Qt not receiving serial data from arduino

Question

I am writing a C++ program using Qt that reads data from the serial line. The "producer" of the data is an Arduino UNO board.

The Arduino code is very simple. Currently, it just generates a random number and sends it over the serial line. Here is the code:

long randNumber;

void setup() {
  Serial.begin(9600);
  randomSeed(analogRead(0));
}

void loop() {
  randNumber = random(300);
  Serial.write(randNumber);
  delay(10);
}

I have used the Arduino's "serial monitor" to verify that data is coming across the serial line.

On the Qt side of things, I have a worker thread that is supposed to read the serial data and update a plot on the UI accordingly. The worker thread is definitely running, and this code is getting executed (I've checked). I only have one device in my "port list" that shows up, and it is the Arduino, so currently I connect right to it. Here is the code:

void doWork ()
{
    QList<QSerialPortInfo> port_list = QSerialPortInfo::availablePorts();
    QSerialPort serialPort;
    serialPort.setPort(port_list[0]);
    serialPort.setBaudRate(QSerialPort::Baud9600);

    if (!serialPort.open(QIODevice::ReadWrite))
    {
        cout << QObject::tr("Failed to open port COM3, error: %1").arg(serialPort.errorString()).toStdString() << endl;
    }

    while(!abort_thread)
    {
        int bytes_available = serialPort.bytesAvailable();
        if (bytes_available >= 4)
        {
            QByteArray byte_array = serialPort.read(4);
            int data = byte_array.toInt();
            emit signalDataReady(data);
        }
    }

    serialPort.close();
}

Unfortunately, there are never any bytes available. It opens the serial port successfully, but no bytes come through. What am I doing wrong here? Any ideas? Thanks!

You seem to have checked pretty much every error I could think of, but are you *absolutely sure* you only have one port on your computer? Did you print out the length of `port_list` or did you use something else to check? I'm just asking because many computers tend to have extra serial ports even though they might not have a physical interface. — Matti Virkkunen, Apr 28 '16 at 01:43
I presume that Qt is running on a PC connected to you arduino board? - or is Qt also running on another arduino board? Anyway, a couple of things to try: First is in your doWork() thread function you will probably be maxing out your processor as you have no switch points. Try adding `QThread::msleep(10);` to add a pause, slow down your polling and allows other processes some cpu time (not sure it will fix it, but....). You will need to `#include ` in your file. — code_fodder, Apr 28 '16 at 06:38
Second thing... ok this is more a stab in the dark then the first, but I always open my QSerialPorts by setting setPortName()... always works for me. So you can try `serialPort.setPortName(port_list[0].portName());` instead of `serialPort.setPort(port_list[0]);` — code_fodder, Apr 28 '16 at 06:41
Finally, have you tried to communicate to a known-good comport output? - i.e. connect to a windows/linux box with a serial terminal (like putty) and look for the ardiuno board output? - If that is ok, then try receiving in your Qt app from the putty terminal and update us with which way around this works / does not work (I know you say you have verified arduino board with its own tools... but I never trust these until I have seen it work with standard tools). — code_fodder, Apr 28 '16 at 06:44

score 1 · Accepted Answer · answered Apr 28 '16 at 05:15

1

You are forgot to add waitForReadyRead() before bytesAvailable().

answered Apr 28 '16 at 05:15

Denis Shienkov

517
3
12

Looking at it closely, this would probably fix it. There's probably no event loop in your thread, and I can't remember how `QSerialPort` behaves without one, but the docs state that this method is useful "when performing I/O operations in a non-GUI thread". At any rate it's good practice to block rather than busy wait. – Matti Virkkunen Apr 28 '16 at 10:12
After having looked at all of the various suggestions made here, it looks like this is the one that did it! Thanks! What's interesting is I have to do this each time before I call bytesAvailable(), and not just once after connecting to the Arduino. – David Apr 28 '16 at 16:14

score 0 · Answer 2 · answered Apr 28 '16 at 01:37

0

From https://www.arduino.cc/en/Serial/Write:

Serial.write(val)

val: a value to send as a single byte

If you write a number Arduino only sends one byte, if you want to send a long then you must decompose it as an array and send the array (or get a pointer to the long's address and send each one of the bytes).

As your Qt code expects to read four bytes the if is never executed.

answered Apr 28 '16 at 01:37

Gusman

14,905
2
34
50

That's probably an error but because the Arduino code is run in a loop it will eventually send 4 and more bytes and this doesn't explain why no data is received by the Qt program. – Matti Virkkunen Apr 28 '16 at 01:38
Well, the loop is not posted on the code, I can't imagine details of code not posted :D (yes, there's a loop() function, but I can't see it to be really a loop as the function just enters and exits). – Gusman Apr 28 '16 at 01:40
For some reason the Arduino/Wiring developers saw fit to protect programmers from having to write loops themselves so they provide a main() function that calls setup() and then keeps calling loop() in a loop for them. Go figure. – Matti Virkkunen Apr 28 '16 at 01:41
My bad then XD, I'm not very used to Arduino, I prefer to program directly on their native frameworks the microcontrollers. – Gusman Apr 28 '16 at 01:44
Same here. I'm not a big fan of the whole Wiring deal. – Matti Virkkunen Apr 28 '16 at 01:45
Thanks for this suggestion. Although it wasn't the root of the problem, it was definitely a bug. It's been fixed! – David Apr 28 '16 at 16:14

Qt not receiving serial data from arduino

2 Answers2