Real-time, ultra-low-latency audio and sensor processing system for BeagleBone Black
 All Classes Namespaces Files Functions Variables Typedefs Enumerations Enumerator Macros Groups

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.

____ _____ _ _
| __ )| ____| | / \
| _ \| _| | | / _ \
| |_) | |___| |___ / ___ \
|____/|_____|_____/_/ \_\
#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)
{, Gpio::INPUT); // Open the pin as an output, Gpio::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->audioFrames; ++n){
if(gCount == (int)(context->audioSampleRate * 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