Reading and Writing Files from an SD Card with an Arduino

You can use an SD card with your Arduino system to store and retrieve information

In some Arduino applications, it is advantageous to be able to store and retrieve information locally. You can do this with a Secure Digital, or SD, card. An SD card is a non-volatile memory card used extensively in portable devices, such as mobile phones, digital cameras, GPS navigation devices, handheld consoles, and tablet computers. Another type of SD Card is the Micro SD card. Measuring only 15 mm x 11 mm x 1 mm, it is the smallest memory card available. It is about one-quarter the size of a normal-sized SD card, or about the size of a fingernail.

To connect a micro SD card to our Arduino Mega, we will use an Ethernet shield with a micro SD slot on it. There are, however, many different shields available for other types of SD cards.

 

As shown in the figure above, a micro SD card has 8 pins. The table below describes the function of each pin.

Pin   Name     Description

1     NC       not connected

2     CS       Chip Select/Slave Select (SS)

3     DI         Master Out/Slave In (MOSI)

4     VDD      Supply voltage

5     CLK      Clock (SCK)

6     VSS      Supply voltage ground

7     DO       Master In/Slave Out (MISO)

8     RSV      Reserved

If you were to try interfacing this SD card yourself, you would have to ensure that you connected the pins of the SD card to the appropriate pins of the Arduino. Since we are using a commercially-available shield, this is not an issue. All we need to do is to declare the default CS (chip select) pin of the Arduino as OUTPUT. This is pin 53 on our Arduino MEGA. On the Ethernet shield, the CS pin is pin number 4. You need to specify this in the code for the SD card to work properly.

 

Experiment 1

In this experiment, we will learn how to read a file from the SD card.

 

Hardware Required

• 1 x micro SD card
  
• 1 x Ethernet shield module
  
• 1 x Arduino Mega2560
  

Arduino MEGA with Ethernet shield installed

Code

To read from the SD card, we will use the SD.h library. This code assumes that the file "ourfile.txt" has already been written to the SD card.

#include <SD.h>

const int cs = 4;

void setup()
{
  Serial.begin(9600);
  



  Serial.print("Initializing card...");
  // make sure that the default chip select pin is declared OUTPUT
  
  pinMode(53, OUTPUT);
  
  // see if the card is present
  if (!SD.begin(cs)) 
  {
    Serial.println("Card failed to initialize, or not present");
  
    return;
  }
  Serial.println("card initialized.");
  
  // open the file named ourfile.txt
  
  File myfile = SD.open("ourfile.txt");

  // if the file is available, read the file
  if (myfile) 
  {
    while (myfile.available())
    {
      Serial.write(myfile.read());
    }
    myfile.close();
  }  
  // if the file cannot be opened give error report
  else {
    Serial.println("error opening the text file");
  } 
}

void loop()
{
}

Reading_and_Writing.zip

Experiment 2

In this experiment, we will learn how to create a file, write it, and then read it from SD card.

Hardware Required

We will use the same hardware as the previous experiment

Code

To write a file to the SD card and to read that file, we will again use the SD.h library.

#include <SD.h>

File myfile;

void setup()
{
 
  Serial.begin(9600);
  
  Serial.print("Initializing card...");
  
  // declare default CS pin as OUTPUT
   pinMode(53, OUTPUT);
   
  if (!SD.begin(4)) {
    Serial.println("initialization of the SD card failed!");
    return;
  }
  Serial.println("initialization of the SDcard is done.");
  
 
  myfile = SD.open("textFile.txt", FILE_WRITE);
  
  
  if (myfile)
  {
    Serial.print("Writing to the text file...");
    myfile.println("Congratulations! You have successfully wrote on the text file.");
	
    myfile.close(); // close the file:
    Serial.println("done closing.");
  } else
  {
    // if the file didn't open, report an error:
    Serial.println("error opening the text file!");
  }
  
  // re-open the text file for reading:
  myfile = SD.open("textFile.txt");
  if (myfile)
  {
    Serial.println("textFile.txt:");
    
    // read all the text written on the file
    while (myfile.available()) 
    {
    	Serial.write(myfile.read());
    }
    // close the file:
    myfile.close();
  } else 
  {
  	// if the file didn't open, report an error:
    Serial.println("error opening the text file!");
  }
}

void loop()
{
}

Reading_and_Writing.zip

Video

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