Using the microSD Card BFF with Arduino involves plugging the breakout into your Arduino-compatible QT Py or Xiao form factor board, installing the Adafruit Fork of the SdFat library, and running the provided example code.

This code has been tested and confirmed working with a QT Py ESP32-S2. Issues may arise with a QT Py RP2040.


Plug a microSD Card BFF into your QT Py or Xiao form factor board exactly as shown below. Here's an example of connecting a QT Py ESP32-S2 to the BFF.

Connect the QT Py ESP32-S2 with pin headers into the microSD Card BFF with socket headers. They should be plugged in with the backs of the boards facing each other.

For more information on soldering socket headers, check out this Learn Guide.

Library Installation

You can install the Adafruit Fork of the SDFat library for Arduino using the Library Manager in the Arduino IDE.

Click the Manage Libraries ... menu item, search for Adafruit SDFat and select the SDFat - Adafruit Fork library:

There are no additional dependencies for the SdFat - Adafruit Fork library.

Read/Write Example

// SPDX-FileCopyrightText: 2023 Liz Clark for Adafruit Industries
// SPDX-License-Identifier: MIT
  SD card read/write

 This example shows how to read and write data to and from an SD card file
 The circuit:
 * SD card attached to SPI bus as follows:
 ** MOSI - pin 11
 ** MISO - pin 12
 ** CLK - pin 13

 created   Nov 2010
 by David A. Mellis
 modified 9 Apr 2012
 by Tom Igoe
 modified 14 Feb 2023
 by Liz Clark

 This example code is in the public domain.


#include <SPI.h>
//#include <SD.h>
#include "SdFat.h"
SdFat SD;

#define SD_FAT_TYPE 3

// default CS pin is TX for microSD BFF
#define SD_CS_PIN TX

File myFile;

void setup() {
  // Open serial communications and wait for port to open:
  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only

  Serial.print("Initializing SD card...");

  if (!SD.begin(SD_CS_PIN)) {
    Serial.println("initialization failed!");
  Serial.println("initialization done.");

  // open the file. note that only one file can be open at a time,
  // so you have to close this one before opening another.
  myFile ="test.txt", FILE_WRITE);

  // if the file opened okay, write to it:
  if (myFile) {
    Serial.print("Writing to test.txt...");
    myFile.println("testing 1, 2, 3.");
    myFile.println("hello bff!");
    // close the file:
  } else {
    // if the file didn't open, print an error:
    Serial.println("error opening test.txt");

  // re-open the file for reading:
  myFile ="test.txt");
  if (myFile) {

    // read from the file until there's nothing else in it:
    while (myFile.available()) {
    // close the file:
  } else {
    // if the file didn't open, print an error:
    Serial.println("error opening test.txt");

void loop() {
  // nothing happens after setup

Upload the sketch to your board and open up the Serial Monitor (Tools -> Serial Monitor) at 115200 baud. A test text file will be created and written to on the SD card. Then, the text file will be read back with its contents printed to the Serial Monitor.

This guide was first published on Feb 14, 2023. It was last updated on May 29, 2024.

This page (Arduino) was last updated on May 29, 2024.

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