GPIO pin numbering (BCM vs BOARD) in Raspberry Pi

Introduction

GPIO pins on a Raspberry Pi can be numbered in two ways. Knowing the difference helps you connect and control devices correctly.

When you want to control LEDs or sensors connected to your Raspberry Pi.

When you follow a tutorial that uses a specific pin numbering system.

When you write code that needs to work with physical pin locations or chip pin numbers.

When you troubleshoot hardware connections on your Raspberry Pi.

Syntax

Raspberry Pi

import RPi.GPIO as GPIO

# Use BCM numbering
GPIO.setmode(GPIO.BCM)

# Use BOARD numbering
GPIO.setmode(GPIO.BOARD)

BCM means Broadcom chip pin numbers, which refer to the processor's pin numbers.

BOARD means the physical pin numbers on the Raspberry Pi's header.

Examples

This uses BCM numbering to turn on the pin connected to GPIO 18.

Raspberry Pi

import RPi.GPIO as GPIO

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

This uses BOARD numbering to turn on the pin at physical pin 12.

Raspberry Pi

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BOARD)
GPIO.setup(12, GPIO.OUT)
GPIO.output(12, GPIO.HIGH)

Sample Program

This program blinks an LED connected to GPIO pin 18 using BCM numbering. It turns the LED on and off three times with half-second pauses.

Raspberry Pi

import RPi.GPIO as GPIO
import time

# Choose BCM numbering
GPIO.setmode(GPIO.BCM)

# Set GPIO 18 as output
GPIO.setup(18, GPIO.OUT)

# Blink LED connected to GPIO 18 three times
for _ in range(3):
    GPIO.output(18, GPIO.HIGH)
    time.sleep(0.5)
    GPIO.output(18, GPIO.LOW)
    time.sleep(0.5)

GPIO.cleanup()

Important Notes

Always call GPIO.cleanup() at the end to reset pins safely.

Using the wrong numbering system can cause your hardware not to work or even damage it.

Check your Raspberry Pi model's pinout diagram to match physical pins with BCM numbers.

Summary

GPIO pins can be numbered by BCM (chip) or BOARD (physical) systems.

Use GPIO.setmode(GPIO.BCM) or GPIO.setmode(GPIO.BOARD) to select the system.

Pick the numbering that matches your hardware setup or tutorial instructions.

Practice

1. Which GPIO pin numbering mode uses the physical pin numbers on the Raspberry Pi header?

easy

A. BOARD numbering mode

B. BCM numbering mode

C. WIRING numbering mode

D. PIN numbering mode

Solution

1. Step 1: Understand GPIO numbering modes

   GPIO pins can be numbered by their physical location (BOARD) or by the Broadcom chip numbering (BCM).

2. Step 2: Identify physical pin numbering

   BOARD mode refers to the physical pin numbers on the Raspberry Pi header.

3. Final Answer:

   BOARD numbering mode -> Option A

4. Quick Check:

   Physical pins = BOARD [OK]

Hint: BOARD mode matches physical pin numbers on the header [OK]

Common Mistakes:

• Confusing BCM with physical pin numbers
• Thinking BCM is the physical numbering
• Assuming WIRING or PIN modes exist

2. Which of the following is the correct way to set GPIO pin numbering to BCM mode in Python?

easy

A. GPIO.setmode(GPIO.BOARD)

B. GPIO.setupmode(GPIO.BCM)

C. GPIO.mode(GPIO.BCM)

D. GPIO.setmode(GPIO.BCM)

Solution

1. Step 1: Recall the correct function to set GPIO mode

   The function to set numbering mode is GPIO.setmode().

2. Step 2: Use the correct constant for BCM mode

   GPIO.BCM is the constant for BCM numbering mode.

3. Final Answer:

   GPIO.setmode(GPIO.BCM) -> Option D

4. Quick Check:

   Use setmode with BCM constant [OK]

Hint: Use GPIO.setmode(GPIO.BCM) to select BCM numbering [OK]

Common Mistakes:

• Using GPIO.mode instead of GPIO.setmode
• Using BOARD instead of BCM constant
• Calling GPIO.setupmode which doesn't exist

3. Given the code snippet:

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BOARD)
GPIO.setup(7, GPIO.OUT)
GPIO.output(7, GPIO.HIGH)

Which physical pin is set HIGH?

medium

A. Pin 11 on the physical header

B. GPIO pin number 7 on the Broadcom chip

C. Pin 7 on the physical header

D. GPIO pin number 11 on the Broadcom chip

Solution

1. Step 1: Identify the numbering mode used

   The code uses GPIO.setmode(GPIO.BOARD), so pin numbers refer to physical header pins.

2. Step 2: Check which pin is set HIGH

   GPIO.setup(7, GPIO.OUT) and GPIO.output(7, GPIO.HIGH) set physical pin 7 HIGH.

3. Final Answer:

   Pin 7 on the physical header -> Option C

4. Quick Check:

   BOARD mode pin 7 = physical pin 7 [OK]

Hint: BOARD mode means pin number is physical header pin [OK]

Common Mistakes:

• Confusing BOARD pin 7 with BCM GPIO 7
• Assuming pin 7 means BCM numbering
• Mixing physical pin numbers with BCM numbers

4. What is wrong with this code snippet?

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(10, GPIO.OUT)
GPIO.output(10, GPIO.HIGH)

Assuming the LED is wired to physical pin 10 on the header.

medium

A. Pin 10 in BCM does not match physical pin 10 wiring

B. GPIO.setmode(GPIO.BOARD) should be used instead

C. GPIO.output() requires pin number as string

D. GPIO.setup() cannot use BCM numbering

Solution

1. Step 1: Understand numbering mismatch

   GPIO.setmode(GPIO.BCM) means pin numbers refer to Broadcom chip pins, not physical pins.

2. Step 2: Check wiring vs numbering

   Physical pin 10 corresponds to BCM GPIO 15. The code uses BCM pin 10 (physical pin 19), so it does not control the wired physical pin 10.

3. Final Answer:

   Pin 10 in BCM does not match physical pin 10 wiring -> Option A

4. Quick Check:

   BCM 10 controls physical 19 ≠ pin 10 [OK]

Hint: Match numbering mode to wiring to avoid pin mismatch [OK]

Common Mistakes:

• Assuming physical pin numbers work with BCM mode
• Using string instead of int for pin number
• Thinking GPIO.setup can't use BCM numbering

5. You want to control an LED connected to physical pin 12 on the Raspberry Pi header. Which code snippet correctly sets the pin HIGH using BCM numbering?

hard

A. GPIO.setmode(GPIO.BOARD)
GPIO.setup(12, GPIO.OUT)
GPIO.output(12, GPIO.HIGH)

B. GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
GPIO.output(18, GPIO.HIGH)

C. GPIO.setmode(GPIO.BCM)
GPIO.setup(12, GPIO.OUT)
GPIO.output(12, GPIO.HIGH)

D. GPIO.setmode(GPIO.BOARD)
GPIO.setup(18, GPIO.OUT)
GPIO.output(18, GPIO.HIGH)

Solution

1. Step 1: Identify physical pin 12's BCM number

   Physical pin 12 corresponds to BCM GPIO 18 on Raspberry Pi.

2. Step 2: Use BCM numbering mode and correct pin

   Set mode to GPIO.BCM and use pin 18 in setup and output.

3. Final Answer:

   GPIO.setmode(GPIO.BCM) GPIO.setup(18, GPIO.OUT) GPIO.output(18, GPIO.HIGH) -> Option B

4. Quick Check:

   Physical pin 12 = BCM 18, use BCM mode [OK]

Hint: Convert physical pin to BCM before coding [OK]

Common Mistakes:

• Using physical pin number with BCM mode
• Using BCM pin number with BOARD mode
• Mixing pin numbers without conversion