What is PWM? Arduino and Pulse Width Modulation

In general, Pulse Width Modulation (PWM) is a modulation technique used to encode a message into a pulsing signal. A PWM is comprised of two key components:  frequency and duty cycle. The PWM frequency dictates how long it takes to complete a single cycle (period) and how quickly the signal fluctuates from high to low. The duty cycle determines how long a signal stays high out of the total period. Duty cycle is represented in percentage.

In Arduino, the PWM enabled pins produce a constant frequency of ~ 500Hz, while the duty cycle changes according to the parameters set by the user. See the following illustration:

Basic Principle of PWM

Pulse Width Modulation or PWM is a common technique used to vary the width of the pulses in a pulse-train. PWM has many applications such as controlling servos and speed controllers, limiting the effective power of motors and LEDs.

Pulse width modulation is basically, a square wave with a varying high and low time. A basic PWM signal is shown in the following figure.

There are various terms associated with PWM −

On-Time − Duration of time signal is high.

Off-Time − Duration of time signal is low.

Period − It is represented as the sum of on-time and off-time of PWM signal.

Duty Cycle − It is represented as the percentage of time signal that remains on during the period of the PWM signal.

Period

As shown in the figure, Ton denotes the on-time and Toff denotes the off-time of signal. Period is the sum of both on and off times and is calculated as shown in the following equation −

T_{total} = T_{on}+T_{off}

Duty Cycle

Duty cycle is calculated as the on-time of the period of time. Using the period calculated above, duty cycle is calculated as −

D = {T_{on}}/{T_{on}+T_{off}} = {T_{on}}/{T_{total}}

AnalogWrite() Function

The analogWrite() function writes an analog value (PWM wave) to a pin. It can be used to light a LED at varying brightness or drive a motor at various speeds. After a call of the analogWrite() function, the pin will generate a steady square wave of the specified duty cycle until the next call to analogWrite() or a call to digitalRead() or digitalWrite() on the same pin. The frequency of the PWM signal on most pins is approximately 490 Hz. On the Uno and similar boards, pins 5 and 6 have a frequency of approximately 980 Hz. Pins 3 and 11 on the Leonardo also run at 980 Hz.

On most Arduino boards (those with the ATmega168 or ATmega328), this function works on pins 3, 5, 6, 9, 10, and 11. On the Arduino Mega, it works on pins 2 - 13 and 44 - 46. Older Arduino boards with an ATmega8 only support analogWrite() on pins 9, 10, and 11.

analogWrite() Function Syntax

analogWrite ( pin , value ) ;

value − the duty cycle: between 0 (always off) and 255 (always on).

Example

/*

  Fade

  This example shows how to fade an LED on pin 9

  using the analogWrite() function.

  The analogWrite() function uses PWM, so if  you

  want to change the pin you're using, be  sure to

  use another PWM capable pin. On most  Arduino,

  the PWM pins are identified with   a "~" sign,

  like ~3, ~5, ~6, ~9, ~10 and ~11.

*/

 

int brightness = 0;

void setup()

{

  pinMode(9, OUTPUT);

}

void loop()

{

  for (brightness = 0; brightness <= 255; brightness += 5) {

    analogWrite(9, brightness);

    delay(30); // Wait for 30 millisecond(s)

  }

  for (brightness = 255; brightness >= 0; brightness -= 5) {

    analogWrite(9, brightness);

    delay(30); // Wait for 30 millisecond(s)

  }

}