SPI with SD card module in Arduino - Mini Project: Build & Apply

SPI with SD card module

📖 Scenario: You want to save some text data to an SD card using an Arduino and an SD card module connected via SPI. This is useful for logging sensor data or storing information without a computer.

🎯 Goal: Build a simple Arduino program that initializes the SD card using SPI, creates a file, writes a line of text to it, and then closes the file.

📋 What You'll Learn

Use the SPI library to communicate with the SD card module

Use the SD library to handle the SD card

Initialize the SD card on chip select pin 10

Create a file named log.txt on the SD card

Write the text Hello, SD card! to the file

Close the file after writing

Print SD card initialized. if initialization succeeds

Print Initialization failed! if initialization fails

💡 Why This Matters

🌍 Real World

Logging sensor data or events to an SD card for later analysis without needing a computer connected.

💼 Career

Many embedded systems and IoT devices use SD cards for data storage. Knowing how to use SPI and SD libraries is essential for hardware programming jobs.

1

Setup SPI and SD card libraries

Include the SPI.h and SD.h libraries at the top of your sketch. Then create a constant integer called chipSelect and set it to 10.

Arduino

# Include SPI and SD libraries
// Your code here

Hint

Use #include <SPI.h> and #include <SD.h> to include the libraries. Then write const int chipSelect = 10;.

2

Initialize the SD card in setup()

Write the setup() function. Inside it, start serial communication at 9600 baud with Serial.begin(9600). Then initialize the SD card with SD.begin(chipSelect). If initialization succeeds, print SD card initialized. to serial. Otherwise, print Initialization failed!.

Arduino

#include 
#include 

const int chipSelect = 10;

void setup() {
  // Your code here
}

void loop() {
  // leave empty
}

Hint

Use Serial.begin(9600); to start serial. Then check if (SD.begin(chipSelect)) to test SD card. Use Serial.println() to print messages.

3

Create and write to a file on the SD card

Inside the setup() function, after SD card initialization, create a File object named myFile by opening log.txt in write mode using SD.open("log.txt", FILE_WRITE). Then write the text Hello, SD card! to myFile. Finally, close myFile.

Arduino

#include 
#include 

const int chipSelect = 10;

void setup() {
  Serial.begin(9600);
  if (SD.begin(chipSelect)) {
    Serial.println("SD card initialized.");
    // Your code here
  } else {
    Serial.println("Initialization failed!");
  }
}

void loop() {
  // leave empty
}

Hint

Use File myFile = SD.open("log.txt", FILE_WRITE); to open the file. Then write with myFile.println("Hello, SD card!"); and close with myFile.close();.

4

Print confirmation after writing to the file

After closing the file, print Data written to log.txt to the serial monitor inside the setup() function.

Arduino

#include 
#include 

const int chipSelect = 10;

void setup() {
  Serial.begin(9600);
  if (SD.begin(chipSelect)) {
    Serial.println("SD card initialized.");
    File myFile = SD.open("log.txt", FILE_WRITE);
    if (myFile) {
      myFile.println("Hello, SD card!");
      myFile.close();
      // Your code here
    }
  } else {
    Serial.println("Initialization failed!");
  }
}

void loop() {
  // leave empty
}

Hint

Use Serial.println("Data written to log.txt"); after closing the file.

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