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.