Blink a LED in C++

Goal

In this tutorial we will explain how to make a program in C++ that will switch on and off one of the robot's LED. The program will be called ledc++:

ledc++ [parameter]

parameter will be either on, off, flash or status.

Material

In order to follow the tutorial you will need:

• The robot Erle
• An 8 GB microSD card (others can be used) with the Ubuntu Linux Distribution that we provide.
• a (male) USB-to-miniUSB
• A computer with a serial terminal installed (you can use PuTTy in Windows or minicom in Unix-based OSs)

Tutorial

Create a blank file called ledc++.cpp:

touch ledc++.cpp

Edit the file (we personally like vim but feel free to learn any editor you like) and add the following content:

/** Simple On-board LED flashing program - written by Derek Molloy
    for the ee402 module

    This program uses USR LED 0 and can be executed in three ways:
         makeLED on
         makeLED off
         makeLED flash  (flash at 100ms intervals - on 50ms/off 50ms)
         makeLED status (get the trigger status)
*/

#include<iostream>
#include<fstream>
#include<string>
using namespace std;

#define LED0_PATH "/sys/class/leds/beaglebone:green:usr0"

void removeTrigger(){
   // remove the trigger from the LED
   std::fstream fs;
   fs.open( LED0_PATH "/trigger", std::fstream::out);
   fs << "none";
   fs.close();
}

int main(int argc, char* argv[]){
   if(argc!=2){
    cout << "Usage is makeLED and one of: on, off, flash or status"
<< endl;
    cout << "e.g. makeLED flash" << endl;
   }

   string cmd(argv[1]);
   std::fstream fs;
   cout << "Starting the LED flash program" << endl;
   cout << "The LED Path is: " << LED0_PATH << endl;

   // select whether it is on, off or flash
   if(cmd=="on"){
    removeTrigger();
    fs.open (LED0_PATH "/brightness", std::fstream::out);
    fs << "1";
    fs.close();
   }
   else if (cmd=="off"){
    removeTrigger();
    fs.open (LED0_PATH "/brightness", std::fstream::out);
    fs << "0";
    fs.close();
   }
   else if (cmd=="flash"){
    fs.open (LED0_PATH "/trigger", std::fstream::out);
    fs << "timer";
    fs.close();
    fs.open (LED0_PATH "/delay_on", std::fstream::out);
    fs << "50";
    fs.close();
    fs.open (LED0_PATH "/delay_off", std::fstream::out);
    fs << "50";
    fs.close();
   }
   else if (cmd=="status"){
    // display the current trigger details
    fs.open( LED0_PATH "/trigger", std::fstream::in);
    string line;
    while(getline(fs,line)) cout << line;
    fs.close();
   }
   else{
    cout << "Invalid command" << endl;
   }
   cout << "Finished the LED flash program" << endl;
   return 0;
}

Now compile the code:

g++ ledc++.cpp -o ledc++

Finally, try calling the program with the different arguments [on, off, flash or status]:

./ledc++ on