synchronous-gpio/render.cpp

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:

• accurate, minimally invasive measurements of CPU time for specific blocks of code (simply toggle a pin before and after the section you want to measure and monitor it with an external oscilloscope)
• performing non-time-critical digital I/O (e.g.: reading buttons or driving LEDs) when you need more than the 16 I/Os provided by the Bela core.
• manually driving the on-board LEDs for BelaMini (requires running with --disable-led).

See here for more details: https://github.com/BelaPlatform/Bela/wiki/Using-arbitrary-GPIO-pins

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).

Preparation:

On Bela:

• Connect an LED in series with a 470ohm resistor between digital pin P8.30 and ground.
• Make sure you run it with Bela's digitals disabled. This is not normally needed when using the Gpio class, unless - like in this case - one of the pins in use would otherwise be used by the Bela Digital I/Os. On BelaMini:
• use the onboard LED, overriding its default behaviour
• make sure you run with --disable-led

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.

/*
 ____  _____ _        _
| __ )| ____| |      / \
|  _ \|  _| | |     / _ \
| |_) | |___| |___ / ___ \
|____/|_____|_____/_/   \_\
http://bela.io
*/
#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 https://github.com/BelaPlatform/Bela/wiki/Pins-used-by-Bela 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)
{
        gpioIn.open(gInputPin, Gpio::INPUT); // Open the pin as an output
        gpioOut.open(gOutputPin, Gpio::OUTPUT); // Open the pin as an output
        return true;
}

void render(BelaContext *context, void *userData)
{
        if(gpioIn.read()) // read the Bela button.
                gShouldBlink = true; // button is not pressed
        else
                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;
                        else
                                gStatus = 0;
                        if(gShouldBlink)
                                gpioOut.write(gStatus); //write the status to the LED (gOutputPin)
                }
                gCount++;
        }
}

void cleanup(BelaContext *context, void *userData)
{
        // Nothing to do here
}