microSD with Seeed XIAO ESP32S3 — module or the Sense slot

Logging to a microSD card works with any XIAO ESP32S3: wire a module to the SPI pins D8 (SCK), D9 (MISO), D10 (MOSI) and CS on D3, and power it from 5V.

The Sense version makes it simpler — its camera board has a card slot on the same SPI lines. The sketch appends a line to /boots.txt at every reset and prints the file, a quick test that writes really persist.

Wiring

Every connection
XIAO ESP32S3	MicroSD card module (SPI)	Wire	Notes
5V	VCC	red	The common module needs 5 V for its regulator — USB power only.
GND	GND	black
D8	SCK	yellow
D9	MISO	blue
D10	MOSI	green
D3	CS	purple

4.5–5.5 V on the common module with regulator (3.3 V-only boards also sold) part, 3.3 V board. The common blue module has an AMS1117 3.3 V regulator and a 74LVC125 level shifter, so it needs 5 V on VCC — 3.3 V leaves the card under-powered. Its signal pins accept 3.3 V or 5 V logic, so ESP32, Pico and Uno all connect directly.

Board quirks

Have a XIAO ESP32S3 Sense? Its expansion board already has a microSD slot on the same D8/D9/D10 lines, with CS on GPIO21. Set SD_CS to 21 and skip the module.
GPIO21 is also the orange user LED, so on the Sense the LED flickers with card access — expected.
The blue module’s regulator needs 5 V, and the XIAO’s 5V pin is dead on battery. For a battery logger use the Sense slot or a 3.3 V-only card module on 3V3.
Cards must be FAT32 (32 GB or smaller works best).

Code

microsd-xiaos3.inoArduino C++Open in Playground Flash

// microSD card module on a Seeed XIAO ESP32S3.
// VCC → 5V (module with regulator), GND → GND, SCK → D8 (GPIO7), MISO → D9 (GPIO8),
// MOSI → D10 (GPIO9), CS → D3 (GPIO4).
// On the XIAO ESP32S3 Sense, the built-in card slot uses the same D8/D9/D10 lines with CS on
// GPIO21: set SD_CS to 21 and skip the external module.
#include <SPI.h>
#include <SD.h>

constexpr uint8_t SD_CS = D3;  // 21 for the Sense board's own slot

void setup() {
  Serial.begin(115200);
  uint32_t t0 = millis();
  while (!Serial && millis() - t0 < 3000) {}
  SPI.begin(D8, D9, D10, SD_CS);  // SCK, MISO, MOSI, SS
  if (!SD.begin(SD_CS, SPI, 4000000)) {
    Serial.println("Card mount failed - check wiring, 5V on VCC and that the card is FAT32");
    while (true) delay(1000);
  }
  Serial.printf("Card size: %llu MB\n", SD.cardSize() / (1024ULL * 1024ULL));
  File f = SD.open("/boots.txt", FILE_APPEND);
  if (f) {
    f.printf("Boot at %lu ms\n", millis());
    f.close();
  }
  f = SD.open("/boots.txt");
  if (f) {
    Serial.println("--- /boots.txt ---");
    while (f.available()) Serial.write(f.read());
    f.close();
  }
}

void loop() {}

// microSD card module on a Seeed XIAO ESP32S3.
// VCC → 5V (module with regulator), GND → GND, SCK → D8 (GPIO7), MISO → D9 (GPIO8),
// MOSI → D10 (GPIO9), CS → D3 (GPIO4).
// On the XIAO ESP32S3 Sense, the built-in card slot uses the same D8/D9/D10 lines with CS on
// GPIO21: set SD_CS to 21 and skip the external module.
#include <SPI.h>
#include <SD.h>

constexpr uint8_t SD_CS = D3;  // 21 for the Sense board's own slot

void setup() {
  Serial.begin(115200);
  uint32_t t0 = millis();
  while (!Serial && millis() - t0 < 3000) {}
  SPI.begin(D8, D9, D10, SD_CS);  // SCK, MISO, MOSI, SS
  if (!SD.begin(SD_CS, SPI, 4000000)) {
    Serial.println("Card mount failed - check wiring, 5V on VCC and that the card is FAT32");
    while (true) delay(1000);
  }
  Serial.printf("Card size: %llu MB\n", SD.cardSize() / (1024ULL * 1024ULL));
  File f = SD.open("/boots.txt", FILE_APPEND);
  if (f) {
    f.printf("Boot at %lu ms\n", millis());
    f.close();
  }
  f = SD.open("/boots.txt");
  if (f) {
    Serial.println("--- /boots.txt ---");
    while (f.available()) Serial.write(f.read());
    f.close();
  }
}

void loop() {}

Library: SD (Arduino, SparkFun (ESP32 and Pico cores ship their own SD library)).

Expected serial output

Serial Monitor · 115200Output

Card size: 15193 MB
--- /boots.txt ---
Boot at 512 ms
Boot at 498 ms

Card size: 15193 MB
--- /boots.txt ---
Boot at 512 ms
Boot at 498 ms

Common errors

“Card mount failed”

Card not FAT32, CS on the wrong pin, or the module on 3V3. Check with the Sense slot (CS 21) if you have one.

Works on USB, fails on battery

The module lost its 5 V. Use a 3.3 V card module or the Sense slot.