SPI with display modules in Raspberry Pi - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this SPI initialization code?

Consider this Python code snippet for initializing SPI on a Raspberry Pi to communicate with a display module. What will be printed when this code runs?

Raspberry Pi

import spidev
spi = spidev.SpiDev()
spi.open(0, 0)
spi.max_speed_hz = 500000
print(spi.max_speed_hz)

A 500000

B 0

C None

D SyntaxError

💡 Hint

Look at the value assigned to max_speed_hz and what is printed.

The code sets spi.max_speed_hz to 500000 and then prints it, so the output is 500000.

🧠 Conceptual

intermediate

Which SPI mode is commonly used for most display modules?

SPI devices use modes 0 to 3 to define clock polarity and phase. Which SPI mode is most commonly compatible with typical SPI display modules?

A Mode 2 (CPOL=1, CPHA=0)

B Mode 1 (CPOL=0, CPHA=1)

C Mode 0 (CPOL=0, CPHA=0)

D Mode 3 (CPOL=1, CPHA=1)

💡 Hint

Most SPI displays use clock polarity and phase starting with clock low and sampling on the first clock edge.

Mode 0 is the default SPI mode for many display modules, meaning clock polarity 0 and clock phase 0.

🔧 Debug

advanced

Why does this SPI display code raise an error?

This code tries to send data to an SPI display but raises an error. What is the cause?

Raspberry Pi

import spidev
spi = spidev.SpiDev()
spi.open(0)
spi.xfer2([0xAA, 0xBB])

A TypeError because open() requires two arguments: bus and device

B AttributeError because xfer2() is not a method of SpiDev

C RuntimeError because SPI device is busy

D ValueError because data list contains invalid bytes

💡 Hint

Check the parameters required by spi.open() method.

spi.open() requires two arguments: bus and device. Calling it with one argument causes a TypeError.

📝 Syntax

advanced

Which option correctly sends a command byte 0x01 followed by data bytes [0x02, 0x03] to an SPI display?

Select the correct Python code snippet using spidev to send a command byte followed by data bytes over SPI.

A spi.writebytes([0x01]); spi.writebytes([0x02, 0x03])

B spi.send([0x01, 0x02, 0x03])

C spi.xfer2(0x01, [0x02, 0x03])

D spi.xfer2([0x01, 0x02, 0x03])

💡 Hint

xfer2() takes a list of bytes to send in one call.

spi.xfer2([0x01, 0x02, 0x03]) sends all bytes in one SPI transaction. Other options are invalid or incorrect method usage.

🚀 Application

expert

How many bytes are sent over SPI in this code snippet?

Given this code sending data to an SPI display, how many bytes are transmitted in total?

Raspberry Pi

import spidev
spi = spidev.SpiDev()
spi.open(0, 0)
data = [0x10, 0x20, 0x30]
for byte in data:
    spi.xfer2([byte])

A 1

B 3

C 0

D 6

💡 Hint

Each call to xfer2 sends one byte in a list; the loop runs three times.

The loop sends one byte per iteration, three iterations total, so 3 bytes are sent.

Practice

1. What is the main purpose of using SPI with display modules on a Raspberry Pi?

easy

A. To power the display module

B. To connect the Raspberry Pi to the internet

C. To store files on the Raspberry Pi

D. To send commands and data quickly to the display

Solution

1. Step 1: Understand SPI role

   SPI is a communication protocol used to send data between devices quickly.

2. Step 2: Connect SPI to display modules

   On Raspberry Pi, SPI sends commands and data to control display modules efficiently.

3. Final Answer:

   To send commands and data quickly to the display -> Option D

4. Quick Check:

   SPI = fast data transfer to display [OK]

Hint: SPI is for fast data transfer to devices like displays [OK]

Common Mistakes:

• Confusing SPI with internet connection
• Thinking SPI stores files
• Assuming SPI powers devices

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

easy

A. spi = spidev.SpiDev(); spi.open(0, 0)

B. spi = spidev.open(1, 0)

C. spi = spidev.open(0, 0)

D. spi = spidev.SpiDev.open(0, 0)

Solution

1. Step 1: Recall spidev usage

   spidev requires creating an SpiDev object first, then calling open(bus, device).

2. Step 2: Check syntax correctness

   spi = spidev.SpiDev(); spi.open(0, 0) correctly creates spi = spidev.SpiDev() then calls spi.open(0, 0).

3. Final Answer:

   spi = spidev.SpiDev(); spi.open(0, 0) -> Option A

4. Quick Check:

   spidev object then open() method [OK]

Hint: Create SpiDev object before calling open() [OK]

Common Mistakes:

• Calling spidev.open() directly without object
• Using wrong bus or device numbers
• Using incorrect method syntax

3. What will be the output of this Python code snippet using spidev?

import spidev
spi = spidev.SpiDev()
spi.open(0, 0)
response = spi.xfer2([0xAA, 0x55])
spi.close()
print(response)

medium

A. [170, 85]

B. Depends on the connected SPI device response

C. An error because xfer2 needs a string

D. [0, 0]

Solution

1. Step 1: Understand xfer2 function

   spi.xfer2 sends bytes and returns the response bytes from the SPI device.

2. Step 2: Recognize device dependency

   The returned list depends on the connected device's response, so output varies.

3. Final Answer:

   Depends on the connected SPI device response -> Option B

4. Quick Check:

   SPI response varies by device [OK]

Hint: SPI transfer output depends on device response [OK]

Common Mistakes:

• Assuming output equals input bytes
• Thinking xfer2 requires string input
• Expecting fixed output without device

4. You wrote this code to send a command to your SPI display but it raises an AttributeError:

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

What is the error and how to fix it?

medium

A. spidev has no open() function; create SpiDev object first

B. xfer2() requires a string, not a list

C. spi.close() should be called before xfer2()

D. Bus number 0 is invalid on Raspberry Pi

Solution

1. Step 1: Identify AttributeError cause

   spidev module does not have open() function; open() is method of SpiDev object.

2. Step 2: Correct usage of spidev

   Create spi = spidev.SpiDev() first, then call spi.open(0, 0) to fix error.

3. Final Answer:

   spidev has no open() function; create SpiDev object first -> Option A

4. Quick Check:

   Use SpiDev() object before open() [OK]

Hint: Always create SpiDev() object before open() [OK]

Common Mistakes:

• Calling spidev.open() directly
• Passing wrong data type to xfer2()
• Closing SPI before transfer

5. You want to send a command byte 0xA5 followed by data bytes [0x01, 0x02, 0x03] to an SPI display using spidev. Which code correctly sends the command first, then the data, assuming spi is already opened?

hard

A. spi.writebytes([0xA5, 0x01, 0x02, 0x03])

B. spi.xfer2([0xA5, 0x01, 0x02, 0x03])

C. spi.xfer2([0xA5]); spi.xfer2([0x01, 0x02, 0x03])

D. spi.writebytes([0xA5]); spi.writebytes([0x01, 0x02, 0x03])

Solution

1. Step 1: Understand command and data separation

   Many SPI displays require sending command and data separately to distinguish them.

2. Step 2: Check code options

   spi.xfer2([0xA5]); spi.xfer2([0x01, 0x02, 0x03]) sends command byte first, then data bytes in separate calls, matching typical display protocol.

3. Final Answer:

   spi.xfer2([0xA5]); spi.xfer2([0x01, 0x02, 0x03]) -> Option C

4. Quick Check:

   Separate command and data transfers [OK]

Hint: Send command and data in separate SPI transfers [OK]

Common Mistakes:

• Sending command and data in one transfer
• Using writebytes() which may not return response
• Not separating command from data