How to interface an SD card with Arduino

This tutorial is about interfacing SD-card with arduino. First i will explain what is SD-Card? Where it can be used? SD-Card applications? Then I will move on to the pin out of SD-card. The protocol used by SD-Card to communicate with external modules(microcontrollers etc). Finally, I will present a simple DIY electronics project on how to interface the SD-Card module with arduino. What are the constraints which must be considered in circuit design (Arduino-SD-card connections) and the software constraints, using the arduino library for SD-Card interface.

What is SD-Card? What are its applications regarding arduino?

SD stands for secure digital and combined with card it stands for secure digital card. It is a non volatile memory in a small thin integrated circuit. You can store still images, videos, files etc on it. In order to move data from one device to another using SD-card you have to remove it from one device and insert it in other. Non volatile attribute made it popular. Unlike floppy discs SD-cards are small in size have greater capacity and are not effected by external magnetic fields.

SD-cards comes in three sizes standard one followed by mini and then the micro. The standard one is biggest in size. With the passage of time the electronic devices becomes smaller in size. so the size of SD-card also reduced to fit in the new devices. For example take the example of mobile phones initially the SD-card supported by the mobile phones are micro sized. Then after 2010 mobile size reduced and the new slot for SD-cards supports only the mini sized SD-card. Another good example is of digital cameras. First digital cameras supported floppy discs. Then comes the cameras who supported SD-card. The size of SD-card at that time was standard one. After a decade cameras supporting micro sized SD-card flooded the market and finally the slot now a days is over taken by mini sized SD-card.

Like physical size the memory capacity of SD-card is also increasing year by year. Though the SD-card standard says a max 128 TB of card is possible but right now card of memory capacity 64 GB is supported by almost all the devices present in market. Digital device support for SD-card also matters. The devices in market doesn’t support the sizes greater then 64 GB though few supports greater then 64 GB but not all. Device address and data bus are not of width to support 512 GB SD-card or greater then 64. The picture below shows the SD-card memory capacity and size.

SD card sizes and memory capacity

Arduio SD-Card Interfacing step by step guide

Their are numerous arduino SD card boards available in market. Each board has an individual SD card slot. SD card size can be varied on each board. Arduino SD card boards can be mounted on arduino boards or they can be used as stand alone shields. Various arduino sd card modules are shown below.

Arduino SD card modules

Commonality in all the Arduino SD card boards

Though all the arduino sd card modules are different in size but one thing is common in all of them. The communication protocol. All the boards communicate with arduino on SPI(serial peripheral interface). From here on i perceive that you all know what is SPI protocol? Where it is used? and what are its advantages when compared to other?

Arduino SD card interface Library

Like all other components their is a dedicated library for SD card interface with arduino. Off-course you can write your own library for sd card or just write code from scratch in order to interface arduino with SD card. I suggest to use the library. Its well written and commented and above all its bug free. Library supports FAT-16 and FAT-32 file system on a sd card. Make sure when you are building this project only use sd card whose file system is FAT-16 or FAT-32.

If you don’t know what it means just format your sd card using your personal computer. Insert the card in sd-card slot in personal computer and then right click on the card icon select the format as FAT-16 or FAT-32 and format it. Some times the format does not change using default window formatter. The solution is to download a small free sd card for formatter software and then format your card with it.

SD card format

Library offers almost all the functions to read to file, write to file, make directory folder, delete folder etc. Their are numerous other functions supported by arduino SD card library. Its better to take a look at official description of the library(https://www.arduino.cc/en/reference/SD).

SD card interface Pin map

The SPI protocol pins on different arduino boards are at different positions/numbers. We don’t need to initialize SPI pins the library does it for us. We just have to select our board from arduino ide boards menu. For this tutorial i am going to interface SD card with arduino uno. SPI pins on arduino uno are present at pin#11,12 and 13.

Arduino uno SPI pins

Important:The SS pin must be left as an OUTPUT pin other wise the library wont work. SS means slave select. We generally don’t use it in case of arduino. Because the SD card modules SS pin is grounded or SD cards arduino boards are always in slave mode.

SD card module pin out

The SD card module which i am going to use in this tutorial is micro SD card module. Its the most popular and cheap sd card module now a days. You can purchase it in less then 0.5$ from aliexpress. The SPI pin layout is visible on the module in the picture below.

Arduino SD card module

Interfacing arduino uno with sd card module using arduino sd card library

Since i explained all the connections and pin out of both the sd card and arduino board above so now we just made physical connections. The circuit diagram is shown below. I used arduino digital pin # 4 for CS(chip select) of arduino sd card module. I powered the SD card module with 5 volts power output regulator present on arduino board.

Arduino SD card interface circuit connections

Direct arduino sd card connections

It is also possible to directly interface the SD card with arduino. We use pre-assembled sd card boards because they are reliable and saves time. If you want to directly interface the sd card with arduino you will require some extra circuit for voltage requirements of sd card pins. Calculating resistance, purchasing them and then soldering building circuit consumes a lot of time and a lot of effort is required to put in. How ever if you want to interface directly a sample circuit is given below. I didn’t tested it but i am confident that in this configuration it will work perfectly with out any mall functioning.

Arduino sd card directly interface

Arduino SD card Coding

At first you have to import arduino SD library in your arduino code. The syntax of SD library is given below.

#include <SD.h>

After importing SD library. You need to import the SPI library. SPI is protocol on which the SD card module talks with arduino. Arduino is master and SD card is slave in our case. Syntax of importing library is given below.

#include <SPI.h>

Most SD cards arduino modules have a CS(chip select) pin. But some come with out chip select pin. If your module has a chip select pin. Then you first need to activate the arduino sd card module using an external arduino digital pin. This digital pin is connected directly to CS pin of sd card module. Syntax of the chip select instruction is.

SD.begin(Arduino digital pin number);

Now to read or write to a file you first need to open the file. Suppose their is a “myfile.txt” file already present on the sd card and you want o read or write to this file. In order to do so you need to first open this file. Syntax of opening the file is

File dataFile = SD.open(“myfile.txt”);

The above command opens the file myfile.txt with a file pointer. The pointer is dataFile.You can now read and write to file using the pointer. Their is an another command which creates new file on sd card.

File dataFile = SD.open(“myfile.txt”, FILE_WRITE);

The above command creates new file with the name myfile.txt on sd card and opens it for writing. If the file already exists then this instruction opens it for writing.

Reading from file

Now you are done with every thing(the initialization part) its time to write to file. In the below code we are writing to file myfile.txt. The while() statement is a continuous loop it ends up only when the file pointer reaches the end of file. One by one characters are read from file and are displayed on the serial monitor of arduino ide. At the end when we reached end of file we close the file using instruction dataFile.close().

Writing to file

Writing to file is simple. Make sure you opened the file in write mode. The command is discussed above. To write text like “www.microcontroller-project.com” the syntax is

dataFile.write(“www.microcontroller-project.com”);

Important: We can only write to file once when we open file. If we again want to write to the same file we have to close it and open it again.

Random file functions

Check if file already exists

SD.exists(“myfile.txt”)

Delete a file

SD.remove(“myfile.txt”);

Create a directory or folder

SD.mkdir(“work”);

The above instruction creates a new folder with name work on sd card.

Create a directory in a directory

SD.mkdir(“work/myfolder/code”)

The above instruction creates a folder myfolder in folder work and then in myfolder an another folder named code.

Remove or delete a directory

SD.rmdir(“work”);

A sample code is presented above. I hope you can easily understand each step and instruction of the code. The fact that loop() function is empty is because if i place the file read write code in loop function it will read and write again and again. The loop function never stops unless some event trigger’s the read write function to stop. So instead of generating an event i just placed the code in setup() function which will execute only once. When you power your circuit the read write code above will execute once. If you reset the circuit it will run again.