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