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Raspberry Pi Gpio Pins Descriptive Essay

 Pin numbering of the BCM2835 GPIO port(s) on the Raspberry Pi has been a source of great confusion since the designs for the Pi were first published. In the early days (even before hardware was available) the default usable GPIO pins were simply referred to by number as GPIO0 through GPIO7. Additionally there were pins for other purposes, SPI, I2C and serial. This was highlighted on the original image on the eLinux Raspberry Pi Wiki site too.

So when initially writing wiringPi, I chose to have the same default pin numbering scheme and numbered them from 0 upwards. This is no different to how the Arduino operates – “Pin 13” on the Arduino is Port B, bit 5 for example. The underlying hardware definitions are hidden by a simplified numbering scheme. On the Pi, using wiringPi, pin 0 is BCM_GPIO pin 17 for example)

However this has subsequently been viewed as “wrong” and several people have expressed concern about my numbering scheme, however I’ve stuck with it (as by then people were using wiringPi). and it’s proven its worth over the hardware board revisions where some pins changed their hardware definitions, however wiringPi was able to hide this from the user. As a result (for example) a program that uses wiringPi pin 2 on a Rev. 1 Pi will work unchanged on a Rev 2. Pi, however someone using BCM_GPIO pin 21 on a Rev 1 Pi will need to change their program to use BCM_GPIO pin 27 on a Rev 2.

So wiringPi supports its own pin numbering scheme as well as the BCM_GPIO pin numbering scheme, and as of Version 2, it also supports the physical hardware pin numbers (for the P1 connector only), but I would like to suggest you stick to the simplified wiringPi pin numbers. That way your programs will be portable over different hardware revisions without needing any changes.

The following tables give the mapping of the Raspberry Pi GPIO Pins to the (P1) GPIO connector in relation to the pin numbers and the physical location on the connector. This is a representation of the GPIO connector as viewed looking at the board from above. The GPIO connector is to the top-right of the board with the Ethernet and USB sockets to the bottom.

Board Revisions: Please note the differences between board revisions 1 and 2 (Rv1 and Rv2 above) The Revision 2 is readily identifiable by the presence of the 2 mounting holes.

The P5 connector is designed to have the header soldered on the underside of the board. Pin 1 is identified by the square solder pad. So if you solder the header on the top of the board be aware that the pin locations will be the other way round!

For a printable version of these tables, click here.

Since the 26-pin GPIO connectors, a new 40-pin connector has appeared on newer Pi’s. There is also the compute Module boards. The best way to get a description of the GPIO connector on whatever Pi you’re currently running on is to use the gpio command:

This will give you a complete picture of your Pi’s GPIO connector(s) with all the numbering schemes present.

The GPIO pins on a Raspberry Pi are a great way to interface physical devices like buttons and LEDs with the little Linux processor. If you’re a Python developer, there’s a sweet library called RPi.GPIO that handles interfacing with the pins. In just three lines of code, you can get an LED blinking on one of the GPIO pins.

Not sure if Raspberry Pi is right for you? Make:’s interactive Board Guide lets you dial into the field to find the best board for your needs.


The newest version of Raspbian has the RPi.GPIO library pre-installed. You’ll probably need to update your library, so using the command line, run:

The current version of RPi.GPIO is 0.5.4 If you need to update to a newer version, run:

If you don’t have the RPi.GPIO library because you’re using an older version of Raspbian, there are great instructions on the Raspberry Pi Spy website on installing the package from scratch.

Using the RPi.GPIO Library

Now that you’ve got the package installed and updated, let’s take a look at some of the functions that come with it. Open the Leafpad text editor and save your sketch as “myInputSketch.py”. From this point forward, we’ll execute this script using the command line:

All of the following code can be added to this same file. Remember to save before you run the above command. To exit the sketch and make changes, press Ctrl+C.

To add the GPIO library to a Python sketch, you must first import it:

Then we need to declare the type of numbering system we’re going to use for our pins:

The main difference between these modes is that the BOARD option uses the pins exactly as they are laid out on the Pi. No matter what revision you’re using, these will always be the same. The BCM option uses the Broadcom SoC numbering, which differs between version 1 and version 2 of the Pi.

from Meltwater’s Raspberry Pi Hardware

In the image above, you’ll see that Pin 5 is GPIO01/03. This means that a v.1 Pi is GPIO 01, while a v.2 Pi is GPIO 03. The BCM numbering is what I’ll be using for the rest of this entry, because it’s universal across other programming languages.

Building a Circuit

Now we’re going to get into inputs and outputs. In the circuit shown below, two momentary switches are wired to GPIO pins 23 and 24 (pins 16 and 18 on the board). The switch on pin 23 is tied to 3.3V, while the switch on pin 24 is tied to ground. The reason for this is that the Raspberry Pi has internal pull-up and pull-down resistors that can be specified when the pin declarations are made.

To set up these pins, write:

This will enable a pull-down resistor on pin 23, and a pull-up resistor on pin 24. Now, let’s check to see if we can read them. The Pi is looking for a high voltage on Pin 23 and a low voltage on Pin 24. We’ll also need to put these inside of a loop, so that it is constantly checking the pin voltage. The code so far looks like this:

The indents in Python are important when using loops, so be sure to include them. You also must run your script as “sudo” to access the GPIO pins. The GPIO.cleanup() command at the end is necessary to reset the status of any GPIO pins when you exit the program. If you don’t use this, then the GPIO pins will remain at whatever state they were last set to.

The Problem With Polling

This code works, but prints a line for each frame that the button is pressed. This is extremely inconvenient if you want to use that button to trigger an action or command only one time. Luckily, the GPIO library has built in a rising-edge and falling-edge function. A rising-edge is defined by the time the pin changes from low to high, but it only detects the change. Similarly, the falling-edge is the moment the pin changes from high to low. Using this definition, let’s change our code slightly:

When you run this code, notice how the statement only runs after the edge detection occurs. This is because Python is waiting for this specific edge to occur before proceeding with the rest of the code. What happens if you try to press button 2 before you let go of button 1? What happens if you try to press button 1 twice without pressing button 2? Because the code is written sequentially, the edges must occur in exactly the order written.

Edge Detection is great if you need to wait for an input before continuing with the rest of the code. However, if you need to trigger a function using an input device, then events and callback functions are the best way to do that.

Events and Callback Functions

Let’s say you’ve got the Raspberry Pi camera module, and you’d like it to snap a photo when you press a button. However, you don’t want your code to poll that button constantly, and you certainly don’t want to wait for an edge because you may have other code running simultaneously.

The best way to execute this code is using a callback function. This is a function that is attached to a specific GPIO pin and run whenever that edge is detected. Let’s try one:

You’ll notice here that button 1 will consistently trigger the printFunction, even while the main loop is waiting for an edge on button 2. This is because the callback function is in a separate thread. Threads are important in programming because they allow things to happen simultaneously without affecting other functions. Pressing button 1 will not affect what happens in our main loop.

Events are also great, because you can remove them from a pin just as easily as you can add them:

Now you’re free to add a different function to the same pin!

Adding Functionality

As convenient as callback functions are for the GPIO pins, it still doesn’t change the fact that the Raspberry Pi is just not ideal for analog inputs or PWM outputs. However, because the Pi has Tx and Rx pins (pins 8 and 10, GPIO 14 and 15), it can easily communicate with an Arduino. If I have a project that requires an analog sensor input, or smooth PWM output, simply writing commands to the serial port to the Arduino can make things seamless.

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