Pulse Width Modulation | Sahithyan's Notes

PWM encodes a value in a digital square wave by varying the fraction of time the signal is HIGH. Frequency stays fixed. Duty cycle varies.

Cycle time ( $T$ )
Total duration of 1 period.
On-time ( $t_{on}$ )
Portion of each cycle the signal is HIGH.

\text{Duty Cycle} = \frac{t_{on}}{T}

The time-averaged voltage:

\bar{V} = V_{supply} \times \text{Duty Cycle}

An MCU timer increments a counter at a fixed clock rate. A compare register holds the duty value. The output goes HIGH at counter reset and LOW at compare match.

An $n$ -bit timer provides $2^n$ duty cycle steps.

Applications:

Motor speed control
Average voltage controls rotational speed.
LED brightness
Perceived brightness tracks duty cycle.
Servo position
Pulse width (1–2 ms within a 20 ms period) encodes angle.
Buzzer tone
PWM frequency sets audible pitch.

Simulated Analog Output

A low-pass RC filter on a PWM signal removes the switching component, leaving a DC level equal to $\bar{V}$ .

Filter cutoff must be far below the PWM frequency. Higher PWM frequency reduces ripple for the same filter. Output accuracy is limited by PWM resolution and filter tolerances.