Real-time, ultra-low-latency audio and sensor processing system for BeagleBone Black
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Controlling a digital pin synchronously

This sketch shows how to control a digital pin synchronously from your code. This is an alternative approach to using the Bela Digital I/Os, which are sampled at audio rate. With this approach, the sampling time is not periodic.

Examples use cases:

See here for more details:

In this example we use as an output GPIO pin 89, which on Bela is P8.30 and on BelaMini is P1.04, but is also connected to the on-board red LED. Additionally, we read the button on the Bela cape. DO NOT press the button for more than 2 seconds at a time, unless you disabled it globally on your system, or you will trigger the default action (system shutdown).


On Bela:

In both cases, you have to disable the default behaviour of the button on the cape by running with --disable-cape-button-monitoring. Again, DO NOT hold-press the button for more than 2 seconds at a time unless it is disabled globally.

The led is blinked on and off by setting the digital pin to 1 and 0 every gInterval seconds, unless the button is pressed, in which case the blinking is temporarily disable.

In setup() the pin is opened and initialized as an output mode via a call to Gpio::open().

In render() the output of the output digital pin is set by Gpio::write(), and the input digital pin is read calling Gpio::read().

To keep track of elapsed time we have a sample counter gCount. When gCount reaches the desired value, it switches the state of the LED pin.

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The platform for ultra-low latency audio and sensor processing
A project of the Augmented Instruments Laboratory within the
Centre for Digital Music at Queen Mary University of London.
(c) 2018 Augmented Instruments Laboratory: Andrew McPherson,
Astrid Bin, Liam Donovan, Christian Heinrichs, Robert Jack,
Giulio Moro, Laurel Pardue, Victor Zappi. All rights reserved.
The Bela software is distributed under the GNU Lesser General Public License
(LGPL 3.0), available here:
#include <Bela.h>
#include <Gpio.h>
Gpio gpioIn;
Gpio gpioOut;
bool gShouldBlink = 1;
int gInputPin = 115; // this is the button on the Bela cape
int gOutputPin = 89; // this is the GPIO number. Check what pin it is on your board
float gInterval = 0.1; // how often to toggle the LED (in seconds)
int gCount = 0; //counts elapsed samples
bool gStatus = false;
bool setup(BelaContext *context, void *userData)
{, INPUT); // Open the pin as an output, OUTPUT); // Open the pin as an output
return true;
void render(BelaContext *context, void *userData)
if( // read the Bela button.
gShouldBlink = true; // button is not pressed
gShouldBlink = false; // button is pressed
for(unsigned int n = 0; n < context->digitalFrames; ++n){
if(gCount == (int)(context->digitalSampleRate * gInterval)){ //if enough samples have elapsed
gCount = 0; //reset the counter
//toggle the status
if(gStatus == 0)
gStatus = 1;
gStatus = 0;
gpioOut.write(gStatus); //write the status to the LED (gOutputPin)
void cleanup(BelaContext *context, void *userData)
// Nothing to do here