RPi Tutorial Series: External Button User Guide

Using a external button to do some control is so cool a thing but knowledge of event interruption is often required in such case. This section gives some examples under different libraries.

BCM2835

#include <bcm2835.h>
#include <stdio.h>

#define  KEY  20

int main(int argc, char **argv)
{
    if (!bcm2835_init())return 1;
    // Sets the pin as input.
    bcm2835_gpio_fsel(KEY, BCM2835_GPIO_FSEL_INPT);
    // Sets the Pull-up mode for the pin.
    bcm2835_gpio_set_pud(KEY, BCM2835_GPIO_PUD_UP);
    printf("Key Test Program!!!!\n");  
    while (1)
    {  
        // Reads the current level on the specified pin and returns either HIGH or LOW (0 or 1).
        if(bcm2835_gpio_lev(KEY) == 0)
        {  
            printf ("KEY PRESS\n") ;
            while(bcm2835_gpio_lev(KEY) == 0)
                bcm2835_delay(100);
        }  
        bcm2835_delay(100);
    }  
    bcm2835_close();
    return 0;
}

Save the file as "key.c" and then compile it. Run the program and give a low level to the pin, the terminal will prompt "KEY PRESS". Hit Ctrl + C to terminate the process.

gcc -Wall key.c -o key -lbcm2835
sudo ./key

WiringPi (Polling)

#include <stdio.h>
#include <wiringPi.h>
 
char KEY = 29;
 
int main()
{
    if (wiringPiSetup() < 0)return 1 ;
    // Sets the pin as input.
    pinMode(KEY,INPUT);
    // Sets the Pull-up mode for the pin.
    pullUpDnControl(KEY, PUD_UP);
    printf("Key Test Program!!!\n");
    while(1)
    {  
        if (digitalRead(KEY) == 0) 
        {  
            printf ("KEY PRESS\n") ;
            // Returns the value read at the given pin. It will be HIGH or LOW (0 or 1).
            while(digitalRead(KEY) == 0)
                delay(100);
        }  
        delay(100);
    }  
}

Save the file as "key.c" and then compile it. Run the program and give a low level to the pin, the terminal will prompt "KEY PRESS". Hit Ctrl + C to terminate the process.

gcc -Wall key.c -o key -wiringPi
sudo ./key

WiringPi (Interruption)

#include <stdio.h>
#include <wiringPi.h>
 
#define button 29

char flag = 0;
// The function will be called when the interrupt triggers.
void myInterrupt()
{
    flag ++;
}
 
int main()
{
    // This initialises the wiringPi system 
    // and assumes that the calling program is going to be using the wiringPi pin numbering scheme.
    if(wiringPiSetup() < 0)return 1;
    // This function registers a function to received interrupts on the specified pin.
    if(wiringPiISR(button, INT_EDGE_FALLING, &myInterrupt) < 0)
    {  
        printf("Unable to setup ISR \n");
    }  
    printf("Interrupt test program\n");
    while(1)
    {  
        if(flag)
        {  
            while(digitalRead(button) ==0);
            printf("button press\n");
            flag = 0;
        }  
    }  
}

Save the file as "Interrupt.c" and then compile and execute it. Hit Ctrl + C to terminate the process.

gcc -Wall Interrupt.c -o key -wiringPi
sudo ./Interrupt

Python (Polling)

#!/usr/bin/python
# -*- coding:utf-8 -*-
import RPi.GPIO as GPIO
import time

KEY = 26
# pin numbers are interpreted as BCM pin numbers.
GPIO.setmode(GPIO.BCM)
# Sets the pin as input and sets Pull-up mode for the pin.
GPIO.setup(KEY,GPIO.IN,GPIO.PUD_UP)
while True:
    time.sleep(0.05)
    # Returns the value read at the given pin. It will be HIGH or LOW (0 or 1).
    if GPIO.input(KEY) == 0:
        print("KEY PRESS")
        while GPIO.input(KEY) == 0:
            time.sleep(0.01)

Run the program and then give a low level to the pin, the terminal will prompt "KEY PRESS", Hit Ctrl + C to terminate the process.

sudo python key.py

Python (Interruption)

#!/usr/bin/python
# -*- coding:utf-8 -*-
import RPi.GPIO as GPIO
import time
 
KEY = 26
# The function will be called when the interrupt triggers.
def MyInterrupt(KEY):
    print("KEY PRESS")
# pin numbers are interpreted as BCM pin numbers.
GPIO.setmode(GPIO.BCM)
GPIO.setup(KEY, GPIO.IN, GPIO.PUD_UP)
# add falling edge detection on a channel (i.e. KEY)
# ignoring further edges for 200ms for switch bounce handling.
GPIO.add_event_detect(KEY, GPIO.FALLING, MyInterrupt, 200)
 
while True:
    time.sleep(1)
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