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MAX3232: IC will send a word itself to DUT

Vincent Su
Prodigy 20 points
Part Number: MAX3232

HI sir,

When the RJ45 cable is not connected, the IC can function normally without sending random messages.
But after connecting it(only cable) and turning DUT on (but not connecting to USB-PC), the text "e" will be entered inside the DUT.

Here is Circuit

    

1.plug RJ45 cable only

Wait for 1 minute (it will not start normally) and then connect RS232 (USB TYPE) to PC

You will find that it keeps sending text "e".

But when we connect RS232 to PC, the text will stop.

2.plug RJ45 cable with RS232(USB type) to PC

normal

thanks!

Vincent

  • 0
    TI__Mastermind 46687 points

    Hi Vincent,

    So the case in which this occurs seems to happen when Pin 11 (input T1) is floating - i.e. you are not driving a signal on this line at this point. This is a problem - the inputs can never be left floating because this device is always active as long as there is a proper VCC and the charge pump has initiated. When the input is left floating in this condition a random bit stream can appear at the output (along with increased supply current typically). The PC may be asserting a value onto that pin when it is connected - since this is UART I'd imagine it defaults high. 

    To fix this issue I'd recommend a pull-up resistor (like 4.7k to 10k range) to the VCC of the IC. The reason being is that this asserts a default value and won't be mistaken for a datastream at the DUT. 

    Is that a possible fix that you can try? 

    Please let me know!

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 20 points

    Hi Parker,

    I added a 4.7K pull-up resistor to pin11 first, 2nd try is add both pin 11&12, but the result remains unchanged.

    Previously, I attempted to capture the signal and discovered that it intermittently sends a signal from the MAX3232 and CPU (Intel denverton) side.



    UART_0_TX_A (CPU to MAX3232) is keep send below signal,

    The RJ45 cable to PC is not connected at this time. The MAX3232 send the signal.

    UART_0_RX_A (MAX3232 to CPU)

    If we not connected the console board and boot up the system, the system can bootup notmaly.

    (CPU side will still send the signal, but will not receive reply.)

    thanks,

    Vincent

  • 0 in reply to Vincent Su
    TI__Mastermind 46687 points

    Hi Vincent,

    To clarify the scope shot you are showing me with the data train is from the unplugged cable? 

    Are you running devices by any high speed comms lines are high current power lines nearby? I ask because if my assumption is correct - you are getting very strong data signal on the "unplugged" CPU line - which means something is either driving it or something is coupling onto it - because the square wave has a high signal quality - which is not what you expect from interference. 

    Can you also please place a picture of the to marking of the device so I can trace the device in our internal system. 

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 20 points

    Hi Parker,

    1.When the MAX3232 and RJ45 cable are connected, the TX of the CPU will continue to transmit signals, and the RX will return signals.

    2.When the MAX3232 and RJ45 cable are connected to PC, the TX of the CPU will continue to transmit signals, but there will be no signal on the RX.

    3.When the MAX3232 is connected, the TX of the CPU will continue to transmit signals, but there will be no signal on the RX.

    4.Without connecting the MAX3232, the TX of the CPU will continue to transmit signals, but there will be no signal on the RX.

    It looks like the MAX3232 responded by itself when it received the CPU's TX.  This is weird.

    I am not sure what you means of "Can you also please place a picture of the to marking of the device so I can trace the device in our internal system."

    Here is the UART trace on Main board (Total path length:8977.9892mil). There is no high speed signal near it.

    And use cable fly to the LED board's RJ45 connector.

    LED Board

    MAX3232

  • 0 in reply to Vincent Su
    TI__Mastermind 46687 points

    Hi Vincent,

    Thanks for the reply - just to clarify I was asking for last picture you added and this device shows up in our system and is legitimate -it isn't a counterfeit device and it looks like its from us; that means that is not an issue. 

    There is a pretty common problem where a lot of commodity type devices are actually counterfeit and just try to mimic the look of better designed devices. 

    So with all that being said - it sounds like it is sending signals because of some signal (even if its just noise) is on the input regardless of setup condition. Since you have now ensured that there are default levels when the PC isn't connected - and since it still seems like there is an issue. Especially considering the fact that you don't have high speed or high current lines nearby - so I doubt something is being coupled in. 

    Unfortunately that means it could be a quality issue with the device - as from an application point of view - nothing that you are doing seems incorrect. Since that is what it is looking like a potential quality issue the next step would be to go to the FA (failure analysis) page on our website - https://www.ti.com/support-quality/additional-information/failure-analysis.html and follow the instructions there. Depending on how you bought the device we may be able to run the part through automated testing equipment to look for failures within device and possibly a bench test depending on automated testing results.  This is the next best step to getting a resolution - as I don't see anything that you are doing wrong at this point that could cause the issue seen. 

    Best,

    Parker Dodson