QSerialPort transmit errors while sending 0xff

tobias423412 · 27 Oct 2019, 05:55

Hello,

i try to implement a communication between an embedded device and my computer. During development, i observed that i get a lot errors during the communication, when the byte array contains magic bytes like 0xff.
It got more strange when i tried to send only characters like 0xff. On my serial adapter i can see, that the TX LED stars stuttering. If i send other bytes like 0x55 or something like that, the LED is smoothly flashing and i have no errors during transmit.
It is really easy to reproduce and to prove that my target has no influence. I disconnected my target and i can still see that the LED stutters during 0xff and not during other characters on my serial adapter.
To make it more confusing: The majority of bytes is transmitted correctly. Just a small percentage of 0xff's is not transmitted correctly.
I am using Mac OS Mojave and tried it with Qt 5.6 and 5.12. Both show exactly the same behavior.

Has someone an idea from where this is coming and how i can fix this?

Greetings
Tobias

aha_1980 · wrote on 27 Oct 2019, 05:55

Hi @tobias423412

I'm using QtSerialPort to transmit binary data to one of my self-built embedded devices using FTDI serial adapters - without problems.

So there are three possible things that could be wrong on your side:

Your hardware - which is that?
Your code - can you show it?
Are you using a software protocol like XON/XOFF?

Regards

tobias423412 · 27 Oct 2019, 08:39

Hi,

this is not the most clean code, but it does the job:

#include "serialportwriter.h"

#include <QCoreApplication>
#include <QDebug>
#include <QThread>
QT_USE_NAMESPACE
#define TEST_BYTE ((uint8_t)0xff)
#define END_ADDR 0xf

SerialPortWriter::SerialPortWriter(QSerialPort *serialPort, QObject *parent)
    : QObject(parent)
    , m_serialPort(serialPort)
    , m_standardOutput(stdout)
    , m_bytesWritten(0)
    , m_testByte(TEST_BYTE)
{
    m_timer.setSingleShot(false);
    connect(m_serialPort, &QSerialPort::bytesWritten, this, &SerialPortWriter::handleBytesWritten);
    connect(m_serialPort, static_cast<void (QSerialPort::*)(QSerialPort::SerialPortError)>(&QSerialPort::error),
            this, &SerialPortWriter::handleError);
    connect(&m_timer, &QTimer::timeout, this, &SerialPortWriter::handleTimeout);
    connect(m_serialPort, &QSerialPort::readyRead, this, &SerialPortWriter::readReady);
    m_timer.start(35);
}

SerialPortWriter::~SerialPortWriter()
{
}

void SerialPortWriter::handleBytesWritten(qint64 bytes)
{

}

void SerialPortWriter::handleTimeout()
{
    static bool read = false;
    static uint32_t readCycle = 0;
    static uint32_t writeCycle = 0;
    static uint16_t addr = 0;
    if (read)
    {
        QByteArray readData;
        readData.push_back((char)0x1); // Read Command
        readData.push_back((char)(addr >> 8)); // 23:16
        readData.push_back((char)(addr & 0xff)); // 15:8
        readData.push_back((char)0x0); // 7:0
        readData.push_back((char)0x40); // Size in DWord


        if (addr < END_ADDR)
        {
            write(readData);
            addr++;
        }
        else
        {
            addr = 0;
            read = false;
            qDebug() << "Read cylce" << readCycle++;
        }
    }
    else
    {
        QByteArray writeData;
        writeData.push_back((char)0x2); // Write Command
        writeData.push_back((char)(addr >> 8)); // 23:16
        writeData.push_back((char)(addr & 0xff)); // 15:8
        writeData.push_back((char)0x0); // 7:0
        writeData.push_back((char)0x40); // Size in DWord

        // Payload
        for (int i = 0; i < 0x40; i++)
        {
            writeData.push_back((char)m_testByte);
            writeData.push_back((char)m_testByte);
            writeData.push_back((char)m_testByte);
            writeData.push_back((char)m_testByte);
        }

        if (addr < END_ADDR)
        {
            write(writeData);
            addr++;
        }
        else
        {
            qDebug() << "Write cylce" << writeCycle++;
            addr = 0;
            read = true;
        }
    }
}

void SerialPortWriter::handleError(QSerialPort::SerialPortError serialPortError)
{
    if (serialPortError == QSerialPort::WriteError) {
        m_standardOutput << QObject::tr("An I/O error occurred while writing the data to port %1, error: %2").arg(m_serialPort->portName()).arg(m_serialPort->errorString()) << endl;
        QCoreApplication::exit(1);
    }
}

void SerialPortWriter::readReady()
{
    QByteArray r = m_serialPort->readAll();
    for (int i = 0; i < r.size(); i++)
    {
        if ((uint8_t)(r.at(i)) != m_testByte)
        {
            qDebug() << "Read Error actual:" << QString::number((uint8_t)(r.at(i))).toInt() << "should:" << QString::number(m_testByte).toInt();
        }
    }
}

void SerialPortWriter::write(const QByteArray &writeData)
{
    m_writeData = writeData;

    qint64 bytesWritten = m_serialPort->write(writeData);

    if (bytesWritten == -1) {
        m_standardOutput << QObject::tr("Failed to write the data to port %1, error: %2").arg(m_serialPort->portName()).arg(m_serialPort->errorString()) << endl;
        QCoreApplication::exit(1);
    } else if (bytesWritten != m_writeData.size()) {
        m_standardOutput << QObject::tr("Failed to write all the data to port %1, error: %2").arg(m_serialPort->portName()).arg(m_serialPort->errorString()) << endl;
        QCoreApplication::exit(1);
    }
    //m_serialPort->flush();
}

What i am doing here? I write 0xff in some memory and when i am done with this, i try to read it back. And that is what i am doing forever.
If i am using for TEST_BYTE 0xAA or something like that, everything works fine. If it is 0xff than something goes wrong. If i read the memory back, i get errors on random places.
The error outputs like this

Read Error actual: 251 should: 255
Read Error actual: 254 should: 255
Read Error actual: 251 should: 255
Read Error actual: 254 should: 255
Read Error actual: 251 should: 255

As a serial converter i am using an ordinary FT232.

Greetings
Tobias

aha_1980 · wrote on 27 Oct 2019, 08:39

@tobias423412

Your code still looks a bit complicated and is missing important pieces, like the serial port set up. Which baud rate are you using? Am I right that you repeat that cycle every 35 ms?

On my serial adapter i can see, that the TX LED stars stuttering.

Ok, then I'd start with a simple program that only transmits a fixed frame, containing 0xff. Does that work? If yes, then do it from a timer with a timeout large enough that the previous frame is completely transmitted (to avoid buffer overflows).

I really can't see why transmitting arbitrary data should lead to problems like you described.

Regards

tobias423412 · wrote on 27 Oct 2019, 09:05

Hi,

yes, the init

#include "serialportwriter.h"

#include <QtSerialPort/QSerialPort>
#include <QCoreApplication>

#include <QString>
#include <QDebug>

QT_USE_NAMESPACE

int main(int argc, char *argv[])
{

    QCoreApplication coreApplication(argc, argv);
    int argumentCount = QCoreApplication::arguments().size();
    QStringList argumentList = QCoreApplication::arguments();

    QTextStream standardOutput(stdout);

    if (argumentCount != 3) {
        standardOutput << "Version 1.0.0 (Compile date: " << __DATE__ << " " << __TIME__ << ")" << endl;
        standardOutput << QObject::tr("Usage: %1 <serialportname> <baudrate>").arg(argumentList.first()) << endl;
        return 1;
    }

    QSerialPort serialPort;
    QString serialPortName = argumentList.at(1);
    serialPort.setPortName(serialPortName);

    int serialPortBaudRate = argumentList.at(2).toInt();
    serialPort.setBaudRate(serialPortBaudRate);

    serialPort.open(QIODevice::ReadWrite);
    serialPort.setParity(QSerialPort::NoParity);
    serialPort.setStopBits(QSerialPort::OneStop);
    serialPort.setFlowControl(QSerialPort::NoFlowControl);
    serialPort.setDataBits(QSerialPort::Data8);

    SerialPortWriter serialPortWriter(&serialPort);
    return coreApplication.exec();
}

I am using 115200 baud.

I use that timer to periodically send one frame of data. After every 15 timer shots i switch from sending payload to reading payload. Theoretically i can send around 500 bytes within this 35ms and currently sending around 261.

I tried also now to just send a byte array containing only 0xff. I also increased the time of the timer. The TX LED on the FT232 is still stuttering.

Greetings
Tobias

tobias423412 · wrote on 27 Oct 2019, 09:23

I guess i found a solution which i don't like.

I observed also two crashes of my USB stack on my Mac which are (obviously) related to this stuff. Sometimes when i start this program all my USB ports are dying and the kernel tasks runs with 100% load. I can only resurrect the USB ports by a reboot of my computer. If i connect the serial adapter on a nativ USB port, it works as intended without any errors.

The USB Hub should have enough power. It is a Hub which is powered from an external power plug.

For sure, i would be really thankful if anyone know a better workaround to this bug instead of utilizing one of the very rare native USB ports on a Mac, but in this case, i guess, it is an Apple problem and not an Qt related one.

Thank you very much for your help

Greetings
Tobias