Sending 0xd byte, but receving 0xa

[jfk214]

I am having a strange problem that I haven't been able to find the cause of.

On the teensy, I am writing the byte "0xd" to USB serial. My teensy code is as follows:

void setup() {

}

void loop() {
  Serial.write(0xd);

}

On my computer running Ubuntu 16.04, I am reading the USB port using the following code and printing the output. I am expecting "d" to get printed, but instead "a" gets printed. I have tried sending other bytes from the teensy and they get printed correctly. It is only happening with 0xd. Does anyone know what could be causing this?


The following was compiled with "g++ d_test.cpp -o d_test" and run with "./d_test"

#include <stdint.h>
#include <iostream>
#include <termios.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>

int main(int argc, char** argv){
  int fd = open("/dev/ttyACM0", O_RDWR | O_NOCTTY | O_NDELAY);
  
  // If the device has been opened successfully, configure it
  if(fd != -1) {
    // set non blocking
    fcntl(fd, F_SETFL, FNDELAY);
    
    struct termios options;
    tcgetattr(fd, &options);
    
    // set baud rate
    cfsetispeed(&options, B9600);
    cfsetospeed(&options, B9600);
    
    // ignore modem lines and enable receiver
    options.c_cflag |= (CLOCAL | CREAD);
    
    // set size of character to 8
    options.c_cflag |= CS8;
    
    // disable hardware flow control
    options.c_cflag &= ~CRTSCTS;
    
    // disable canonaical mode, echo, and signal generation
    options.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
    
    
    // apply the changes
    tcsetattr(fd, TCSANOW, &options);
  }
  else{
    std::cout << "Failed to open teensy" << std::endl;
    return 1;
  }  

  while(true){
    // try reading a byte from the teensy
    uint8_t byte = 0;
    int ret = read(fd, &byte, sizeof(uint8_t));

    // if a byte was read, print it
    if(ret == 1){
      std::cout << std::hex << (unsigned int)byte << std::endl;
    }

  }

  return 0;
}

 

[PaulStoffregen]

Sounds like a unix line discipline is "helpfully" changing stuff.

Use cfmakeraw(&options) to configure for raw mode, so you actually receive the same bytes Teensy transmits. For details, type "man termios".

 

[jfk214]

That worked. Thank you!