Real-time, ultra-low-latency audio and sensor processing system for BeagleBone Black
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Using the second PRU for your own code

The Bela environment uses one of the Programmable Real-time Units (PRUs) on the BeagleBone Black to handle the data transfer to and from the analog, digital and audio pins. The BeagleBone Black has two PRUs, leaving the second one free for your use!

This example shows a simple GPIO sketch running on the second PRU, with communication back to the main CPU. Because the 16 GPIOs used for the Bela digital pins are already in use by the first PRU, we need to choose a different pin for this example. In this case, GPIO 30 is used as an output to drive an LED.

To run this example, attach a resistor and LED in series between GPIO 30 and ground. You should see the LED blink progressively faster, and when it becomes fast enough that it cannot be seen blinking anymore, it resets to its original speed.

The PRU code for this exmaple can be found in pru_gpio.p. You need the program "pasm" (PRU assembler) from the TI prussdrv library to compile it. The compiled version is found in pru_gpio_bin.h. Which PRU this code runs on depends on which PRU is used by Bela. This can be selected by passing the –pru-number flag to Bela; e.g. –pru-number=1 makes Bela use PRU 1 for its core code, leaving PRU 0 for the user code.

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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) 2016 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 <GPIOcontrol.h>
#include <cmath>
#include "prussdrv.h"
#include "pruss_intc_mapping.h"
#include "pru_gpio_bin.h"
// The definitions below are locations in PRU memory
// that need to match the PRU code
uint32_t *gPRUCommunicationMem = 0;
int gpioNumber0 = 30; // must match PRU code
float gFrequency = 440.0;
float gPhase;
float gInverseSampleRate;
int gUpdateCount = 0;
int64_t gPeriodNS = 500000000LL; // 0.5 seconds
bool load_pru(int pru_number); // Load the PRU environment
bool start_pru(int pru_number); // Start the PRU
void set_period(uint64_t period_ns); // Set the period of the blink
int gPruNumber;
// Set user PRU to the opposite number as the default Bela PRU
if(settings->pruNumber == 0)
gPruNumber = 1;
gPruNumber = 0;
bool setup(BelaContext *context, void *userData)
if(gpio_export(gpioNumber0)) {
printf("Warning: couldn't export GPIO pin %d\n", gpioNumber0);
if(gpio_set_dir(gpioNumber0, OUTPUT_PIN)) {
printf("Warning: couldn't set direction on GPIO pin\n");
// Load PRU environment and map memory
if(!load_pru(gPruNumber)) {
printf("Error: could not initialise user PRU code.\n");
return false;
if(!start_pru(gPruNumber)) {
printf("Error: could not start user PRU code.\n");
return false;
gInverseSampleRate = 1.0 / context->audioSampleRate;
gPhase = 0.0;
return true;
void render(BelaContext *context, void *userData)
for(unsigned int n = 0; n < context->audioFrames; n++) {
float out = 0.8 * sinf(gPhase);
gPhase += 2.0f * (float)M_PI * gFrequency * gInverseSampleRate;
if(gPhase > M_PI)
gPhase -= 2.0f * (float)M_PI;
for(unsigned int channel = 0; channel < context->audioOutChannels; channel++) {
audioWrite(context, n, channel, out);
if(++gUpdateCount >= 1024) {
gUpdateCount = 0;
gPeriodNS -= 1000000LL;
if(gPeriodNS <= 0)
gPeriodNS = 500000000ULL;
void cleanup(BelaContext *context, void *userData)
if(gpio_unexport(gpioNumber0)) {
printf("Warning: couldn't unexport GPIO pin %d\n", gpioNumber0);
/* Disable PRU */
// Load environment for the second PRU, but don't run it yet
bool load_pru(int pru_number)
void *pruMemRaw;
uint32_t *pruMemInt;
/* There is less to do here than usual for prussdrv because
* the core code will already have done basic initialisation
* of the library. */
/* Allocate and initialize memory */
if(prussdrv_open(PRU_EVTOUT_1)) {
rt_printf("Failed to open user-side PRU driver\n");
return false;
/* Map shared memory to a local pointer */
prussdrv_map_prumem(PRUSS0_SHARED_DATARAM, (void **)&pruMemRaw);
/* The first 0x800 is reserved by Bela. The next part is available
for our application. */
pruMemInt = (uint32_t *)pruMemRaw;
gPRUCommunicationMem = &pruMemInt[0x800/sizeof(uint32_t)];
return true;
// Start the second PRU running
bool start_pru(int pru_number)
if(prussdrv_exec_code(pru_number, PRUcode, sizeof(PRUcode))) {
rt_printf("Failed to execute user-side PRU code\n");
return false;
return true;
void set_period(uint64_t period_ns)
// Set the period in nanoseconds (with a resolution of 20ns)
// Write the value to the PRU which will use it in its loop
// Each delay macro in the PRU code has 10ns on top of whatever
// value is written, so subtract that off here. (Note that this
// doesn't consider the time for the GPIO code itself to run.)
uint64_t delay = period_ns / 20ULL - 20;
gPRUCommunicationMem[PRU_COMM_USER_DELAY] = (uint32_t)delay;