SPI library usage in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this SPI transfer code?

Consider the following Arduino code snippet using the SPI library. What value will be stored in receivedData after the transfer?

Arduino

#include <SPI.h>
byte dataToSend = 0x3C;
byte receivedData;

SPI.begin();
SPI.beginTransaction(SPISettings(4000000, MSBFIRST, SPI_MODE0));
receivedData = SPI.transfer(dataToSend);
SPI.endTransaction();

A Always 0x00 because no slave is connected

B The value received from the SPI slave device during transfer

C The same value as <code>dataToSend</code> (0x3C)

D Compilation error due to missing SPI library include

💡 Hint

Remember that SPI.transfer sends data and simultaneously receives data from the slave device.

The SPI.transfer function sends a byte and returns the byte received from the slave device at the same time. The received value depends on the slave device's response.

🧠 Conceptual

intermediate

Which SPI mode is used in this code snippet?

Given the following SPISettings constructor call, which SPI mode is being set?

SPISettings(1000000, MSBFIRST, SPI_MODE3)

A Clock polarity (CPOL) = 1, Clock phase (CPHA) = 0

B Clock polarity (CPOL) = 0, Clock phase (CPHA) = 0

C Clock polarity (CPOL) = 1, Clock phase (CPHA) = 1

D Clock polarity (CPOL) = 0, Clock phase (CPHA) = 1

💡 Hint

SPI modes are numbered 0 to 3, combining CPOL and CPHA bits.

SPI_MODE3 means CPOL=1 and CPHA=1, which means clock idles high and data is sampled on the trailing edge.

🔧 Debug

advanced

Why does this SPI code cause a runtime error?

Examine this Arduino code snippet:

SPI.beginTransaction(SPISettings(8000000, MSBFIRST, SPI_MODE0));
SPI.transfer(0x55);
SPI.endTransaction();
SPI.transfer(0xAA);

Why does the last SPI.transfer call cause a problem?

A Because <code>SPI.endTransaction()</code> was called, SPI bus is not active for transfer

B Because the SPI clock speed is too high for the device

C Because <code>SPI.begin()</code> was never called

D Because <code>SPI.transfer</code> requires a chip select pin to be set manually

💡 Hint

Check the order of SPI transaction calls and when transfers are allowed.

After SPI.endTransaction(), the SPI bus transaction ends and transfers should not be done outside a transaction.

📝 Syntax

advanced

Which option correctly initializes SPI with a custom chip select pin?

You want to use SPI with a chip select pin connected to pin 10. Which code snippet correctly sets this up?

A pinMode(10, OUTPUT); digitalWrite(10, HIGH); SPI.begin();

B pinMode(10, INPUT); SPI.begin();

C SPI.setCS(10); SPI.begin();

D SPI.begin(10); pinMode(10, OUTPUT);

💡 Hint

Chip select pins must be outputs and set HIGH before SPI communication.

The chip select pin must be set as an output and set HIGH before starting SPI. SPI.begin() does not take parameters.

🚀 Application

expert

How many bytes are transferred in this SPI sequence?

Given this Arduino code:

byte buffer[4] = {0x01, 0x02, 0x03, 0x04};
SPI.beginTransaction(SPISettings(2000000, MSBFIRST, SPI_MODE0));
for (int i = 0; i < 4; i++) {
  buffer[i] = SPI.transfer(buffer[i]);
}
SPI.endTransaction();

How many bytes are sent and received over SPI during this code execution?

A 0 bytes sent and 4 bytes received

B 4 bytes sent and 0 bytes received

C 1 byte sent and 1 byte received

D 4 bytes sent and 4 bytes received

💡 Hint

Each SPI.transfer sends one byte and receives one byte simultaneously.

The loop calls SPI.transfer 4 times, each sending and receiving one byte, so total 4 bytes sent and 4 bytes received.

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