Communication between Raspberry Pico and QtSerialPort not working on Windows

[Dragoner]

I have a Raspberry Pico connect via USB to my Windows Computer. On the Pico the following program is running:

int main() {

    stdio_init_all();

    while(true) {
        printf("HelloWorld");
        sleep_ms(1000);
    }
 }

I try to read the output via QSerialPort. My Qt program is:

int main(int argc, char *argv[])
{
    QSerialPort serialPort("COM3");

    qDebug() << "Opening";
    if (!serialPort.open(QIODevice::ReadWrite)) {
        qDebug() << "Failed to open " << serialPort.portName();
        exit(-1);
    }

    qDebug() << "Open!";
    while (true) {
        if (serialPort.waitForReadyRead(2000)) {
            QByteArray response = serialPort.readAll();
            qDebug().noquote() << "R: " << response;
        } else {
            qDebug() << "Got Nothing";
        }
    }
}

If I plug in the Pico via USB and start the Qt program, the output I get is:

Opening
Open!
Got Nothing
Got Nothing
Got Nothing
...

But, If I open the COM port using a different tool, in my case the SerialMonitor from the Arudino IDE, I see

HelloWorldHelloWorldHelloWorldHelloWorldHelloWorldHelloWorld

as excpected. If I start my Qt program after I opened the COM port with the Arudio IDE, I can also see the output:

Opening        
Open!
R:  HelloWorld
R:  HelloWorld
R:  HelloWorld

The same issue is also reported on Stackoverflow, but using PyQt.
This issue appears only with Windows. On a Raspberry Pi everything works as expected.

• Why does QtSerialPort works as expected if I open the Port with a different tool before?
• What does the Arudino IDE do, that QtSerialPort is (maybe) missing?

[J.Hilk]

@Dragoner you fail to specify baudrate, parity, data & stopbit, flow control etc of your QSerialPort, maybe thats the issue here.

[Dragoner]

@J-Hilk While searching for proof that values like baudrate are irrelevant for USB-CDC (e. g. here ), I found a thread shows the answer: Windows COM access to PICO via USB trouble.

The Data-Terminal-Ready flag needs to be set to DTR_CONTROL_ENABLE.

This can be done via

QSerialPort::setDataTerminalReady(true)

after opening the Port.