SPI library usage in Arduino - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does SPI stand for and what is its main purpose?

SPI stands for Serial Peripheral Interface. It is used to send data quickly between microcontrollers and small peripherals like sensors, SD cards, or displays.

beginner

How do you start using the SPI library in an Arduino sketch?

You include the SPI library with #include <SPI.h> and then call SPI.begin(); in the setup() function to initialize the SPI bus.

beginner

What is the role of the SPI.transfer() function?

SPI.transfer() sends a byte of data to the SPI device and simultaneously receives a byte back. It is the main way to communicate over SPI.

intermediate

Why do you need to control the Chip Select (CS) pin manually in SPI communication?

The CS pin tells the SPI device when to listen or ignore data. You set it LOW before communication and HIGH after to start and stop talking to the device.

intermediate

How can you change the SPI clock speed and data order in Arduino?

Use SPI.beginTransaction() with an SPISettings object to set clock speed, bit order (MSBFIRST or LSBFIRST), and data mode (clock polarity and phase).

Which Arduino function initializes the SPI bus?

A SPI.init()

B SPI.begin()

C SPI.start()

D SPI.setup()

The correct function to initialize SPI communication is SPI.begin().

What does SPI.transfer() do?

A Only receives data without sending

B Only sends data without receiving

C Sends and receives one byte over SPI

D Resets the SPI bus

SPI.transfer() sends one byte and simultaneously receives one byte from the SPI device.

Why do you set the Chip Select (CS) pin LOW before SPI communication?

A To tell the device to listen

B To reset the device

C To power off the device

D To end communication

Setting CS LOW signals the device to listen and start communication.

Which of these is NOT a parameter you can set with SPISettings?

A Clock speed

B Bit order

C Data mode

D Baud rate for UART

Baud rate is for UART, not SPI. SPISettings controls clock speed, bit order, and data mode.

What must you do after finishing SPI communication with a device?

A Set CS pin HIGH

B Call SPI.end()

C Reset the Arduino

D Call SPI.transfer(0)

Setting the CS pin HIGH tells the device communication is finished.

Explain the basic steps to communicate with a device using the SPI library on Arduino.

Think about setup, starting communication, sending data, and ending communication.

Describe how to configure SPI speed and data order for a device that requires specific settings.

Focus on SPISettings and transaction control.

Practice

1. What is the main purpose of the SPI.begin() function in Arduino's SPI library?

easy

A. To close the SPI communication

B. To initialize the SPI bus and prepare it for communication

C. To set the SPI clock speed

D. To send a byte of data over SPI

Solution

1. Step 1: Understand the role of SPI.begin()

   The SPI.begin() function sets up the SPI hardware and pins for communication.

2. Step 2: Differentiate from other SPI functions

   Functions like SPI.transfer() send data, while SPI.beginTransaction() sets speed and mode, but SPI.begin() only initializes.

3. Final Answer:

   To initialize the SPI bus and prepare it for communication -> Option B

4. Quick Check:

   SPI.begin() initializes SPI [OK]

Hint: Remember: begin() always starts or initializes [OK]

Common Mistakes:

• Confusing SPI.begin() with SPI.transfer()
• Thinking SPI.begin() sets speed or mode
• Assuming SPI.begin() closes communication

2. Which of the following is the correct syntax to send a byte of data using the SPI library in Arduino?

easy

A. SPI.send(0xFF);

B. SPI.sendByte(0xFF);

C. SPI.write(0xFF);

D. SPI.transfer(0xFF);

Solution

1. Step 1: Identify the correct SPI function for sending data

   The Arduino SPI library uses SPI.transfer() to send and receive a byte simultaneously.

2. Step 2: Check syntax correctness

   Only SPI.transfer(0xFF); matches the correct function and parameter format.

3. Final Answer:

   SPI.transfer(0xFF); -> Option D

4. Quick Check:

   SPI.transfer() sends data byte [OK]

Hint: Use transfer() to send and receive SPI bytes [OK]

Common Mistakes:

• Using non-existent functions like send() or sendByte()
• Confusing SPI.transfer() with SPI.begin()
• Using write() which is not part of SPI library

3. What will be the output of this Arduino code snippet using SPI?

SPI.begin();
byte response = SPI.transfer(0xAA);
Serial.println(response, HEX);

Assuming the SPI slave device echoes back the sent byte.

medium

A. AA

B. 55

C. 00

D. FF

Solution

1. Step 1: Understand SPI.transfer() behavior

   The SPI.transfer() sends a byte and simultaneously receives a byte from the slave device.

2. Step 2: Analyze the slave device response

   The slave echoes back the sent byte 0xAA, so response will be 0xAA.

3. Final Answer:

   AA -> Option A

4. Quick Check:

   Sent byte echoed back = AA [OK]

Hint: SPI.transfer() returns the received byte immediately [OK]

Common Mistakes:

• Assuming transfer() only sends without receiving
• Expecting a different response byte
• Confusing HEX output with decimal

4. Identify the error in this Arduino SPI code snippet:

SPI.beginTransaction(SPISettings(4000000, MSBFIRST, SPI_MODE0));
SPI.transfer(0x01);
SPI.endTransaction();
SPI.begin();

medium

A. SPI.begin() should be called before SPI.beginTransaction()

B. SPI.endTransaction() is missing

C. SPI.transfer() cannot be used inside a transaction

D. SPISettings parameters are incorrect

Solution

1. Step 1: Check SPI initialization order

   SPI.begin() must be called once before starting any transaction to initialize SPI hardware.

2. Step 2: Verify transaction usage

   The code calls SPI.beginTransaction() before SPI.begin(), which is incorrect and causes errors.

3. Final Answer:

   SPI.begin() should be called before SPI.beginTransaction() -> Option A

4. Quick Check:

   Initialize SPI first with begin() [OK]

Hint: Always call SPI.begin() before transactions [OK]

Common Mistakes:

• Calling beginTransaction() before begin()
• Forgetting to call endTransaction()
• Misunderstanding SPISettings parameters

5. You want to communicate with two SPI devices on the same Arduino. How should you manage SPI transactions to avoid conflicts?

hard

A. Use SPI.transfer() without transactions for both devices

B. Call SPI.begin() twice, once for each device

C. Call SPI.beginTransaction() with each device's settings before communication and SPI.endTransaction() after

D. Connect both devices to the same chip select pin

Solution

1. Step 1: Understand SPI multi-device communication

   Each SPI device may require different speed or mode settings, so transactions help manage these safely.

2. Step 2: Use transactions properly

   Call SPI.beginTransaction() with the device's settings before communicating, then SPI.endTransaction() after to avoid conflicts.

3. Final Answer:

   Call SPI.beginTransaction() with each device's settings before communication and SPI.endTransaction() after -> Option C

4. Quick Check:

   Use transactions per device to avoid conflicts [OK]

Hint: Use beginTransaction/endTransaction per device [OK]

Common Mistakes:

• Calling SPI.begin() multiple times
• Sharing chip select pins for different devices
• Skipping transactions causing wrong settings