Using GPIO#

Todo

Add information about software image used for this demo.

GPIO stands for General-Purpose Input/Output. It’s a set of programmable pins that you can use to connect and control various electronic components.

You can set each pin to either read signals (input) from things like buttons and sensors or send signals (output) to things like LEDs and motors. This lets you interact with and control the physical world using code!

A great resource for understanding pin numbering can be found at pinout.beagley.ai

Warning

BeagleY-AI GPIOs are 3.3V tolerant, using higher voltages WILL DAMAGE the processor!

Pin Numbering#

You will see pins referenced in several ways. While this is confusing at first, in reality, we can pick our favorite way and stick to it.

The two main ways of referring to GPIOs is by their number, so GPIO 2, 3, 4 etc. as seen in the diagram below. This corresponds to the SoC naming convention. For broad compatibility, BeagleY-AI re-uses the Broadcom GPIO numbering scheme used by RaspberryPi.

The second (and arguably easier) way we will use for this tutorial is to use the actual pin header number (shown in dark grey)

So, for the rest of the tutorial, if we refer to hat-08-gpio we mean the 8th pin of the GPIO header. Which, if you referenced the image below, can see refers to GPIO 14 (UART TX)

If you are curious about the “real” GPIO numbers on the Texas Instruments AM67A SoC, you can look at the board schematics.

Required Hardware#

For the simple blink demo, all that is needed is an LED, a Resistor (we use 2.2K here) and 2 wires.

Similarly, a button is used for the GPIO read example, but you can also just connect that pin to 3.3V or GND with a wire to simulate a button press.

Todo

Add fritzing diagram and chapter on Pin Binding here

GPIO Write#

Fig. 115 LED connected to HAT Pin8 (GPIO14)#

At it’s most basic, we can set a GPIO using the gpioset command.

To set HAT Pin 8 (GPIO14) to ON:

gpioset $(gpiofind GPIO14)=1

To set HAT Pin 8 (GPIO14) to OFF:

gpioset $(gpiofind GPIO14)=0

Blink an LED#

Let’s create a script called blinky.sh,

Create the file,

touch blinky.sh

Open the file using nano editor,

nano blinky.sh

Copy paste the code below to blinky.sh file,

#!/bin/bash

while :
do
      gpioset $(gpiofind GPIO14)=1
      sleep 1
      gpioset $(gpiofind GPIO14)=0
      sleep 1
done

Close the editor by pressing Ctrl + O followed by Enter to save the file and then press to Ctrl + X exit
Now execute the blinky.sh script by typing:

bash blinky.sh

You can exit the blinky.sh progrm by pressing CTRL + C on your keyboard.

Understanding the code#

#!/bin/bash

while :
do
   gpioset $(gpiofind GPIO14)=1 ①
   sleep 1 ②
   gpioset $(gpiofind GPIO14)=0 ③
   sleep 1 ④
done

The script is an infinite while loop in which we do the following:

① set the HAT Pin 8 (GPIO14) as 1 (HIGH)

② Wait 1 Second

③ set the HAT Pin 8 (GPIO14) as 0 (LOW)

④ Wait 1 Second

Read a Button#

A push button simply completes an electric circuit when pressed. Depending on wiring, it can drive a signal either “Low” (GND) or “High” (3.3V).

We will connect our Button between HAT Pin 12 (GPIO18) and Ground (GND).

Fig. 119 Button connected to HAT Pin12 (GPIO18)#

The cool part is since we have an internal pull-up resistor, we don’t need an external one! The pull resistor guarantees that the Pin stays in a known (HIGH) state unless the button is pressed, in which case it will go LOW.

Reading GPIOs can be done using the gpioget command

gpioget --bias=pull-up $(gpiofind GPIO18)

Results in 1 if the Input is held HIGH or 0 if the Input is held LOW

Let’s create a script called button.sh to continuously read an input pin connected to a button and print out when it’s pressed!

Create the file,

touch button.sh

Open the file using nano editor,

nano button.sh

Copy paste the code below to button.sh file,

#!/bin/bash

while :
do
   if (( $(gpioget --bias=pull-up $(gpiofind GPIO18)) == 0))
   then
      echo "Button Pressed!"
   fi
done

Close the editor by pressing Ctrl + O followed by Enter to save the file and then press to Ctrl + X exit
Now execute the button.sh script by typing:

bash button.sh

You can exit the button.sh by pressing Ctrl + C on your keyboard.

Combining the Two#

Fig. 120 Button connected to HAT Pin12 (GPIO18) & LED connected to HAT Pin8 (GPIO14)#

Now, logically, let’s make an LED match the state of the button.

Let’s create a script called blinkyButton.sh:

Create the file,

touch blinkyButton.sh

Open the file using nano editor,

nano blinkyButton.sh

Copy paste the code below to blinkyButton.sh file,

#!/bin/bash

while :
do
   if (( $(gpioget --bias=pull-up $(gpiofind GPIO18)) == 0))
   then
      gpioset $(gpiofind GPIO14)=1
   else
      gpioset $(gpiofind GPIO14)=0
   fi
done

Close the editor by pressing Ctrl + O followed by Enter to save the file and then press to Ctrl + X exit
Now execute the blinkyButton.sh script by typing:

bash blinkyButton.sh

This means when we see HAT Pin 12 (GPIO18) go LOW, we know the button is pressed, so we set HAT Pin 8 (GPIO14) (our LED) to ON, otherwise, we turn it OFF.

Fig. 121 LED is ON when button is pressed#

You can exit the blinkyButton.sh program by pressing Ctrl + C on your keyboard.

Understanding Internal Pull Resistors#

Pull-up and pull-down resistors are used in digital circuits to ensure that inputs to logic settle at expected levels.

Internal pull-up resistors connects the pin to a high voltage level (e.g., 3.3V) to ensure the pin input reads as a logic high (1) when no active device is pulling it low.
Internal pull-down resistors connects the pin to ground (GND) to ensure the input reads as a logic low (0) when no active device is pulling it high.

These resistors prevent floating inputs and undefined states.

By default, all GPIOs on the HAT Header are configured as Inputs with Pull-up Resistors Enabled.

This is important for something like a button, as without it, once a button is released, it goes in an “undefined” state!

To configure Pull-ups on a per-pin basis, we can use pass the following arguments within gpioget or gpioset:

-B, --bias=[as-is|disable|pull-down|pull-up] (defaults to 'as-is')

The “Bias” argument has the following options:

as-is - This leaves the bias as-is… quite self explanatory
disable - This state is also known as High-Z (high impedance) where the Pin is left Floating without any bias resistor
pull-down - In this state, the pin is pulled DOWN by the internal 50KΩ resistor
pull-up - In this state, the pin is pulled UP by the internal 50KΩ resistor

For example, a command to read an input with the Bias intentionally disabled would look like this:

gpioget --bias=disable $(gpiofind GPIO14)

Pull resistors are a foundational block of digital circuits and understanding when to (and not to) use them is important.

This article from SparkFun Electronics is a good basic primer - Link

Troubleshooting#

My script won’t run!

Make sure you gave the script execute permissions first and that you’re executing it with a ./ before

To make it executable:

chmod +X scriptName.sh

To run it:

./scriptName.sh

Bonus - Turn all GPIOs ON/OFF#

Copy and paste this with the button on the right to turn all pins ON.

gpioset $(gpiofind GPIO14)=1 ;\ gpioset $(gpiofind GPIO15)=1 ;\ gpioset $(gpiofind GPIO17)=1 ;\ gpioset $(gpiofind GPIO18)=1 ;\ gpioset $(gpiofind GPIO27)=1 ;\ gpioset $(gpiofind GPIO22)=1 ;\ gpioset $(gpiofind GPIO23)=1 ;\ gpioset $(gpiofind GPIO24)=1 ;\ gpioset $(gpiofind GPIO10)=1 ;\ gpioset $(gpiofind GPIO9)=1 ;\ gpioset $(gpiofind GPIO25)=1 ;\ gpioset $(gpiofind GPIO11)=1 ;\ gpioset $(gpiofind GPIO8)=1 ;\ gpioset $(gpiofind GPIO7)=1 ;\ gpioset $(gpiofind GPIO1)=1 ;\ gpioset $(gpiofind GPIO6)=1 ;\ gpioset $(gpiofind GPIO12)=1 ;\ gpioset $(gpiofind GPIO13)=1 ;\ gpioset $(gpiofind GPIO19)=1 ;\ gpioset $(gpiofind GPIO16)=1 ;\ gpioset $(gpiofind GPIO26)=1 ;\ gpioset $(gpiofind GPIO21)=1

Similarly, copy and paste this to turn all pins OFF.

gpioset $(gpiofind GPIO14)=0 ;\ gpioset $(gpiofind GPIO15)=0 ;\ gpioset $(gpiofind GPIO17)=0 ;\ gpioset $(gpiofind GPIO18)=0 ;\ gpioset $(gpiofind GPIO27)=0 ;\ gpioset $(gpiofind GPIO22)=0 ;\ gpioset $(gpiofind GPIO23)=0 ;\ gpioset $(gpiofind GPIO24)=0 ;\ gpioset $(gpiofind GPIO10)=0 ;\ gpioset $(gpiofind GPIO9)=0 ;\ gpioset $(gpiofind GPIO25)=0 ;\ gpioset $(gpiofind GPIO11)=0 ;\ gpioset $(gpiofind GPIO8)=0 ;\ gpioset $(gpiofind GPIO7)=0 ;\ gpioset $(gpiofind GPIO1)=0 ;\ gpioset $(gpiofind GPIO6)=0 ;\ gpioset $(gpiofind GPIO12)=0 ;\ gpioset $(gpiofind GPIO13)=0 ;\ gpioset $(gpiofind GPIO19)=0 ;\ gpioset $(gpiofind GPIO16)=0 ;\ gpioset $(gpiofind GPIO26)=0 ;\ gpioset $(gpiofind GPIO21)=0

Going Further#

License Terms