Real-time, ultra-low-latency audio and sensor processing system for BeagleBone Black
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Blinking 8 LEDs with 4 GPIOs (and a PWM)

Bicolor LEDs often come in a package with two terminals only. Internally, the LEDs are connected back-to-back with opposite polarity. By using a tri-state pin and a PWM generator, you can dim and mix each color at will. A PWM channel can be shared across multiple LED pairs, as long as it provides enough current. In this example we control 8 LEDs (or 4 bi-color LEDs) using a single PWM generator and 4 tri-state GPIOs.

Wiring: from each GPIO channels, add a 100ohm series resistor and then connect two LEDs back-to-back with opposite orientation from the resistor to the PWM output.

|---|>|---| 100ohm
PWM --| |---/\/\/--- GPIOpin

Remember that PWM oscillates between HIGH and LOW at a high rate. LED0 turns on only when GPIO is LOW. LED1 turns only when GPIO is HIGH. When GPIO is set to an INPUT, it goes to high impedance and acts as an open circuit, therefore both LEDs are off.

Bela allows to switch GPIO between OUTPUT-HIGH, OUTPUT-LOW, INPUT for every sample at 44.1kHz. For instance, to control the brightness of LED0 we can set an arbitrary period and set GPIO to OUTPUT-LOW for a number of samples (<= period) proportional to the brightness, setting it to INPUT for the remainder samples. If we keep it to OUTPUT-LOW for the whole period, it will have the maximum brightness, if we set it to INPUT for the whole period, it will be dark. Intermediate values will produce intermediate brightness levels. Similarly for LED1, except that we would alternate between OUTPUT-HIGH and INPUT.

Larger values of period allow for more resolution for the dimming, however if the value gets too large, you may have visible flickering.

As we need to control both LEDs from a single GPIO pin, we decide to alternate between the two LEDs for every sample, so that during even samples we control LED0 and during odd samples we control LED1. This way, for even samples the GPIO can only be OUTPUT-LOW or INPUT; for odd samples it can only be OUTPUT-HIGH or INPUT. The balance between OUTPUT-LOW and INPUT on even samples controls the brightness of LED0, while the balance between OUTPUT-HIGH and INPUT on odd samples controls the brightness of LED1. This way we can have independent control over the brightness of two LEDs with a single tri-state GPIO + a shared PWM generator.

When using a hardware PWM generator, this switches at a very high frequency, many times per sample. If this is not available, we can use a software PWM generator, toggling one of Bela's digital outputs. For this to be effective, the GPIO needs to hold the state for at least one full PWM period. We toggle the soft PWM output every sample (generating a 50% pulse wave at context->digitalSampleRate / 2 Hz), therefore we need to hold the value of an LED for 2 samples.

Varying the duty cycle of the PWM generator allows to balance between the different brightness of different colors of LEDs. However, to achieve this with the soft PWM, you would need to slow down the frequency of the pulse wave. This would in turn lower the resolution of the dimming, unless the period is increased, again with the risk of causing visible flickering.

When useSequencer = true, then analogIn0 affects the speed of the steps and analogIn1 affects the overall brightness. When useSequencer = false, then each of the analogIns affects the brightness of the corresponding LED.

To use a hardware PWM, e.g.: on P9_14:

cp MY-PWM-01-00A0.dtbo /lib/firmware
echo MY-PWM-01 > $SLOTS
config-pin P9.14 pwm
echo 3 > /sys/class/pwm/export
echo 0 > /sys/class/pwm/pwm3/duty_ns
echo 1000 > /sys/class/pwm/pwm3/period_ns
echo 700 > /sys/class/pwm/pwm3/duty_ns # adjust this to make sure the brightness is balanced between differnet LED colors
echo 1 > /sys/class/pwm/pwm3/run

otherwise you can use soft PWM using the Bela digtal specified in the code.

Also, you will need your pins not to have any pull-up or pull-down (0x2f) otherwise they may be lightly dim when they should be off.

____ _____ _ _
| __ )| ____| | / \
| _ \| _| | | / _ \
| |_) | |___| |___ / ___ \
|____/|_____|_____/_/ \_\
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>
const int numPins = 4;
int pins[numPins] = {0, 1, 2, 3}; // Digital pins 0 to 3
bool useHardwarePwm = false; // if you want to use hardware PWM, check out the help text at the bottom - check the pin diagram in the IDE
int softPwmPin = 4; // Digital pin 4 - check the pin diagram in the IDE
int period = 176; // duration (in samples) of a "brightness period", affects resolution of dimming. Larger values will cause flickering.
bool useSequencer = true; // if disabled, control the brightness of each LED from the respective analog input
const int numLeds = numPins * 2;
const int numSteps = 16;
float steps[numSteps][numLeds]; // built-in sequencer
bool setup(BelaContext *context, void *userData)
int n = 0;
int j = 0;
// fill the first numLeds positions with one LED at a time
for(; j < numLeds; ++j)
for(int led = 0; led < numPins * 2; ++led)
steps[j][led] = (j == led);
// fill the subsequent steps with one LED full-on, the other half-on and viceversa
n += j;
j = 0;
for(; j < numLeds * 2; j += 2)
for(int led = 0; led < numPins * 2; ++led)
if(led == j)
steps[n+j][led] = 1;
steps[n+j][led+1] = 0.2;
steps[n+j+1][led] = 0.2;
steps[n+j+1][led+1] = 1;
pinMode(context, 0, softPwmPin, OUTPUT);
return true;
void render(BelaContext *context, void *userData)
float brightness[numPins][2];
// read the analog input at block rate to determine each LED brightness
for(unsigned int n = 0; n < numPins; ++n)
if(context->analogInChannels >= n * 2 + 1)
brightness[n][0] = analogRead(context, 0, n * 2) / 0.84f;
brightness[n][1] = analogRead(context, 0, n * 2 + 1) / 0.84f;
} else {
brightness[n][0] = 1;
brightness[n][1] = 1;
} else {
// use the step sequencer
static int step = 0;
static int stepCounter = 0;
float stepLength;
float masterBrightness;
if(context->analogInChannels >= 1)
stepLength = analogRead(context, 0, 0) * context->digitalSampleRate;
masterBrightness = analogRead(context, 0, 1) / 0.84f;
} else {
stepLength = 0.5f * context->digitalSampleRate;
masterBrightness = 1;
if(stepCounter >= stepLength)
// select the next step
stepCounter = 0;
if(step == numSteps)
step = 0;
for(unsigned int n = 0; n < numPins; ++n)
brightness[n][0] = steps[step][n * 2] * masterBrightness;
brightness[n][1] = steps[step][n * 2 + 1] * masterBrightness;
stepCounter += context->digitalFrames;
for(unsigned int n = 0; n < context->digitalFrames; ++n){
static unsigned int count = 0;
if(useHardwarePwm == false)
// do the software PWM if needed, toggling every sample
int pwmValue = n & 1;
digitalWriteOnce(context, n, softPwmPin, pwmValue);
int led; // find which LED in a pair we are controlling in this sample
if(useHardwarePwm == false)
led = (count >> 1) & 1; // hold a given led for two samples, so that the soft PWM is effective
} else {
led = count & 1; // hardware PWM runs at much higher clock speed: we can change LED every sample
for(unsigned int channel = 0; channel < numPins; ++channel)
float bright = brightness[channel][led];
int direction = count >= bright * period ? INPUT : OUTPUT; // if the pin is set to INPUT, the LED will be OFF
int pin = pins[channel];
int value = led;
pinModeOnce(context, n, pin, direction);
if(direction == OUTPUT)
digitalWriteOnce(context, n, pin, value);
if(count == period)
count = 0;
void cleanup(BelaContext *context, void *userData)