Writing data to SD card in Arduino

Introduction

Writing data to an SD card lets your Arduino save information even when it is turned off. This is useful for keeping records or logs.

You want to save sensor readings over time to check later.

You need to store user settings that stay after power off.

You want to log events or errors during your Arduino program.

You want to save data for a project that runs without a computer.

Syntax

Arduino

File dataFile = SD.open("filename.txt", FILE_WRITE);
if (dataFile) {
  dataFile.println("Your data here");
  dataFile.close();
} else {
  // error opening file
}

Use SD.open() with FILE_WRITE to open or create a file for writing.

Always close the file after writing to save data properly.

Examples

This writes a line of text to a file named log.txt on the SD card.

Arduino

File dataFile = SD.open("log.txt", FILE_WRITE);
if (dataFile) {
  dataFile.println("Hello, SD card!");
  dataFile.close();
}

This writes two numbers separated by a comma, like a CSV row.

Arduino

File dataFile = SD.open("data.csv", FILE_WRITE);
if (dataFile) {
  dataFile.print(23);
  dataFile.print(",");
  dataFile.println(42);
  dataFile.close();
}

Sample Program

This program initializes the SD card, then writes a line of text to a file named test.txt. It prints messages to the Serial Monitor to show progress.

Arduino

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

const int chipSelect = 10;

void setup() {
  Serial.begin(9600);
  while (!Serial) {}

  Serial.print("Initializing SD card...");
  if (!SD.begin(chipSelect)) {
    Serial.println("Initialization failed!");
    return;
  }
  Serial.println("Initialization done.");

  File dataFile = SD.open("test.txt", FILE_WRITE);
  if (dataFile) {
    dataFile.println("Data saved to SD card.");
    dataFile.close();
    Serial.println("Data written to test.txt");
  } else {
    Serial.println("Error opening test.txt");
  }
}

void loop() {
  // nothing here
}

Important Notes

Make sure your SD card module is connected correctly to the Arduino pins.

Use the correct chip select pin number for your hardware.

Always check if the file opened successfully before writing.

Summary

Use SD.open() with FILE_WRITE to write data to a file on the SD card.

Always close the file after writing to save your data.

Check SD card initialization and file opening to avoid errors.

Practice

1. What is the correct way to open a file for writing data on an SD card in Arduino?

easy

A. Use SD.close(filename)

B. Use SD.read(filename)

C. Use SD.open(filename, FILE_WRITE)

D. Use SD.format()

Solution

1. Step 1: Understand file opening modes

   To write data, the file must be opened in write mode, which is done using FILE_WRITE.

2. Step 2: Identify the correct function call

   The function SD.open(filename, FILE_WRITE) opens the file for writing. Other options do not open files for writing.

3. Final Answer:

   Use SD.open(filename, FILE_WRITE) -> Option C

4. Quick Check:

   Open file for writing = SD.open(filename, FILE_WRITE) [OK]

Hint: Use FILE_WRITE with SD.open() to write files [OK]

Common Mistakes:

• Using SD.read() to open file for writing
• Forgetting FILE_WRITE mode
• Trying to write without opening file

2. Which of the following is the correct syntax to write the string "Hello" followed by a newline to a file named "log.txt" on the SD card?

easy

A. file.write("Hello");

B. file.println("Hello");

C. file.print("Hello");

D. file.read("Hello");

Solution

1. Step 1: Identify the write functions

   To write text with a newline, file.println() is used. file.print() writes without newline, file.write() writes bytes.

2. Step 2: Choose the function that writes string with newline

   file.println("Hello") writes "Hello" followed by a newline, which is common for logging.

3. Final Answer:

   file.println("Hello"); -> Option B

4. Quick Check:

   Write string with newline = file.println() [OK]

Hint: Use file.println() to write string with newline [OK]

Common Mistakes:

• Using file.read() to write data
• Confusing file.write() with file.print()
• Forgetting to add newline if needed

3. What will be the output on the SD card file after running this code snippet?

File file = SD.open("data.txt", FILE_WRITE);
if (file) {
  file.println("Line 1");
  file.println("Line 2");
  file.close();
}

medium

A. No output because file.close() is missing

B. Line 1\nLine 2\n

C. Line 1Line 2

D. Line 1
Line 2

Solution

1. Step 1: Understand file writing with println()

   Each call to file.println() writes the string plus a newline character.

2. Step 2: Check file closing

   The file is closed properly with file.close(), so data is saved.

3. Final Answer:

   Line 1 Line 2 -> Option D

4. Quick Check:

   file.println() adds newline, file.close() saves data [OK]

Hint: file.println() adds newline; always close file to save [OK]

Common Mistakes:

• Thinking file.close() is missing
• Assuming println() does not add newline
• Confusing escape sequences with actual newlines

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

if (!SD.begin()) {
  Serial.println("Initialization failed!");
}
File file = SD.open("test.txt", FILE_WRITE);
file.println("Data");
// Missing file.close()

medium

A. File is not closed after writing, risking data loss

B. Serial.println() cannot be used with SD cards

C. FILE_WRITE is not a valid mode for SD.open()

D. SD.begin() should be called after opening the file

Solution

1. Step 1: Check SD initialization

   SD.begin() is correctly called before opening the file.

2. Step 2: Identify missing file.close()

   The file is opened and written to, but not closed, which can cause data not to be saved properly.

3. Final Answer:

   File is not closed after writing, risking data loss -> Option A

4. Quick Check:

   Always close file after writing to save data [OK]

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

Common Mistakes:

• Forgetting to close the file
• Calling SD.begin() after opening file
• Misunderstanding FILE_WRITE mode

5. You want to write multiple sensor readings to a file on the SD card, but only if the SD card initializes successfully. Which of the following code snippets correctly implements this logic?

hard

A. if (SD.begin()) {
File dataFile = SD.open("readings.txt", FILE_WRITE);
if (dataFile) {
dataFile.println(sensorValue);
dataFile.close();
}
} else {
Serial.println("SD init failed");
}

B. File dataFile = SD.open("readings.txt", FILE_WRITE);
if (dataFile) {
if (SD.begin()) {
dataFile.println(sensorValue);
}
dataFile.close();
}

C. if (!SD.begin()) {
Serial.println("SD init failed");
} else {
File dataFile = SD.open("readings.txt");
dataFile.println(sensorValue);
dataFile.close();
}

D. SD.begin();
File dataFile = SD.open("readings.txt", FILE_WRITE);
dataFile.println(sensorValue);
dataFile.close();

Solution

1. Step 1: Check SD initialization before file operations

   The correct code first calls and checks SD.begin() before attempting to open the file.

2. Step 2: Proper file open, write, close, and error handling

   Open with SD.open("readings.txt", FILE_WRITE), verify with if (dataFile), write dataFile.println(sensorValue), dataFile.close(), and print error if init fails.

3. Final Answer:

   if (SD.begin()) { File dataFile = SD.open("readings.txt", FILE_WRITE); if (dataFile) { dataFile.println(sensorValue); dataFile.close(); } } else { Serial.println("SD init failed"); } -> Option A

4. Quick Check:

   Check SD.begin() first, then open, write, close file [OK]

Hint: Always check SD.begin() before file operations [OK]

Common Mistakes:

• Opening file before SD.begin()
• Not closing file after writing
• Ignoring SD initialization failure