如何最大程度地减少来自Arduino的串行通信读取错误

Question

我试图建立计算机和arduino之间的串行通信通道。 当我看着ArduinoIDE时，我从arduino收到了一条完美的消息-3个相同的数字。 现在，我试图创建一个c ++应用程序以在运行Ubuntu的计算机上读取该数据，但是在字符串上我得到了很多垃圾。 我一直在阅读和搜索，没有成功。 谁能帮助我找到问题的根源？

码：

SerialComm.h：

#ifndef SERIALCOMM_HPP
#define SERIALCOMM_HPP

#include <fstream>
#include <string>

#include <stdio.h> // standard input / output functions
#include <string.h> // string function definitions
#include <unistd.h> // UNIX standard function definitions
#include <fcntl.h> // File control definitions
#include <errno.h> // Error number definitions
#include <termios.h> // POSIX terminal control definitionss

class SerialComm {
public:

    SerialComm() noexcept {
    }

    virtual ~SerialComm() noexcept {
        tcsetattr(fd, TCSANOW, &port_settings);
        close(fd);
    }

    void begin(std::string port, speed_t baudrate);
    std::string read_data();

private:
    int fd;
    speed_t _baudrate;
    std::string _port;
    static constexpr int BUFFER_SIZE = 256;
    char buffer[BUFFER_SIZE];
    termios port_settings;
};

SerialComm.cpp

#include "SerialComm.hpp"

#include <iostream>

using namespace std;

void SerialComm::begin(string porta, speed_t baudrate) {
    _port = porta;
    _baudrate = baudrate;

    // abre a porta
    fd = open(_port.c_str(), O_RDWR | O_NOCTTY | O_NDELAY);

    if (fd == -1) {
        printf((string("Unable to open port ") + _port).c_str());
    } else {
        fcntl(fd, F_SETFL, 0);
        printf("port is open.\n");
    }

    cfsetispeed(&port_settings, _baudrate); // set baud rates
    cfsetospeed(&port_settings, _baudrate);

    port_settings.c_cflag &= ~PARENB; // set no parity, stop bits, data bits
    port_settings.c_cflag &= ~CSTOPB;
    port_settings.c_cflag &= ~CSIZE;
    port_settings.c_cflag |= CS8;

    tcsetattr(fd, TCSANOW, &port_settings); // apply the settings to the port   
}

string SerialComm::read_data() {
    int state = 1;

    while (true) {
        state = read(fd, buffer, BUFFER_SIZE);
        if (state > 0) 
        {
            return string( buffer );
        }
    }
}

main.ccp

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

    SerialComm serial;

    serial.begin("/dev/ttyACM0", B115200);

    for(auto i = 0; i < 100; ++i)
    {
        cout << serial.read_data() << endl;
    }
}

serial.ino：

double sinal = 0;

void setup()
{
  Serial.begin( 115200 );
}

void loop()
{
  sinal = analogRead( A0 ) * ( 5.0 / 1024.0 );
  Serial.print( "$" );
  Serial.print( sinal, 5 );
  Serial.print( "," );
  Serial.print( sinal, 5 );
  Serial.print( "," );
  Serial.print( sinal, 5 );
  Serial.print( "#\n" );
}

Arduino IDE输出：

$2.24121,2.24121,2.24121#
$2.24609,2.24609,2.24609#
$2.24121,2.24121,2.24121#
$2.24121,2.24121,2.24121#
$2.24609,2.24609,2.24609#

电脑输出：

$2.24609,2.24?�̯m#
$2.
09375#
$2.2412109375,2.2412109937500#
$2.2460937500,2.2460937500,2.2460937500#
375#
$2.2460937500,2.2460937500,2.2460937500#
$2.
375,2.2412109375#
$2.241210937937500#
$2.2460937500,2.2460937500,2.2460937500#

PS：以上是我能得到的最漂亮的输出。

Answer 1

除了未终止的字符串缓冲区存在的问题外，您也不能说您将在一次read调用中收到完整的消息。 相反，您必须循环阅读，直到消息末尾（发送的换行符）为止。

当然，这会给您带来另一个问题，即您可以在同一read调用中收到一条消息的末尾和下一条消息的末尾。 这意味着您必须保存下一条消息的开头，并在下一次调用read之前将其放入缓冲区。

Answer 2

我认为这是您的错误：

string SerialComm::read_data() {
    int state = 1;
    int receivedbyte = 0;  // never used!

    while (true) {
        state = read(fd, buffer, BUFFER_SIZE);
        if (state > 0) 
        {
            return string( buffer );
        }
    }
    buffer[receivedbyte + 1] = '\0';  // never reached!  And "off-by-one" if it were...
}

这可能会更好：

string SerialComm::read_data() {
    int receivedbyte = 0;

    while (true) {
        receivedbyte = read(fd, buffer, BUFFER_SIZE - 1);
        if (receivedbyte > 0) 
        {
            buffer[receivedbyte] = '\0';
            return string( buffer );
        }
    }
}

那应该消除由于未终止的字符串而导致的所有垃圾。 也就是说，要获得漂亮的以换行符结尾的字符串，您可能需要一个外部循环来查找这些边界并以这种方式正确地划分流。

一种实现方法是：声明一个在类中string received的string received ，以保存所有尚未返回给调用者的缓冲输入。 然后，如下重写read_data（）：

string SerialComm::read_data()
{
    while (true)
    {
        size_t pos = received.find_first_of('\n', 0);
        if (pos != string::npos)
        {
            string result = received.substr(0, pos);
            received.erase(0, pos);
            return result;
        }

        int receivedbytes;
        do
        {
            receivedbytes = read(fd, buffer, BUFFER_SIZE - 1);
        } while (receivedbytes == 0);

        if (receivedbytes < 0)
            abort();  // error... you might want to handle it more cleanly, though

        buffer[receivedbytes] = 0;
        received += string( buffer );
    }
}            

如果您想要的版本在没有完整的行可供查看时返回空字符串，而不是永远等待数据，则可以使用此版本的代码。 注意：如果缓冲的数据没有终止换行符，它将一直保留直到它看到终止换行符为止。 您可能想要添加单独的flush方法以使该数据可见。

string SerialComm::read_data()
{
    while (true)
    {
        size_t pos = received.find_first_of('\n', 0);
        if (pos != string::npos)
        {
            string result = received.substr(0, pos);
            received.erase(0, pos);
            return result;
        }

        int receivedbytes = read(fd, buffer, BUFFER_SIZE - 1);

        if (receivedbytes < 0)
            abort();  // error... you might want to handle it more cleanly, though

        if (receivedbytes == 0)
            return string();  // nothing to see yet

        // Add received data to buffer and loop to see if we have a newline yet.           
        buffer[receivedbytes] = 0;
        received += string( buffer );
    }
}