PWM (Pulse Width Modulation)

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Visualization of PWM duty cycle

Introduction

PWM or "Pulse Width Modulation" is the continuous fast-switching of a pin from '0' to '1' and back, creating a square wave signal.

By varying the ratio between "on-time" (the time during which the pin is "on" or 1) and "off-time" (the time during which the pin is "off" or 0), we can precisely control the amount of power delivered to a device, without ever changing the voltage.

More details about PWM can be found on Wikipedia.

Typical application

Typical applications for PWM include:

  • motor speed control
  • LED intensity control ("dimming")
  • servo control

This tutorial presents the 'pwm' library, which allows us to control the PWM pins of the ioNode.

Note: not all pins are PWM-capable. Check out the ioNode documentation to see which pins can do PWM.

How to use

First, we need to add the 'pwm' library to our dependencies in dfe.conf:

name: example_app
type: app
mmcu: atmega1284p
freq: 10000000
deps:
  - pwm

Then we can set the PWM ratio on any PWM-enabled pin with 'pwm_set' (and disable it completely with 'pwm_off').

The PWM value (ratio) is a single byte. Therefore:

  • 0 -> 0%
  • 127 -> 50%
  • 255 -> 100%

etc...

 1 #include <pwm/pwm.h>
 2 
 3 void main()
 4 {
 5     // Set PWM @ 50% on pin 11
 6     pwm_set(11, 127);
 7 
 8     // ...
 9 
10     // Disable PWM on pin 11
11     pwm_off(11);
12 
13     // ...
14 }