spidev library usage in Raspberry Pi - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the spidev library in Raspberry Pi?

The spidev library allows Python programs to communicate with SPI devices connected to the Raspberry Pi. It provides an interface to send and receive data over the SPI bus.

beginner

How do you open an SPI connection using spidev?

You create an instance of spidev.SpiDev() and call its open(bus, device) method, where bus is the SPI bus number (usually 0) and device is the chip select (CS) line (usually 0 or 1).

intermediate

What does the xfer2() method do in spidev?

The xfer2() method sends a list of bytes to the SPI device and simultaneously receives a list of bytes back. It performs a full-duplex SPI transfer.

intermediate

How can you set the SPI speed using spidev?

You set the max_speed_hz attribute of the SpiDev object to the desired speed in hertz. For example, spi.max_speed_hz = 500000 sets the speed to 500 kHz.

beginner

Why is it important to close the SPI connection in spidev?

Closing the SPI connection with spi.close() frees the SPI device and system resources. It helps avoid conflicts if other programs want to use SPI later.

Which method opens the SPI device in spidev?

A start(bus, device)

B open(bus, device)

C connect(bus, device)

D init(bus, device)

The open(bus, device) method is used to open the SPI device on the specified bus and chip select.

What type of data does xfer2() send and receive?

A List of bytes

B String

C Integer

D Boolean

xfer2() sends and receives a list of bytes for SPI communication.

How do you set the SPI clock speed in spidev?

A spi.max_speed_hz = value

B spi.speed = value

C spi.clock = value

D spi.set_speed(value)

The max_speed_hz attribute sets the SPI clock speed in hertz.

Which of these is NOT a typical SPI bus number on Raspberry Pi?

A 0

B 2

C 3

D 1

Raspberry Pi usually has SPI buses 0 and 1; bus 3 is not standard.

Why should you call spi.close() after SPI communication?

A To increase data transfer size

B To speed up the SPI bus

C To reset the SPI device

D To free system resources and avoid conflicts

Closing the SPI connection frees resources and prevents conflicts with other programs.

Explain how to set up and use the spidev library to send data to an SPI device on Raspberry Pi.

Think about the steps from starting communication to ending it.

Describe the role of the xfer2() method in SPI communication using spidev.

Focus on what happens during data transfer.

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