Need explanation QSerialPort vs Pyserial

  • I'm trying to reset an Arduino Leonardo into boot loader mode by opening and closing the usb serial with a baudrate of 1200. This does not seem to work with Qt. When I tried this with PySerial, just opening and closing the port it works. I also tested with stty and that also works.

    Does anybody have any ideas why QSerialPort does not work ?

    void Programmer::resetAvr()
        if( == true)
    #!/usr/bin/env python
    import serial, sys
    serialPort = sys.argv[1]
    print serialPort
    ser = serial.Serial(
    ser.close()             # close port
    stty -F /dev/ttyACM0 1200 cs8 -cstopb -parenb

  • Qt Champions 2018

    Hi @moscowbob,

    just opening and closing a serial port without sending data will not change anything for the other side (independent of the baud rate).

    So there is most likely a control line (RTS, CTS, RTR) involved that actually does the reset? Do you have a description how the reset is supposed to work?

    And if it is working like this, there can be indeed a difference between pyserial and QSerialPort.

  • @aha_1980 , thanks for response.
    The connection is a USB CDC connection, the AT32U4 chip has an embedded usb that is configured as a virtual comport. So there are no control lines connected. As I understand, when the host opens the port, it will configure the device endpoint and it is there where the 1200 baud is used to reset the device into bootloader mode.

    I have captured the usb conversation between the host pc and device for all 3 methods and the QSerialPort conversation looks completely different - unfortunately my knowledge is very limited when it comes to this, so if you want, I can post the three Wireshark capure files for viewing.

  • Qt Champions 2018

    Hi @moscowbob,

    yeah, you can post them, but I have to admit that my knowledge is also limited here :)

    But maybe someone else can help you here...

  • What is the accepted method for the forum when posting files - I do not have privilege to upload here ?


  • Lifetime Qt Champion

    It only wants
    alt text
    What type are you trying ?

  • Thanks @mrjj ,
    The capture files I have are wireshark .pcapng or a zip file containing all 3 captures.

  • Lifetime Qt Champion

    Not sure it will accept those.

  • No there is something else wrong, I just tried to upload a simple text file with .c extension and I just get the same message

    " ERROR
    You do not have enough privileges for this action."

  • Well after spending many hours digging through usb protocol documentation I have finally got to a point where I can say the problem is with QSerialPort. Since I cannot upload files I will try and explain.

    PySerial and linux stty both exhibit the same behavior when viewed in wireshark usb capture:

    1. Host send command 0x22 with value 0x03 to enable RTS and DTR
    2. Host send comand 0x20 with 7 bytes data to configure BAUD, DATABITS, PARITY and STOPBITS
      1200, 8, n, 1
    3. Host send command 0x22 with value 0x00 to disable RTS/DTR

    Now the device resets and goes into boot mode.

    QSerialPort behaves as follows - Open/Close :

    1. Host send command 0x22 with value 0x03 to enable RTS and DTR
    2. Host send command 0x20 with 7 bytes data to configure BAUD, DATABITS, PARITY and STOPBITS
      (all these configurations are sent)
      400000, 8, n, 1
      9600, 8, n, 1
      1200, 8, n, 1
      115200, 8, n, 1
    3. Host send command 0x22 with value 0x00 to disable RTS and DTR
    #include <QCoreApplication>
    #include <QDebug>
    #include <QSerialPort>
    int main(int argc, char *argv[])
        QCoreApplication a(argc, argv);
        QSerialPort mPort;
        qDebug() << "Open Port";;
            qDebug() << "Close Port";
    //    return a.exec();

    Anybody ??

  • Well, finally a breakthrough !

    It turn out that the deprecated function


    Does the trick.

    Although the Arduino now does reset, since QSerialPort does not send the restored values before closing the port, it still sends a garbage packet before setting the 1200 baud. I have not figured that one out yet, and I am now getting tired.

    So I will mark this as solved but will raise another post to query the strange behavior when QSerialPort opens.

  • Qt Champions 2018


    Please report this at (and provide a link to the bug here).

    You will have better possibilities to discuss the behavior with the developers there and you should also be able to attach the Wireshark logs.


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