PWM Expansion Node Module

The Onion PWM Node Module, node-pwm-exp is a wrapper around the libonionpwmexp dynamic C library that provides functions to setup the servo Expansion and generate PWM signals.


Programming Flow

After each power-cycle, the chip that controls the PWM Expansion must be programmed with an initialization sequence to enable the on-board oscillator so that PWM signals can be generated.

After the initialization, the other functions can be used to generate PWM signals on specific channels or to change the PWM signal frequency.

Additionally, it is possible to disable to the oscillator, disabling the generation of all PWM signals at once.

Before generating new PWM signals, the initialization sequence must be run again.

PWM Signal Refresher

Pulse Width Modulated signals can be described with duty cycle percentages and frequencies/periods:

Duty Cycle Graph

The duty cycle indicates what percentage of time a signal is on or high voltage.

The frequency determines the overall period of the pulse.

For a more detailed explanation, see the guide on using the Servo Expansion.

Channels on the Expansion

The PWM Expansion has 16 channels that can generate distinct PWM signals. Note that they will all be running on the same frequency.

The Node Module

The node-pwm-exp exposes a series of methods that perform all of the actions specified in the Programming Flow section.

Install the Module

Install the module on your Omega:

opkg update
opkg install node-pwm-exp

NodeJS will need to be installed to run any Node programs:

opkg install nodejs

Importing the Module

To use the module within your script you have to import it into your node program as you would a module:

var pwmModule = require("/usr/bin/node-pwm-exp");

Example Code

Example code that uses the node-pwm-exp module can be found here in the i2c-exp-node-addons Onion GitHub Repo.

Return Values

All of the functions will either return a 0 indicating success or 1 indicating failure.

Calling Methods

Methods are called in the following format.


Replace method with your funcion of interest.

Available Methods

Refer to the table below for a list and brief description of available PWM methods.

Method Inputs Description
driverInit() none Initialize the PWM expansion for use
setupDriver(int driverNum, float duty, float delay ) 0-15 or -1, 0-100, 0-100 Generates the specified PWM signal on the specified channel
setFrequency(float freq) 24-1526 Sets the frequency for the oscillator chip
disableChip() none Disables the oscillator chip and stops all PWM signals

Initialization Function

This function programs the initialization sequence on the PWM Expansion, after this step is completed, the functions to generate PWM signals or change the signal frequency can be used with success:


Check for Initialization

This function performs several reads to determine if the PWM Expansion has been initialized and the oscillator is running.



Let’s check if the oscillator is initialized.

if (pwmExp.checkInit()) {
  console.log('Oscillator sucessfull initialized');
} else {
  console.error('Error with oscillator initializing');

Generate a PWM Signal

Here we go! Use this function to generate a PWM signal on a specified channel:

pwmExp.setupDriver(int driverNum, float duty, float delay);


Argument Input Meaning
channel -1 or 0 - 15 Detemines on which channel to generate the PWM signal
duty 0.00 - 100.00 Specifies duty cycle percentage for the PWM signal (decimial allowed)
delay 0.00 - 100.00 Specifies the percentage delay before the PWM signal goes high (decimial allowed)

Note that in normal use with servos, delay should be set to 0.

This table describes how to use channel argument:

Value Meaning
0 - 15 Matches the label on the Servo Expansion
-1 Generates the same signal on all channels


Set channel 0 to a PWM signal with a 50% duty cycle:


Generate a 3.55% duty cycle PWM signal with a 45% delay on channel 7:

pwmExp.setupDriver(7, 3.55f, 45);

Set channel 0 to a PWM signal with a 50% duty cycle:

pwmExp.setupDriver(15, 100, 0);

Set channel 8 to always off:

pwmExp.setupDriver(8, 0, 0);

Set all channels to a 15.65% duty cycle PWM signal:

pwmExp.setupDriver(-1, 15.65f, 0.0f);

Set PWM Signal Frequency

The oscillator can be reprogrammed to generate a variety of different frequencies:

pwmExp.setFrequency(float freq);

The default frequency is 50 Hz.

This method changes the frequency of the PWM signals generated on all of the channels. The oscillator can generate frequencies between 24 Hz and 1526 Hz.


Argument Input Meaning
frequency 24.00 - 1526.00 Specifies the frequency in Hz (will be clamped to given range if outside)


Change the frequency to 60 Hz and generate a 40% duty cycle signal on channel 14:


Generate a signal on channel 13, change the chip frequency to 105.45 Hz, and generate a new signal on channel 13:


Disabling the Oscillator

The oscillator can also be disabled, automatically stopping all PWM signal generation:


This might be useful for disabling PWM signal-driven devices while not powering off the Omega. The initialization function will have to be run before new PWM signals can be generated