How to use interrupts with Python on the Raspberry Pi and RPi.GPIO


The latest big news in the world of Raspberry Pi Python GPIO programming is that Ben Croston has released an update for RPi.GPIO. Why is that a big deal? Because this version has interrupts. “What’s an interrupt?” I hear you say. It’s a way of waiting for something to happen without checking constantly whether or not it’s happening.

Imagine that you’re waiting for a delivery – something you’re really excited about – like a Pi camera.You spend far too much time looking down the street in eager anticipation of the postman’s arrival. You can’t fully focus on what you’re supposed to be doing because you know it’s imminent. Every time you hear something in the street, you jump up and look out of the window. Woe betide any door-knocking salesman who calls when you’re expecting a delivery.

What I’ve just described in human terms is a bit like polling. Polling is continually checking for something. For example, if you want to make sure your program reacts as quickly as possible to a button press, you can check the button status about ten thousand times per second. This is great if you need a quick reaction, but it uses quite a bit of the computer’s processing power. In the same way that you can’t fully focus when you’re expecting a delivery, a large part of your CPU is used up with this polling.

How to use interrupts with Python on the Raspberry Pi and RPi.GPIOThere has to be a better way, right?

Yes. And there is. It’s interrupts. This is the first in a series of articles which aim to show you how to use this new interrupt facility in Python.

Interrupts are a much more efficient way of handling the “wait for something to happen and react immediately when it does” situation. They free up the resources you would have wasted on polling, so that you can use them for something else. Then, when the event happens, the rest of your program is “interrupted” and your chosen outcome occurs.

So, to carry on our human example…
An interrupt is like having an automatic postman detector that will tell you for sure when the postman arrives, so you can get on with something else. You now know you will not miss that knock on the door and end up with one of those “we tried to deliver your item but you were out and the collection office is closed for the next two days, so enjoy the wait” cards.

So interrupts are good, as you can set them up to wait for events to occur without wasting system resources.

So how do you code them?

I’m going to show a simple “wait for a button press” example in this blog article and follow up with other examples in subsequent articles. But before you try this, you will quite likely need to update your RPi.GPIO package. You can check what version of RPi.GPIO you have in the command line with…

sudo python
import RPi.GPIO as GPIO
GPIO.VERSION

This should show you what RPi.GPIO version you have. You need 0.5.1 or higher for this example.
You can exit the python environment with CTRL+Z

Install RPi.GPIO version 0.5.1 for simple interrupts

If you need to, you can install 0.5.1 or later with
sudo apt-get update
sudo apt-get dist-upgrade
(This will update all your Raspbian packages and may take up to an hour)

or, from the command line prompt (this will only update RPi.GPIO)…
wget http://raspberry-gpio-python.googlecode.com/files/python-rpi.gpio_0.5.1a-1_armhf.deb
wget http://raspberry-gpio-python.googlecode.com/files/python3-rpi.gpio_0.5.1a-1_armhf.deb
sudo dpkg -i python-rpi.gpio_0.5.1a-1_armhf.deb
sudo dpkg -i python3-rpi.gpio_0.5.1a-1_armhf.deb

For more detail: How to use interrupts with Python on the Raspberry Pi and RPi.GPIO


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