Software PWM with RPi.GPIO in Raspberry Pi - Step-by-Step Execution

Concept Flow - Software PWM with RPi.GPIO

Start Program

↓

Setup GPIO Pin

↓

Enter Loop

↓

Set Pin HIGH

↓

Wait ON Duration

↓

Set Pin LOW

↓

Wait OFF Duration

↓

Repeat Loop

↩Back to Enter Loop

The program sets up a GPIO pin, then repeatedly turns it ON and OFF with delays to create a PWM signal.

Execution Sample

Raspberry Pi

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)

while True:
    GPIO.output(18, GPIO.HIGH)
    time.sleep(0.001)
    GPIO.output(18, GPIO.LOW)
    time.sleep(0.009)

This code creates a software PWM on pin 18 with 10% duty cycle by turning the pin ON for 1ms and OFF for 9ms repeatedly.

Execution Table

Step	Action	Pin 18 State	Delay (seconds)	Loop Iteration
1	Set pin 18 HIGH	HIGH	0	1
2	Wait ON duration	HIGH	0.001	1
3	Set pin 18 LOW	LOW	0	1
4	Wait OFF duration	LOW	0.009	1
5	Set pin 18 HIGH	HIGH	0	2
6	Wait ON duration	HIGH	0.001	2
7	Set pin 18 LOW	LOW	0	2
8	Wait OFF duration	LOW	0.009	2
...	...	...	...	...

💡 Loop runs indefinitely until program is stopped manually.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	After Step 8	...final
Pin 18 State	LOW (default)	HIGH	LOW	HIGH	LOW	Repeats HIGH/LOW
Loop Iteration	0	1	1	2	2	Increments each cycle

Key Moments - 3 Insights

Why does the pin state change between HIGH and LOW repeatedly?

Because the code sets the pin HIGH, waits (ON time), then sets it LOW, waits (OFF time), creating a pulse. See execution_table steps 1-4.

How does the duty cycle relate to the ON and OFF delays?

Duty cycle is ON time divided by total period (ON + OFF). Here, ON is 0.001s and OFF is 0.009s, so duty cycle is 10%. See delays in execution_table.

Why is this called software PWM?

Because the timing is controlled by software delays (time.sleep), not hardware PWM modules. The loop manually toggles the pin.

Visual Quiz - 3 Questions

Look at the execution_table, what is the pin state at Step 3?

A HIGH

B LOW

C Undefined

D Still HIGH

💡 Hint

Refer to Step 3 in execution_table where pin 18 is set LOW.

At which step does the first OFF delay happen?

A Step 2

B Step 1

C Step 4

D Step 3

💡 Hint

Check execution_table for delay after pin is set LOW, which is Step 4.

If the ON delay is increased to 0.005 seconds, how does the duty cycle change?

A Duty cycle increases

B Duty cycle decreases

C Duty cycle stays the same

D Duty cycle becomes zero

💡 Hint

Duty cycle = ON time / (ON + OFF). Increasing ON time increases duty cycle.

Concept Snapshot

Software PWM with RPi.GPIO:
- Setup GPIO pin as output
- Loop: set pin HIGH, wait ON time
- Set pin LOW, wait OFF time
- Repeat for continuous PWM
- Duty cycle = ON time / (ON + OFF)

Full Transcript

This example shows how to create software PWM on a Raspberry Pi using the RPi.GPIO library. The program sets a GPIO pin as output and enters a loop. In each loop, it sets the pin HIGH, waits for the ON duration, then sets the pin LOW and waits for the OFF duration. This cycle repeats indefinitely, creating a pulse-width modulated signal. The duty cycle is controlled by adjusting the ON and OFF wait times. This method uses software delays, so timing is less precise than hardware PWM but is simple to implement.