Tutorial: Interrupt-Driven Event-Counter on the Raspberry Pi

--D. Thiebaut (talk) 19:57, 23 July 2013 (EDT)

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This page is under construction. Its purpose is to illustrate how to implement a user-level interrupt in C on a Raspberry Pi to count events. You may want to start with the very first tutorial of this series, here.

Page under construction!

Getting the Environment Ready

Install the WiringPi Library

• Get the WiringPi library from drogon.net
• Follow the directions on the Web site to download to the Pi. In my case the Pi is connected to my Mac through an ethernet cable, so I downloaded the tgz archive from https://git.drogon.net/?p=wiringPi;a=summary and sftp it over to the pi. I renamed the actual library to wiring.tgz, which is easier to type than its actual name.

This section is only visible to computers located at Smith College

sftp pi@169.254.0.2
sftp> pwd 
Remote working directory: /home/pi
sftp> put wiring.tgz
Uploading wiring.tgz to /home/pi/wiring.tgz
wiring.tgz                                                                            100%  108KB 107.8KB/s   00:00    
sftp> quit

• Connect to the RPI and build the library

 tar -xzvf wiring.tgz 
 cd wiringPi-cbf6d64/
 ./build 
wiringPi Build script
=====================


WiringPi Library
make: Warning: File `Makefile' has modification time 1.4e+07 s in the future
[UnInstall]
[Compile] wiringPi.c
[Compile] wiringSerial.c
...
[Compile] drc.c
[Link (Dynamic)]
[Install Headers]
[Install Dynamic Lib] 
make: warning:  Clock skew detected.  Your build may be incomplete.

WiringPi Devices Library
make: Warning: File `Makefile' has modification time 1.4e+07 s in the future
[UnInstall]
[Compile] ds1302.c
...
[Link (Dynamic)]
[Install Headers]
[Install Dynamic Lib] 
make: warning:  Clock skew detected.  Your build may be incomplete.

GPIO Utility
make: Warning: File `Makefile' has modification time 1.4e+07 s in the future
[Compile] gpio.c
[Compile] extensions.c
[Compile] readall.c
[Link] 
make: warning:  Clock skew detected.  Your build may be incomplete.
make: Warning: File `Makefile' has modification time 1.4e+07 s in the future
[Install]
make: warning:  Clock skew detected.  Your build may be incomplete.

All Done.

NOTE: This is wiringPi v2, and if you need to use the lcd, Piface,
  Gertboard, MaxDetext, etc. routines then you must change your
  compile scripts to add -lwiringPiDev

• Add the new library to the libray path, as explained in the INSTALL file of the wiringPi distribution.

 sudo nano /etc/ld.so.conf

and add the following line to it:

  /usr/local/lib

• Tell the system to configure the libraries:

  sudo ldconfig

Hardware Setup

• Our hardware setup is the same as that presented in Introduction to accessing the Raspberry Pi’s GPIO in C++ (Linux Way / SYSFS) on hertaville.com. We have connected the switch only. The printf statements will provide the feedback about whether activating the button triggers the ISR or not.
• We use PIN 17 of the GPIO, available on the RPI 26-pin connector Pin 11.
• Connecting the momentary switch is simple:
  • 1 lead connected to GND
  • other lead connected to two places:
    1. to one side of a a 10Kω resistor, the other side of the resistor to 3.3V
    • to Pin 17 of the GPIO

Interrupt Service Routine

Picking the Right Constant for GPIO Pin 17

• The wiringPi library labels GPIO Pin 17 as Pin 0 (see drogon.net), as illustrated in the table below taken from their Web site:

• Our switch is connected to Pin 17 of the GPIO, so we'll use 0 to refer to this pin when using the wiringPi library.

ISR Code

The code for the Interrupt Service Routine is given below. Its operation is simple:

1. it defines Pin 0 (GPIO Pin 17) as the pin which will receive the events
2. it defines a function that will be called by the interrupt triggered by Pin 0.
3. it initializes the wiringPi library
4. it attaches

/*
isr4pi.c
D. Thiebaut
based on isr.c from the WiringPi library, authored by Gordon Henderson
https://github.com/WiringPi/WiringPi/blob/master/examples/isr.c

Compile as follows:

    gcc -o isr4pi isr4pi.c -lwiringPi

Run as follows:

    sudo ./isr4pi

 */
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>
#include <wiringPi.h>


// Use GPIO Pin 17, which is Pin 0 for wiringPi library

#define BUTTON_PIN 0


// the event counter 
volatile int eventCounter = 0;


/*
 * myInterrupt:
 */

void myInterrupt (void) {
   eventCounter++;
}


/*
* main
*/

int main (void) {

  if (wiringPiSetup () < 0) {
      fprintf (stderr, "Unable to setup wiringPi: %s\n", strerror (errno));
      return 1;
  }

  if ( wiringPiISR (BUTTON_PIN, INT_EDGE_FALLING, &myInterrupt) < 0 ) {
      fprintf (stderr, "Unable to setup ISR: %s\n", strerror (errno));
      return 1;
  }

  while ( 1 ) {
    // display # of events in past second
    printf( "%d\n", eventCounter );

    // reset counter to 0 
    eventCounter = 0;

    // wait 1 second doing nothing...
    delay( 1000 ); // wait 1 second
  }

  return 0;
}

Compilation

• Compile the code above against the WiringPi library as follows:

 gcc -o isr4pi isr4pi.c -lwiringPi