Raspberry Pi C++ IO

Enable Hardware

On Raspberry Pi OS-Desktop:

• access the start menu
• Preferences->Raspberry Pi Configuration

On the command line:

• sudo raspi-config
• enable/disable various IO in interface options

Check Pinout

Use pinout to identify GPIO pin numbers from terminal.

pigpio

The pigpio library provides a C API to many common Raspberry Pi interfaces.

Install

On Raspberry Pi OS:

sudo apt update
sudo apt install pigpio

Some Examples

See PiGPIO C Interface for full interface.

The library also includes serial, SPI and i2c libraries (not covered here).

Turning on an LED

#include <pigpio.h>

int main(int argc, char *argv[]) {
  if (gpioInitialise() < 0) {
    return 1; // Failed to initialize.
  }

  gpioSetMode(26, PI_OUTPUT);

  gpioWrite(26, PI_HIGH);
  time_sleep(1.0);
  gpioWrite(26, PI_LOW);

  gpioTerminate(); // Compulsary.
  return 0;
}

Reading from a Button

#include <stdio.h>
#include <pigpio.h>

int main(int argc, char *argv[]) {
  if (gpioInitialise() < 0) {
    return 1;
  }

  gpioSetMode(21, PI_INPUT);
  gpioSetPullUpDown(21, PI_PUD_UP);

  if (gpioRead(21) == PI_LOW) {
    fprintf(stdout, "<PRESSED>\n");
  } else {
    fprintf(stdout, "<NONE>\n");
  }

  gpioTerminate(); // Compulsary.
  return 0;
}

Commanding a Servo

#include <pigpio.h>

int main(int argc, char *argv[]) {
  if (gpioInitialise() < 0) {
    return 1; // Failed to initialize.
  }

  const double start = time_time();
  while ((time_time() - start) < 60.0) {
    time_sleep(0.5);
    gpioServo(24, 1500);

    time_sleep(0.5);
    gpioServo(24, 1800);
  }

  gpioTerminate(); // Compulsary.
  return 0;
}

• If gpioServo(24, 0) is called, then it receives no power

Pin Interrupts

#include <stdio.h>
#include <pigpio.h>

void triggered(int gpio, int level, uint32_t tick) {
  if (level == 0) {
    fprintf(stdout, "Pressed");
  } else if (level == 1) {
    fprintf(stdout, "Released");
  } else {
    fprintf(stdout, "Timeout");
  }
  fprintf(stdout, "at %f s\n", (double)tick/1e6);
}

int main(int argc, char *argv[]) {
  if (gpioInitialise() < 0) {
    return 1;
  }

  gpioSetMode(21, PI_INPUT);
  int timeout_ms = 1000; // Set to 0 to disable.
  gpioSetISRFunc(21, EITHER_EDGE, timeout_ms, triggered);

  time_sleep(60.0);

  gpioTerminate(); // Compulsary.
  return 0;
}

• The timeout allows us to trigger events when no edge change is detected.
• The level values are:
  • 0 = Falling Edge
  • 1 = Rising Edge
  • 2 = Watchdog Timeout (optional)

Potentially Useful .bashrc Shortcuts

# Compiles and runs a pigpio c file.
hpigpio_c(){
  mkdir -p bin
  gcc -o ./bin/${1//.c} $1 -lpigpio -lrt -lpthread
  sudo ./bin/${1//.c}
}

# Compiles and runs a pigpio c++ file.
hpigpio_cpp(){
  mkdir -p bin
  g++ -Wall -pthread -o bin/${1//.cpp} $1 -lpigpio -lrt 
  sudo ./bin/${1//.cpp}
}

Dealing with the daemon

You may wish to run the pigpio daemon if:

• You interface with pigpio in python
• You don’t have sudo permissions
  • This is requied when using ROS
• You want remote pin access

In these cases the cpp code above won’t work.

Instead, we need to use the pigpio daemon interface

Running the daemon

The best way is to use systemctl:

systemctl is-active pigpiod # check if active
systemctl start pigpiod     # run in background
systemctl enable pigpiod    # always run

Driving a servo from the daemon

#include <iostream>
#include <csignal>

#include <pigpiod_if2.h>

#define SERVO_FREQ (50) // Hz
#define SERVO_PIN  (24)

static int pi_id = -1;

void at_signal(int i) {
  std::cout << "Shutting down connection to pigpiod\n";
  if (pi_id >= 0) {
    pigpio_stop(pi_id);
  }
}

int main(int argc, char *argv[]) {
  std::signal(SIGINT, at_signal);

  pi_id = pigpio_start("localhost", "8888");
  if (pi_id < 0) {
    std::cout << "Connecting to pigpiod failed\n";
    return 1;
  }

  set_mode(pi_id, SERVO_PIN, PI_OUTPUT);
  set_PWM_frequency(pi_id, SERVO_PIN, SERVO_FREQ);
  set_PWM_range(pi_id, SERVO_PIN, 20000); // 1us/count

  const double start = time_time();
  while ((time_time() - start) < 60.0) {
    time_sleep(0.5);
    set_PWM_dutycycle(pi_id, SERVO_PIN, 1500);

    time_sleep(0.5);
    set_PWM_dutycycle(pi_id, SERVO_PIN, 1800);
  }

  return 0;
}

• we must manage daemon connections now
• servo drivers are not provided, we write our own

Compiling

The following is a bash script I keep in .bashrc for compiling/running single files.

# Compiles and runs a pigpiod_if2 c++ file.
hpigpiod_cpp(){
  mkdir -p bin
  g++ -Wall -pthread -o bin/${1//.cpp} $1 -lpigpiod_if2 -lrt 
  sudo ./bin/${1//.cpp}
}