Why is serial port skipping data when sending data?

Question

I have written some C++ code to talk to my arduino via serial. It just tries to make oscillations on two servo motors using sine and cosine, but it is skipping data. I'm not sure why this is happening. I am using the termios.h for the serial stuff. The output from C++ is something like "V180H90" i.e. Vertical 180, Horizontal 90. I was using fstream and usleep() to send data before and it was working, but I'd like to use a better method than delaying by some arbitrary number.

Thanks for any help or guidance.

My arduino code

#include <Servo.h>
typedef enum { NONE, GOT_V, GOT_H } states;
states state = NONE;
Servo pan;
Servo tilt;
int laser = 11;
unsigned int currentValue;

int v_pan = 0;
int v_tilt = 0;

void setup()
{
  pan.attach(10);
  tilt.attach(9);

  Serial.begin(9600);
  state = NONE;
}

void processVertical(const unsigned int value)
{
  Serial.print("Vertical = ");
  Serial.println(value);
  int result = 1300 + (value - 90) * 2;
  //Serial.println(result);
  tilt.writeMicroseconds(result);
}

void processHorizontal(const unsigned int value)
{
  Serial.print("Horizontal = ");
  Serial.println(value);
  int result = 1500 + (value - 180) * 1;
  //Serial.println(result);
  pan.writeMicroseconds(result);
}

void handlePreviousState()
{
  switch(state)
  {
    case GOT_V:
      processVertical(currentValue);
      break;
    case GOT_H:
      processHorizontal(currentValue);
      break;
  }
  currentValue = 0;
}

void processIncomingByte (const byte c)
{
  if (isdigit(c))
  {
    currentValue *=10;
    currentValue += c - '0';
  }
  else
  {
    handlePreviousState();

    switch (c)
    {
      case 'V':
        state = GOT_V;
        break;
      case 'H':
        state = GOT_H;
        break;
      default:
        state = NONE;
        break;
    }
  }
}

void loop()
{
  if(Serial.available() > 0) 
  { 
    processIncomingByte(Serial.read());
  } 
  digitalWrite(laser, HIGH);
}

//check out writeMicroseconds

My C++ code

// Program for sending data to serial

#include <iostream>
#include <sstream>
#include <string>
#include <termios.h>
#include <fcntl.h>
#include <math.h>

using namespace std;

//open serial port
int openPort(string path)
{
  int fd; //file descriptor for port
  fd = open(path.c_str(), O_RDWR | O_NOCTTY | O_NDELAY);
  if (fd == -1)
    cerr << "Cannot open port" << endl;
  else
    fcntl(fd, F_SETFL, 0);
  return (fd);
}

//set options for an open serial port
void setOptions(int fd)
{
  struct termios options;
  tcgetattr(fd, &options);
  cfsetispeed(&options, B9600);
  cfsetospeed(&options, B9600);

  //No parity 8N1
  options.c_cflag &= ~PARENB;
  options.c_cflag &= ~CSTOPB;
  options.c_cflag &= ~CSIZE;
  options.c_cflag |= CS8;

  //No flow control
  options.c_cflag &= ~CRTSCTS;

  //Turn off s/w flow control
  options.c_iflag &= ~(IXON | IXOFF | IXANY);

  //Turn on read and ignore ctrl lines
  options.c_cflag |= (CLOCAL | CREAD); 

  if( tcsetattr(fd, TCSANOW, &options) < 0) { 
    cerr << "Could not set attributes" << endl; 
  }
}

//write to serial port
void writePort(int fd, string data)
{
  int n = write(fd, data.c_str(), 9);
  if (n < 0)
    cerr << "Cannot write to port" << endl;
}

int main() {
  string path = "/dev/tty.usbmodemfd131";
  //string path = "/dev/tty.usbmodemfa141";
  int fd = openPort(path);
  setOptions(fd);

  stringstream ss;
  string output;
  unsigned short vertical = 0;
  unsigned short horizontal = 0; 
  unsigned short freq = 10;

  for(int i = 0; i < 360; i++) {
    vertical = ((cos(i * freq * ((M_PI)/180))) + 1) * 90;
    horizontal = ((sin(i * freq * ((M_PI)/180))) + 1) * 90;
    ss << "V" << vertical << "H" << horizontal << endl; 
    output = ss.str();
    ss.str("");
    writePort(fd, output);
//    cout << output; //DEBUG
  }

  close(fd);
  return 0;
}

Answer 1

The "processIncomingByte" loop inside the device may have suffered a speed problem as you are processing the previous state (handlePreviousState) immediately after you receive a new mode.

The problem may be caused by doing a Serial.print in the corresponding function while the value-data bytes are still incoming continuously from the PC. Serial print is a relatively slow process in micro-controller logic.

I am not familiar with Arduino hardware, but some lower end micro-controller board is performing software serial interface using bitbanging method, so when you transmit, the receiving is completely stopped. To verify this you can remark the Serial.print to see whether it helps.

Anyway, doing lengthy processing in the middle of incoming data stream is alway problematic, unless you have a hardware serial interface in the device with lots of FIFO buffers.

A proper way to this problem is to receive the whole message inside a buffer first and then process it only when a end-of-message marker is received. For example, insert your message inside the [] pair like [V180H90]. Reset the buffer upon the "[" and process the buffer after you receive the "]". When you are collecting bytes into the buffer, make sure you also check for buffer overflow.

Answer 2

If you just shove data down the port's throat, it'll do its best not to set on fire, but the excess data isn't going to be sent. After all, the port operates at a finite speed and is a pretty limited and dump device.

So, before sending a character to the port you need to check the status of the port to see if it's actually ready to accept another character of data for transmission. Some serial ports can even generate interrupts when they can take more data to help you avoid wasteful status polling.

Also, sometimes two serial ports on the two devices can be connected with an extra pair of non-data signals (RTS and CTS) to indicate whether the receiving side is ready to receive more data. If you have those connected and your device is using them to indicate its readiness, your program should take the state of the device's CTS into account as well.

Answer 3

Clearly your device reads/process data slower than you send it via serial port. I see few possible solutions here:

1) Implement flow control and send data via serial port in blocking mode. You still have to wait after sending, but only as much as it is needed for your device to read and process data.

2) Implement two way communication so your device sends confirmation message (i.e. any single ASCII symbol) to indicate that it is ready to accept data.

3) Divide your code into two parallel parts i.e. : main loop (or an ISR) only reads data from serial port and stores it in a ring buffer, another loop polls the ring buffer and takes/process data from it as soon as there is some data available. This is the most difficult solution of the three as you need two separate threads (or a thread and an ISR) and protect ring buffer from concurrent access, but also the most powerful and flexible.

Answer 4

You are writing data out too quickly to the serial device and the device itself is spitting out data faster than you can read it back in on the other side of the device.

The correct way to cope with this is to throttle the speed of writes to the serial device to avoid flooding it with data.