PWM frequency and duty cycle relationship in Raspberry Pi - Step-by-Step Execution

Concept Flow - PWM frequency and duty cycle relationship

Start PWM Setup

↓

Set Frequency

↓

Set Duty Cycle

↓

PWM Signal Output

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Signal ON time = Duty Cycle * Period

↓

Signal OFF time = Period - ON time

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Repeat Signal Continuously

This flow shows how PWM starts by setting frequency and duty cycle, then outputs a repeating signal with ON and OFF times based on these settings.

Execution Sample

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
pwm = GPIO.PWM(18, 1000)  # 1kHz frequency
pwm.start(50)  # 50% duty cycle

time.sleep(2)
pwm.stop()
GPIO.cleanup()

This code sets up a PWM signal on pin 18 with 1kHz frequency and 50% duty cycle, runs it for 2 seconds, then stops.

Execution Table

Step	Action	Frequency (Hz)	Duty Cycle (%)	Period (ms)	ON Time (ms)	OFF Time (ms)
1	Set frequency	1000	-	1	-	-
2	Set duty cycle	1000	50	1	0.5	0.5
3	PWM signal cycle start	1000	50	1	0.5	0.5
4	Signal ON for ON time	1000	50	1	0.5	0.5
5	Signal OFF for OFF time	1000	50	1	0.5	0.5
6	Repeat cycle	1000	50	1	0.5	0.5
7	Stop PWM	-	-	-	-	-

💡 PWM stopped after 2 seconds, ending signal output.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 7
Frequency (Hz)	-	1000	1000	-
Duty Cycle (%)	-	-	50	-
Period (ms)	-	1	1	-
ON Time (ms)	-	-	0.5	-
OFF Time (ms)	-	-	0.5	-

Key Moments - 2 Insights

Why does changing frequency affect the period but not the duty cycle percentage?

Frequency sets the total period length (1/frequency). Duty cycle is a percentage of that period, so changing frequency changes ON and OFF times but duty cycle percentage stays the same (see execution_table rows 1 and 2).

What happens to ON and OFF times if duty cycle is 0% or 100%?

At 0% duty cycle, ON time is 0 ms (signal always OFF). At 100%, ON time equals the full period (signal always ON). This is implied by the calculation ON Time = Duty Cycle * Period in execution_table.

Visual Quiz - 3 Questions

Look at the execution_table at Step 2, what is the ON time in milliseconds?

A 0.5 ms

B 1 ms

C 0 ms

D 2 ms

💡 Hint

Check the 'ON Time (ms)' column at Step 2 in the execution_table.

At which step does the PWM signal start repeating its ON and OFF cycle?

A Step 1

B Step 3

C Step 6

D Step 7

💡 Hint

Look for the first mention of PWM signal cycle start in the execution_table.

If the frequency is doubled to 2000 Hz but duty cycle stays 50%, how does the ON time change?

A ON time doubles

B ON time stays the same

C ON time halves

D ON time becomes zero

💡 Hint

Recall ON Time = Duty Cycle * Period and that period is 1/frequency from variable_tracker.

Concept Snapshot

PWM frequency sets the signal's period (Period = 1/Frequency).
Duty cycle is the percent of the period signal is ON.
ON time = Duty Cycle * Period.
Changing frequency changes period and ON/OFF times but not duty cycle %.
PWM repeats ON and OFF cycles continuously until stopped.

Full Transcript

PWM (Pulse Width Modulation) works by setting a frequency which determines the total period of the signal. The duty cycle is a percentage that controls how long the signal stays ON during each period. For example, a 1kHz frequency means the period is 1 millisecond. A 50% duty cycle means the signal is ON for 0.5 milliseconds and OFF for 0.5 milliseconds. The code example sets up PWM on a Raspberry Pi pin with these values, runs it for 2 seconds, then stops. The execution table shows each step, including setting frequency, duty cycle, and the ON/OFF times. Key points include that changing frequency changes the period and ON/OFF times but not the duty cycle percentage. If frequency doubles, ON time halves because the period is shorter. This visual trace helps understand how PWM signals are generated and controlled.