How to Read and Write to SD Card Using Arduino

To read and write to an SD card using Arduino, use the SD library which provides functions like SD.begin() to initialize the card, SD.open() to open files, and file methods like read() and write(). Connect the SD card module to Arduino SPI pins, then use these functions to create, write, and read files on the card.

Syntax

The SD library is used to interact with the SD card. Key functions include:

• SD.begin(chipSelectPin): Initializes the SD card on the specified chip select pin.
• SD.open(filename, mode): Opens a file with a given name and mode (FILE_WRITE or FILE_READ).
• file.print() or file.println(): Writes text to the file.
• file.read(): Reads data from the file.
• file.close(): Closes the file to save changes.

Always check if SD.begin() and SD.open() succeed before proceeding.

arduino

if (!SD.begin(10)) {
  Serial.println("Initialization failed!");
  return;
}
File dataFile = SD.open("example.txt", FILE_WRITE);
if (dataFile) {
  dataFile.println("Hello, SD card!");
  dataFile.close();
} else {
  Serial.println("Error opening file");
}

Example

This example writes a line to a file named test.txt on the SD card, then reads and prints its content to the Serial Monitor.

arduino

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

const int chipSelect = 10;

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

  if (!SD.begin(chipSelect)) {
    Serial.println("SD card initialization failed!");
    return;
  }
  Serial.println("SD card initialized.");

  File myFile = SD.open("test.txt", FILE_WRITE);
  if (myFile) {
    myFile.println("Hello from Arduino!");
    myFile.close();
    Serial.println("Data written to test.txt");
  } else {
    Serial.println("Error opening test.txt for writing");
  }

  myFile = SD.open("test.txt");
  if (myFile) {
    Serial.println("Reading from test.txt:");
    while (myFile.available()) {
      Serial.write(myFile.read());
    }
    myFile.close();
  } else {
    Serial.println("Error opening test.txt for reading");
  }
}

void loop() {
  // Nothing here
}

Output

SD card initialized.
Data written to test.txt
Reading from test.txt:
Hello from Arduino!

Common Pitfalls

Common mistakes when working with SD cards on Arduino include:

• Not initializing the SD card with SD.begin() before accessing files.
• Using the wrong chip select pin; it depends on your Arduino model and wiring.
• Forgetting to close files with file.close(), which can cause data loss.
• Trying to open files without checking if the file opened successfully.
• Using incompatible SD cards (some cards may not work well with Arduino).

Always verify wiring and test with simple read/write code first.

arduino

/* Wrong way: Not checking SD.begin() and file open */
#include <SD.h>
void setup() {
  Serial.begin(9600);
  SD.begin(10); // No check for success
  File f = SD.open("file.txt", FILE_WRITE); // No check
  f.println("Test");
  f.close();
}

/* Right way: Check initialization and file open */
#include <SD.h>
void setup() {
  Serial.begin(9600);
  if (!SD.begin(10)) {
    Serial.println("SD init failed");
    return;
  }
  File f = SD.open("file.txt", FILE_WRITE);
  if (f) {
    f.println("Test");
    f.close();
  } else {
    Serial.println("File open failed");
  }
}

Quick Reference

Here is a quick summary of key functions and their purpose:

Function	Description
SD.begin(chipSelectPin)	Initialize SD card, returns true if successful
SD.open(filename, mode)	Open a file for reading or writing
file.print()/file.println()	Write text to the file
file.read()	Read data from the file
file.close()	Close the file to save changes

Key Takeaways

Always initialize the SD card with SD.begin() and check its success before file operations.

Use SD.open() with proper mode and always check if the file opened successfully.

Close files after writing to ensure data is saved correctly.

Connect the SD card module to the correct SPI pins and chip select pin on your Arduino.

Test with simple read/write examples before adding complexity.