SPI with display modules in Raspberry Pi - Step-by-Step Execution

Concept Flow - SPI with display modules

Start SPI Setup

↓

Configure SPI Pins

↓

Initialize SPI Bus

↓

Send Commands/Data to Display

↓

Display Updates

↓

End or Repeat

This flow shows how SPI communication is set up and used to send data to a display module step-by-step.

Execution Sample

Raspberry Pi

import spidev
spi = spidev.SpiDev()
spi.open(0, 0)
spi.max_speed_hz = 500000
spi.xfer2([0xAE])  # Display OFF command

This code opens SPI bus 0, device 0, sets speed, and sends a command to turn the display off.

Execution Table

Step	Action	SPI Bus State	Data Sent	Result
1	Open SPI bus 0, device 0	Bus open, mode default	None	SPI ready for communication
2	Set max speed to 500000 Hz	Bus open, speed=500000	None	Speed configured
3	Send command 0xAE (Display OFF)	Bus open, speed=500000	[0xAE]	Display receives OFF command
4	Close or continue communication	Bus open	None	Ready for next commands or close

💡 Communication ends or repeats for more display updates

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
spi	None	SpiDev object open	Speed set to 500000	Data sent [0xAE]	Ready for next commands

Key Moments - 3 Insights

Why do we set spi.max_speed_hz before sending data?

Setting spi.max_speed_hz configures how fast data is sent over SPI. Without it, the default speed might be too slow or too fast for the display. See execution_table step 2.

What does spi.xfer2([0xAE]) do exactly?

spi.xfer2 sends the list of bytes over SPI and waits for the transfer to finish. Here, it sends the command 0xAE to turn the display off. See execution_table step 3.

Can we send data without opening the SPI bus first?

No, the SPI bus must be opened with spi.open before sending data. Otherwise, communication will fail. See execution_table step 1.

Visual Quiz - 3 Questions

Look at the execution_table, what is the SPI bus speed after step 2?

A Default speed

B 500000 Hz

C 1000000 Hz

D 0 Hz

💡 Hint

Check the 'SPI Bus State' column at step 2 in execution_table

At which step is the command to turn the display off sent?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Data Sent' column in execution_table

If spi.open was not called, what would happen when sending data?

A An error occurs because SPI bus is not open

B Data sends normally

C Data sends but very slowly

D SPI bus opens automatically

💡 Hint

Refer to key_moments about opening SPI bus before sending data

Concept Snapshot

SPI with display modules:
- Open SPI bus with spi.open(bus, device)
- Set speed with spi.max_speed_hz
- Send commands/data with spi.xfer2([bytes])
- Commands control display behavior
- Always open bus before sending data

Full Transcript

This visual execution shows how to use SPI communication on a Raspberry Pi to control a display module. First, the SPI bus is opened with spi.open(0, 0). Then, the speed is set to 500000 Hz using spi.max_speed_hz. Next, a command byte 0xAE is sent to the display using spi.xfer2, which turns the display off. The variable 'spi' changes from None to an open SPI object, then has speed set, and finally sends data. Key points include setting speed before sending data, sending commands with xfer2, and opening the SPI bus first. The quiz checks understanding of these steps and their order.

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