struct termios setting for serial communication with arduino

Question

on a unix based software, which must send a number between 0 and 179 to arduino and arduino will apply that number as an angle to a servo motor, but i do not know what parameters i have to change in the terminos struct to permit the serial communication.

this is the c++ code:

#include <iostream>
#include <unistd.h>
#include <fstream>
#include <termios.h>

using namespace std;

int main()
{
    unsigned int angle;
    ofstream arduino;
    struct termios ttable;

    //cout<<"test-1";

    arduino.open("/dev/tty.usbmodem3a21");

    //cout<<"test-2";

    if(!arduino)
    {
        cout<<"\n\nERR: could not open port\n\n";
    }
    else
    {

        if(tcgetattr(arduino,&ttable)<0)
        {
            cout<<"\n\nERR: could not get terminal options\n\n";
        }
        else
        {

            //there goes the terminal options setting for the output;

            ttable.c_cflag = -hupcl //to prevent the reset of arduino

            cfsetospeed(&ttable,9600);

            if(tcsetattr(arduino,TCSANOW,&ttable)<0)
            {
                cout<<"\n\nERR: could not set new terminal options\n\n";
            }
            else
            {
                do
                {
                    cout<<"\n\ninsert a number between 0 and 179";
                    cin>>angle;
                    arduino<<angle;
                }while(angle<=179);

                arduino.close();
            }
        }
    }

}

and this is arduino’s :

#include <Servo.h>

Servo servo;
const int pinServo = 2;
unsigned int angle;

void setup()
{
    Serial.begin(9600);
    servo.attach(pinServo);

    servo.write(0);

}

void loop()
{
    if(Serial.available()>0)
    {  
       angle = Serial.read();

       if(angle <= 179)
       {
         servo.write(angle);
       }
    }
}

So would you kindly tell me what do i have to change of "ttable" ?

Answer 1

In general, talking to an Arduino from C/C++ is easiest with the serial port in 'raw' mode. This is basically 8N1, byte-at-a-time, with the TTY doing the minimal amount of futzing about with the data. An easy way to set the various flags in a termios struct for this mode is to use cfmakeraw(3). Per the man pages this does the following:

struct termios config;

config.c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP |
  INLCR | IGNCR | ICRNL | IXON);
config.c_oflag &= ~OPOST;
config.c_lflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
config.c_cflag &= ~(CSIZE | PARENB);
config.c_cflag |= CS8;

For good measure, explicitly set receive enable and ignore modem control with config.c_cflag |= (CLOCAL | CREAD);. If the termios structure you are using is a copy of the existing one (obtained with tcgetattr(), for example), then also disable flow control altogether with config.c_iflag &= ~(IXOFF | IXANY);. So all-in-all, it will look like:

struct termios config;

cfmakeraw(&config);
config.c_cflag |= (CLOCAL | CREAD);
config.c_iflag &= ~(IXOFF | IXANY);

// set vtime, vmin, baud rate...
config.c_cc[VMIN] = 0;  // you likely don't want to change this
config.c_cc[VTIME] = 0; // or this

cfsetispeed(&config, B9600);
cfsetospeed(&config, B9600);

// write port configuration to driver
tcsetattr(fd, TCSANOW, &config;

Using C++ file streams is a bit tricky as well. You very likely want non-blocking read/write and no controlling TTY, so it is usually easier to use open(2) with the O_NOCTTY and O_NDELAY flags set.