both V+ waveforms look good to me, although one (5.4V) is a little weaker than the other one (5.8V). 

    What's the distance between ports? You could try 115200 with shorter cable. If you have, can you check any DTR/DSR, RTS/CTS signals as well? But I think it could be most helpful if you check TD signal directly.

    Regards,

    Hao

  • 0 in reply to Hao L
    TI__Guru 62910 points
    Hi Poh Kok,

    Were you able to resolve your issue? Or, were you able to probe the RS-232 signals like Hao suggested? This would be a useful test, since the issue becoming worse at higher data rates suggests there may be some marginality with the physical layer.

    Regards,
    Max
  • 0 in reply to Max Robertson
    Genius 5112 points
    hi Max,

    I suspected the issue could be their Rasberry Pi CM3 firmware issue. Based on the captured UART output, 99% of the chance the garbage apperar before the xx$ sign. This show strong signal to me that is more firmware related.
    At this moment, they plan to go with lower baud rate as temporary solution since they need to get ready for their production.
    FYI, they are using 3 lines, TX, RX, GND with 8-n-1 without CTS/RTS/DTR/DSR. I have ask them to relook into their PCB routing as well since the Rasbettery PI CM3 module is a plug in module to the motherboard.

    We can temporary put this case to close at the moment until further investigation on the issue.

    Best Regards,
    kpk
  • 0 in reply to Poh Kok Keng
    TI__Guru 62910 points
    Kpk,

    Understood, and thanks for your update! Just let us know if you need further support later on.

    Max