1.4 SD Card (SDMMC)

Read and write files on a Micro SD card using the SDMMC 1-bit interface.

Overview

This example teaches file system operations on the ESP32-S3 using the high-speed SDMMC interface. It mounts an SD card, detects its type and size, then runs through a full file I/O test: creating, reading, writing, appending, renaming, and deleting files.

Note

This example is based on the official open-source SD card file system example available in Arduino IDE. To make the program structure clearer for this kit, we separated the file management functions into a dedicated .ino file so the main sketch stays easier to read.

What You’ll Learn

  • Mounting an SD card with the SD_MMC library in 1-bit mode

  • Configuring custom SDMMC pins with setPins()

  • File system operations: list, create, read, write, append, rename, delete

  • Measuring read/write throughput

Before You Begin

Complete the setup steps in Preparation before continuing.

Note

Use the recommended Arduino IDE settings in Recommended Board Settings unless this lesson says otherwise.

Open the example sketch from the code package:

LAFVIN-ESP32-S3-Multimedia-Kit-main/1.Arduino/4.SDMMC/4.SDMMC.ino

Note

You need a Micro SD card formatted as FAT32. Insert it into the onboard card slot before powering the board.

Signal

ESP32-S3

CLK

GPIO 39

CMD

GPIO 38

D0

GPIO 40

The board uses SDMMC 1-bit mode. Only one data line (D0) is connected. This keeps more GPIOs free for other peripherals while still providing adequate speed for audio and image applications.

Upload and Test

  1. Insert a FAT32-formatted Micro SD card into the slot.

  2. Connect the board with a USB Type-C data cable.

  3. Open 4.SDMMC.ino in Arduino IDE.

  4. In the Tools menu, select ESP32S3 Dev Module as the board and choose the correct serial port.

  5. Click Upload.

  6. Open Serial Monitor at 115200 baud and observe the file operation test:

    SD Card Type: SDHC
SD Card Size: 8192MB
--- Starting file operation test ---
Listing directory: /
  FILE: hello.txt  SIZE: 24
---
Creating Dir: /mydir
Dir created
---
51200 bytes read for 45 ms
1048576 bytes written for 120 ms
Total space: 7580MB
Used space: 12MB

How the Code Works

Pin Configuration and Mount

Unlike SPI mode, the SDMMC interface requires explicit pin assignment before mounting.

#include "SD_MMC.h"

int clk = 39;
int cmd = 38;
int d0 = 40;

SD_MMC.setPins(clk, cmd, d0);
SD_MMC.begin("/sdcard", true);   // true = 1-bit mode

Card Detection

The sketch checks whether a card is present and then reads its type and capacity.

uint8_t cardType = SD_MMC.cardType();
// CARD_MMC, CARD_SD (SDSC), CARD_SDHC, or CARD_NONE

uint64_t cardSize = SD_MMC.cardSize() / (1024 * 1024);

File Operations

The file operation test demonstrates the most common filesystem actions.

writeFile(SD_MMC, "/hello.txt", "Hello ");
appendFile(SD_MMC, "/hello.txt", "World!\n");
readFile(SD_MMC, "/hello.txt");
renameFile(SD_MMC, "/hello.txt", "/foo.txt");
deleteFile(SD_MMC, "/foo.txt");

In the provided sketch:

  • sd_helpers.ino contains reusable helper functions for each file operation

  • The 1-bit mode is set by the second parameter of SD_MMC.begin()

  • Performance test writes 1MB (2048 x 512-byte blocks) and measures elapsed time

Try This Next

  • Create folders to organize your own files (e.g., /music, /photos)

  • Try logging sensor data to a CSV file on the SD card