Abnormal output when reading from serial port in QT - c++

Question

I am using QT and QCustomPlot to create a real time plotting tool, and the data of the plot is read from the Arduino UNO board. My application succeeded in plotting while the data is a total mess. Here is my code below (Some code is from QCustomPlot website):
void Dialog::realtimeDataSlot()
{
bool currentPortNameChanged = false;
QString currentPortName;
if (currentPortName != portName) {
currentPortName = portName;
currentPortNameChanged = true;
}
QString currentRequest = request;
QSerialPort serial;
if (currentPortNameChanged) {
serial.close();
serial.setPortName(currentPortName);
if (!serial.open(QIODevice::ReadOnly)) {
return;
}
}
static QTime time(QTime::currentTime());
// calculate two new data points:
double key = time.elapsed()/1000.0;
static double lastPointKey = 0;
if (key-lastPointKey > 0.002) // at most add point every 2 ms
{
// add data to lines:
if(serial.waitForReadyRead(-1)){
data = serial.readAll();
QTextStream(stdout) << "HERE:" << data.toDouble() << endl;
customPlot->graph(0)->addData(key, data.toDouble());
customPlot->graph(0)->rescaleValueAxis(); //rescale value (vertical) axis to fit the current data:
lastPointKey = key;
customPlot->xAxis->setRange(key, 8, Qt::AlignRight);
customPlot->replot();
static double lastFpsKey;
static int frameCount;
++frameCount;
if (key-lastFpsKey > 2) // average fps over 2 seconds
{
lastFpsKey = key;
frameCount = 0;
}
}
}
// calculate frames per second:
if (currentPortName != portName) {
currentPortName = portName;
currentPortNameChanged = true;
} else {
currentPortNameChanged = false;
}
}
When I Tried to print out the data I read from the serial port, I found the following:
HERE:1
HERE:15
HERE:150
HERE:149
HERE:149
HERE:149
HERE:150
HERE:150
HERE:15
HERE:150
HERE:149
HERE:49
HERE:150
HERE:150
HERE:1
HERE:150
The values around 150 are normal while the value that are 0, 1 to others are not. Also it is not print out at a stable speed. I don't know what happened to this, and thanks to whoever may help, and I would appreciate it if there is any better ways to implement this.

Answer 1

The problem here is that it is not guaranteed that the serial transmission is received all at once. So it is better to let the serial to be processed somewhere else, for instance:
// in the class definition
QSerialPort serialPort;
private slots:
void handleReadyRead();
private:
QByteArray serialBuffer;
volatile double lastSerialValue;
// In the initialization part (not the realtimeDataSlot function)
lastSerialValue = qQNaN();
serialPort.setPortName(currentPortName);
connect(&serialPort, &QSerialPort::readyRead, this, &Dialog::handleReadyRead, Qt::UniqueConnection);
if (!serialPort.open(QIODevice::ReadOnly)) {
return;
}
serialBuffer.clear();
// Other functions:
void Dialog::realtimeDataSlot()
{
...
if (key-lastPointKey > 0.002) // at most add point every 2 ms
{
if (!qIsNaN(lastSerialData))
{
// use lastSerialValue as the data.toDouble() you had before, then, at the end
lastSerialValue = qQNaN();
}
...
}
void Dialog::handleReadyRead()
{
serialBuffer.append(serialPort.readAll());
int serPos;
while ((serPos = serialBuffer.indexOf('\n')) >= 0)
{
bool ok;
double tempValue = QString::fromLatin1(serialBuffer.left(serPos)).toDouble(&ok);
if (ok) lastSerialValue = tempValue;
serialBuffer = serialBuffer.mid(serPos+1);
}
}
Explanation: whenever you receive something from the arduino the bytes are appended to a buffer. Then the byte array is parsed looking for a terminator, and if found the byte array is split and analysed. When the other function needs the data, it simply pulls the most recent one saved in the variable.
NOTE 1: I saw that you used a binary transmission. The problem is that you do not have any way to determine where the data begins and end in this way. For instance, if you receive 0x01 0x02 0x03 0x04 and you know that there are 3 bytes, are they 01..03 or 02..04 or 03, 04 and a missing one or...? The version I implemented requires you to send data in string format with a new-line terminator (simplest version, you just have to write Serial.println(doubleValue); in the arduino code), but if you need the binary version I can give you some hints
NOTE 2: The code I wrote is NOT thread safe. It will work only if the realtimeDataSlot and the handleReadyRead are called in the same thread. Note that if they belong to the same object and are called through signals this is guaranteed.
Now, this should work. But I highly discourage you from doing this. I don't know who needs to call the realtimeDataSlot(), but I think that the most correct version is something like this:
// in the class definition
QSerialPort serialPort;
private slots:
void handleReadyRead();
void receivedData(double val);
private:
QByteArray serialBuffer;
signals:
void newData(double data);
// In the initialization part (not the realtimeDataSlot function)
serialPort.setPortName(currentPortName);
connect(&serialPort, &QSerialPort::readyRead, this, &Dialog::handleReadyRead, Qt::UniqueConnection);
connect(this, &Dialog::newData, this, &Dialog::receivedData, Qt::UniqueConnection);
if (!serialPort.open(QIODevice::ReadOnly)) {
return;
}
serialBuffer.clear();
// Other functions:
void Dialog::receivedData(double val)
{
double key = time.elapsed()/1000.0;
static double lastPointKey = 0;
if (key-lastPointKey > 0.002) // at most add point every 2 ms
{
QTextStream(stdout) << "HERE:" << data.toDouble() << endl;
customPlot->graph(0)->addData(key, data.toDouble());
customPlot->graph(0)->rescaleValueAxis();
...
}
}
void Dialog::handleReadyRead()
{
serialBuffer.append(serialPort.readAll());
int serPos;
while ((serPos = serialBuffer.indexOf('\n')) >= 0)
{
bool ok;
double tempValue = QString::fromLatin1(serialBuffer.left(serPos)).toDouble(&ok);
if (ok) emit newData(tempValue);
serialBuffer = serialBuffer.mid(serPos+1);
}
}
So keep the graph responsive to events (received a new data) instead of to a timer.
One more thing: I removed the port change on purpose. I suggest you to handle it in another way: put a button to start and stop the serial, and when the serial port is started prevent the user from changing the port name. This way the user will explicitely need to shut it down when he needs to change the port. If you want your version, however, don't include it in your code, but make a slot on its own to call whenever you need to change the port name:
void changeSerialPortName(QString newName)
{
if (newName != serialPort.portName()) {
if (serialPort.isOpen())
serialPort.close();
serialPort.setPortName(newName);
if (!serialPort.open(QIODevice::ReadOnly)) {
return;
}
}
}