SPI with SD card module in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this SPI initialization code?

Consider the following Arduino code snippet initializing SPI for an SD card module. What will be printed on the Serial Monitor?

Arduino

void setup() {
  Serial.begin(9600);
  if (!SD.begin(10)) {
    Serial.println("Initialization failed!");
  } else {
    Serial.println("Initialization done.");
  }
}

void loop() {}

A Initialization done.

B Initialization failed!

C No output, program stuck.

D Compilation error.

💡 Hint

Check if the SD card module is connected correctly and the chip select pin matches.

If the SD card is not connected or the chip select pin is wrong, SD.begin() returns false, printing "Initialization failed!".

🧠 Conceptual

intermediate

Which SPI mode is typically used by SD card modules?

SPI communication has four modes (0 to 3) defined by clock polarity and phase. Which mode is standard for SD card modules?

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

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

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

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

💡 Hint

SD cards use SPI mode where clock idles low and data is sampled on the rising edge.

SD cards use SPI Mode 0, where clock polarity is 0 (idle low) and clock phase is 0 (data sampled on rising edge).

🔧 Debug

advanced

Why does this SD card write code fail to save data?

Examine the code below that attempts to write "Hello" to a file on the SD card. Why does it fail to save the data?

Arduino

#include <SPI.h>
#include <SD.h>

File myFile;

void setup() {
  Serial.begin(9600);
  if (!SD.begin(10)) {
    Serial.println("SD init failed");
    return;
  }
  myFile = SD.open("test.txt", FILE_WRITE);
  if (myFile) {
    myFile.print("Hello");
    Serial.println("Write done");
    myFile.close();
  } else {
    Serial.println("Error opening file");
  }
}

void loop() {}

A The SPI library is missing initialization.

B The chip select pin 10 is incorrect for SD.begin().

C The file mode FILE_WRITE is invalid.

D The file is not closed after writing, so data is not saved.

💡 Hint

Think about what happens after writing to a file on SD cards.

Data is buffered and only saved when the file is closed. Missing myFile.close() causes data loss.

📝 Syntax

advanced

What error does this SPI transfer code produce?

Look at the code snippet below. What error will it cause when compiled?

Arduino

byte data = SPI.transfer();

A Runtime error: null pointer exception

B No error, returns 0 by default

C Compilation error: missing argument for SPI.transfer()

D Compilation error: SPI library not included

💡 Hint

Check the required parameters for SPI.transfer().

SPI.transfer() requires one byte argument to send. Calling it without arguments causes a compile error.

🚀 Application

expert

How many files are created by this SD card code?

Given the code below, how many files will be created on the SD card after running setup() once?

Arduino

#include <SPI.h>
#include <SD.h>

void setup() {
  Serial.begin(9600);
  SD.begin(10);
  for (int i = 0; i < 3; i++) {
    String filename = "file" + String(i) + ".txt";
    File f = SD.open(filename.c_str(), FILE_WRITE);
    if (f) {
      f.println("Data " + String(i));
      f.close();
    }
  }
}

void loop() {}

A 3 files named file0.txt, file1.txt, file2.txt

B No files created due to missing SD card check

C 1 file named file2.txt only

D Infinite files created due to loop error

💡 Hint

Look at the loop and how filenames are generated.

The loop runs 3 times creating 3 different files with names file0.txt, file1.txt, and file2.txt.

Practice

1. What is the main purpose of using SPI when working with an SD card module on Arduino?

easy

A. To enable fast communication between Arduino and the SD card

B. To power the SD card module

C. To store data permanently on the Arduino board

D. To convert analog signals to digital

Solution

1. Step 1: Understand SPI role

   SPI is a communication protocol that allows fast data transfer between devices.

2. Step 2: Connect Arduino and SD card

   SPI connects Arduino to the SD card module to send and receive data quickly.

3. Final Answer:

   To enable fast communication between Arduino and the SD card -> Option A

4. Quick Check:

   SPI = fast communication [OK]

Hint: SPI is for fast data transfer, not power or storage [OK]

Common Mistakes:

• Confusing SPI with power supply
• Thinking SPI stores data on Arduino
• Mixing SPI with analog-to-digital conversion

2. Which of the following is the correct way to initialize the SD card module using SPI on Arduino if the chip select pin is 10?

easy

A. SD.start(10);

B. SD.begin(10);

C. SD.open(10);

D. SD.init(10);

Solution

1. Step 1: Recall SD library initialization

   The SD library uses SD.begin(chipSelectPin) to start communication.

2. Step 2: Match correct function and parameter

   SD.begin(10) correctly initializes the SD card with pin 10 as chip select.

3. Final Answer:

   SD.begin(10); -> Option B

4. Quick Check:

   SD.begin(pin) initializes SD card [OK]

Hint: Use SD.begin() with chip select pin to start SD card [OK]

Common Mistakes:

• Using SD.start() which does not exist
• Confusing SD.open() with initialization
• Using SD.init() which is not a valid method

3. What will be the output on the Serial Monitor after running this Arduino code snippet?

if (SD.begin(10)) {
  Serial.println("SD card initialized.");
} else {
  Serial.println("Initialization failed.");
}

medium

A. SD card initialized.

B. Initialization failed.

C. Compilation error

D. No output

Solution

1. Step 1: Understand SD.begin() return value

   SD.begin(10) returns true if the SD card initializes successfully.

2. Step 2: Check the if condition and output

   If true, it prints "SD card initialized." to Serial Monitor.

3. Final Answer:

   SD card initialized. -> Option A

4. Quick Check:

   SD.begin(10) true prints initialization message [OK]

Hint: SD.begin() returns true if SD card is ready [OK]

Common Mistakes:

• Assuming SD.begin() always fails
• Expecting no output without Serial.begin() (assumed present)
• Confusing SD.open() with SD.begin()

4. Identify the error in this Arduino code snippet for writing to an SD card:

SD.begin(10);
File dataFile = SD.open("log.txt", FILE_WRITE);
if (dataFile) {
  dataFile.println("Hello World");
}
// Missing dataFile.close();

medium

A. SD.open() is used incorrectly

B. Missing SD.begin() before opening file

C. println() cannot be used with File

D. File is not closed after writing

Solution

1. Step 1: Check file handling steps

   After writing to a file, it must be closed to save data properly.

2. Step 2: Identify missing close()

   The code writes but does not call dataFile.close(), risking data loss.

3. Final Answer:

   File is not closed after writing -> Option D

4. Quick Check:

   Always close files after writing [OK]

Hint: Always close the file after writing to save data [OK]

Common Mistakes:

• Forgetting to call SD.begin() before open
• Thinking println() is invalid for File
• Ignoring file close causes data loss

5. You want to write multiple sensor readings to an SD card file named "data.txt" every 5 seconds. Which approach correctly uses SPI with the SD card module to do this efficiently?

hard

A. Call SD.begin(10) once in setup(), open the file, write readings, close file each time in loop()

B. Call SD.begin(10) once in setup(), open file once in setup(), write readings and close file each time in loop()

C. Call SD.begin(10) once in setup(), open and close the file each time in loop() before writing readings

D. Call SD.begin(10) inside loop(), open file once in setup(), write readings without closing file

Solution

1. Step 1: Understand SD.begin() usage

   SD.begin() should be called once in setup() to initialize the card.

2. Step 2: Manage file open/close in loop()

   Opening and closing the file each time in loop() ensures data is saved and file is not locked.

3. Step 3: Avoid keeping file open across loops

   Keeping file open without closing risks data loss or corruption if power fails.

4. Final Answer:

   Call SD.begin(10) once in setup(), open and close the file each time in loop() before writing readings -> Option C

5. Quick Check:

   Initialize once, open/close file each write [OK]

Hint: Init SD once; open and close file each write cycle [OK]

Common Mistakes:

• Calling SD.begin() repeatedly wastes time
• Leaving file open across loop iterations
• Opening file only once and never closing