spidev library usage in Raspberry Pi - Step-by-Step Execution

Concept Flow - spidev library usage

Import spidev

↓

Create SPI object

↓

Open SPI bus and device

↓

Set SPI parameters (mode, speed)

↓

Transfer data (write/read)

↓

Close SPI connection

This flow shows how to use spidev: import, open SPI bus, set parameters, transfer data, then close.

Execution Sample

Raspberry Pi

import spidev
spi = spidev.SpiDev()
spi.open(0, 0)
spi.max_speed_hz = 50000
resp = spi.xfer2([0x01, 0x02, 0x03])
spi.close()

This code opens SPI bus 0 device 0, sets speed, sends 3 bytes, receives response, then closes SPI.

Execution Table

Step	Action	SPI Object State	Data Sent	Data Received	Notes
1	Import spidev	No SPI object yet	-	-	Library ready to use
2	Create SPI object	spi created, not open	-	-	spi = spidev.SpiDev()
3	Open SPI bus 0, device 0	spi open on bus 0, device 0	-	-	spi.open(0, 0)
4	Set max speed to 50000 Hz	spi max_speed_hz=50000	-	-	spi.max_speed_hz = 50000
5	Transfer data [0x01,0x02,0x03]	spi open	[1, 2, 3]	[response bytes]	spi.xfer2 sends and receives data
6	Close SPI connection	spi closed	-	-	spi.close()
7	End	spi closed	-	-	No further SPI communication

💡 SPI connection closed, no more data transfer possible

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6
spi	None	SpiDev object created	Opened on bus 0, device 0	max_speed_hz=50000 set	Data transferred, response stored	Closed

Key Moments - 3 Insights

Why do we need to call spi.open(0, 0) before transferring data?

Because spi.open(0, 0) connects to the specific SPI bus and device. Without opening, the SPI object can't communicate. See execution_table step 3 and 5.

What does spi.xfer2([0x01, 0x02, 0x03]) do exactly?

It sends the list of bytes [1, 2, 3] over SPI and simultaneously reads the response bytes from the device. See execution_table step 5.

Why should we call spi.close() at the end?

Closing releases the SPI device so other programs can use it and cleans up resources. See execution_table step 6.

Visual Quiz - 3 Questions

Look at the execution_table at step 5, what data is sent over SPI?

A [1, 2, 3]

B [0x00, 0x00, 0x00]

C [255, 255, 255]

D No data sent

💡 Hint

Check the 'Data Sent' column at step 5 in the execution_table.

At which step is the SPI bus actually opened for communication?

A Step 2

B Step 4

C Step 3

D Step 6

💡 Hint

Look at the 'Action' and 'SPI Object State' columns in execution_table.

If we skip spi.close(), what happens according to the execution flow?

A Data transfer fails

B SPI connection remains open, possibly blocking others

C SPI bus closes automatically immediately

D Program crashes

💡 Hint

Refer to key_moments about why spi.close() is important.

Concept Snapshot

spidev usage steps:
1. import spidev
2. create spi object
3. open SPI bus/device
4. set parameters (speed, mode)
5. transfer data with xfer2()
6. close SPI connection
Always open before transfer and close after done.

Full Transcript

This visual execution shows how to use the spidev library on Raspberry Pi. First, import spidev and create an SPI object. Then open the SPI bus and device you want to communicate with. Set parameters like max speed. Use xfer2() to send and receive data bytes. Finally, close the SPI connection to free resources. The execution table traces each step, showing the SPI object's state and data sent or received. Key moments clarify why opening and closing are necessary and how data transfer works. The quiz tests understanding of these steps and their order.

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