spidev library usage in Raspberry Pi

Introduction

The spidev library helps your Raspberry Pi talk to devices using SPI, a way to send data quickly between chips.

You want to read data from a sensor connected via SPI.

You need to control an SPI-based display or LED driver.

You want to send commands to an SPI flash memory chip.

You are building a project that communicates with an SPI device like an ADC or DAC.

You want to learn how to connect and communicate with hardware using Raspberry Pi.

Syntax

Raspberry Pi

import spidev

spi = spidev.SpiDev()
spi.open(bus, device)
spi.max_speed_hz = speed

response = spi.xfer2(data_bytes)

spi.close()

bus and device select which SPI channel and device you use (usually bus 0, device 0 or 1).

spi.xfer2() sends and receives data at the same time, returning the response from the device.

Examples

This example opens SPI bus 0 device 0, sends three bytes, and prints the response.

Raspberry Pi

import spidev

spi = spidev.SpiDev()
spi.open(0, 0)  # Open SPI bus 0, device 0
spi.max_speed_hz = 50000

response = spi.xfer2([0x01, 0x02, 0x03])
print(response)

spi.close()

This example communicates with device 1 on bus 0, sending one byte and printing the reply.

Raspberry Pi

import spidev

spi = spidev.SpiDev()
spi.open(0, 1)  # Open SPI bus 0, device 1
spi.max_speed_hz = 1000000

response = spi.xfer2([0xAA])
print(response)

spi.close()

Sample Program

This program opens SPI bus 0 device 0, sends one byte (0x10), prints what it sends and receives, then closes the connection safely.

Raspberry Pi

import spidev
import time

# Create SPI object
spi = spidev.SpiDev()

# Open SPI bus 0, device 0
spi.open(0, 0)

# Set SPI speed to 100kHz
spi.max_speed_hz = 100000

try:
    # Send a byte and receive response
    send_data = [0x10]
    print(f"Sending: {send_data}")
    response = spi.xfer2(send_data)
    print(f"Received: {response}")

    # Wait a bit
    time.sleep(1)

finally:
    # Close SPI connection
    spi.close()

Important Notes

Make sure SPI is enabled on your Raspberry Pi using raspi-config before using spidev.

Use spi.xfer2() to send and receive data in one call; it returns a list of bytes received.

Always close the SPI connection with spi.close() to free the device.

Summary

spidev lets your Raspberry Pi talk to SPI devices by sending and receiving bytes.

Open the SPI bus and device, set speed, use xfer2() to communicate, then close it.

Enable SPI on your Pi first and always close the connection when done.

Practice

1. What is the primary purpose of the spidev library on a Raspberry Pi?

easy

A. To communicate with SPI devices by sending and receiving bytes

B. To control the Raspberry Pi's GPIO pins directly

C. To manage Wi-Fi connections on the Raspberry Pi

D. To create graphical user interfaces on the Raspberry Pi

Solution

1. Step 1: Understand the role of spidev

   The spidev library is designed to allow communication with SPI devices by sending and receiving bytes.

2. Step 2: Compare with other options

   Options B, C, and D relate to GPIO, Wi-Fi, and GUI, which are not the main functions of spidev.

3. Final Answer:

   To communicate with SPI devices by sending and receiving bytes -> Option A

4. Quick Check:

   spidev = SPI communication [OK]

Hint: Remember spidev is for SPI device communication only [OK]

Common Mistakes:

• Confusing spidev with GPIO control libraries
• Thinking spidev manages network or GUI tasks
• Assuming spidev handles I2C communication

2. Which of the following is the correct way to open SPI bus 0, device 0 using spidev in Python?

easy

A. spi.connect(0, 0)

B. spi.start(0, 0)

C. spi.init(0, 0)

D. spi.open(0, 0)

Solution

1. Step 1: Recall spidev method to open SPI

   The correct method to open SPI bus and device is open(bus, device).

2. Step 2: Verify method names

   Methods like start, connect, and init are not valid for spidev.

3. Final Answer:

   spi.open(0, 0) -> Option D

4. Quick Check:

   Open SPI with open() method [OK]

Hint: Use spi.open(bus, device) to start SPI communication [OK]

Common Mistakes:

• Using incorrect method names like start or connect
• Confusing open() with init()
• Not specifying both bus and device numbers

3. What will be the output of the following code snippet?

import spidev
spi = spidev.SpiDev()
spi.open(0, 0)
response = spi.xfer2([0x01, 0x02, 0x03])
spi.close()
print(response)

medium

A. [1, 2, 3]

B. [0, 0, 0]

C. A list of bytes received from the SPI device

D. SyntaxError

Solution

1. Step 1: Understand spi.xfer2() behavior

   The xfer2() method sends bytes and returns a list of bytes received from the SPI device.

2. Step 2: Analyze the code output

   The output depends on the connected SPI device's response, so it returns a list of received bytes, not necessarily the same as sent.

3. Final Answer:

   A list of bytes received from the SPI device -> Option C

4. Quick Check:

   xfer2() returns received bytes list [OK]

Hint: xfer2() returns device response bytes list [OK]

Common Mistakes:

• Assuming output equals input list
• Expecting zeros if no device connected
• Thinking code causes syntax error

4. Identify the error in this code snippet using spidev:

import spidev
spi = spidev.SpiDev()
spi.xfer2([0x01, 0x02])
spi.open(0, 0)
spi.close()

medium

A. Not closing SPI device after use

B. Calling xfer2() before opening SPI device

C. Using wrong method name xfer2()

D. Missing import statement

Solution

1. Step 1: Check method call order

   The xfer2() method requires the SPI device to be opened first with open().

2. Step 2: Identify the error

   Here, xfer2() is called before open(), causing a runtime error.

3. Final Answer:

   Calling xfer2() before opening SPI device -> Option B

4. Quick Check:

   Open SPI before xfer2() call [OK]

Hint: Always open SPI before using xfer2() [OK]

Common Mistakes:

• Calling xfer2() before open()
• Forgetting to close SPI device
• Confusing method names

5. You want to send the bytes [0x10, 0x20, 0x30] to an SPI device on bus 1, device 0, at 500000 Hz speed, and receive the response. Which code snippet correctly does this using spidev?

hard

A. spi = spidev.SpiDev()
spi.open(1, 0)
spi.max_speed_hz = 500000
response = spi.xfer2([0x10, 0x20, 0x30])
spi.close()
print(response)

B. spi = spidev.SpiDev()
spi.open(0, 1)
spi.speed = 500000
response = spi.xfer([0x10, 0x20, 0x30])
spi.close()
print(response)

C. spi = spidev.SpiDev()
spi.open(1, 0)
spi.max_speed = 500000
response = spi.xfer2([0x10, 0x20, 0x30])
spi.close()
print(response)

D. spi = spidev.SpiDev()
spi.open(1, 0)
spi.max_speed_hz = 500000
response = spi.transfer([0x10, 0x20, 0x30])
spi.close()
print(response)

Solution

1. Step 1: Check SPI open parameters and speed setting

   Bus 1, device 0 is opened with spi.open(1, 0). Speed is set with spi.max_speed_hz = 500000.

2. Step 2: Verify transfer method and closing

   xfer2() is the correct method to send and receive bytes. The device is closed after communication.

3. Final Answer:

   Correct code snippet with spi.open(1, 0), max_speed_hz, xfer2(), and close() -> Option A

4. Quick Check:

   Use open(1,0), max_speed_hz, xfer2(), close() [OK]

Hint: Set max_speed_hz and use xfer2() after open() [OK]

Common Mistakes:

• Using wrong bus/device numbers
• Setting speed with wrong attribute name
• Using incorrect transfer method name
• Not closing SPI device