MicroSD card module with Arduino Uno — SPI wiring and data logging

A microSD module turns the Uno into a data logger. Temperature every minute, the hours a pump ran, or a GPS track — all go to a CSV file you can open in Excel on a PC.

The usual module from Indian sellers has a 3.3 V regulator and a level-shifter chip, so it wires straight to the Uno’s 5V and its hardware SPI pins (D11, D12, D13), with CS on D10. The real constraint is RAM: the SD library needs a 512-byte buffer, which is a quarter of the Uno’s 2 KB. Plan the rest of the project with that in mind.

The sketch appends a line with the uptime and the A0 reading to LOG.TXT every 2 seconds. It closes the file after each write so nothing is lost if the power goes.

Wiring

Board quirks

• This wiring is for the common blue module with a regulator and level-shifter chip. Bare 3.3 V-only adapters (just a socket and resistors) must be powered from 3.3V and need a level shifter on MOSI, SCK and CS.
• The SD library keeps a 512-byte sector buffer in RAM — a quarter of the Uno’s 2 KB. Combined with an OLED’s 1 KB buffer, you will run out; use a character LCD instead.
• Format the card as FAT32 (FAT16 for 2 GB and smaller) with the official SD Card Formatter. Cards over 32 GB come as exFAT, which the SD library can’t read.
• File names must fit the old 8.3 format (LOG.TXT, DATA2026.CSV). Longer names fail to open.
• Always close() or flush(). If power cuts while a file is open, the last data is lost and the card can need a check on a PC.

Code

// microSD card module on an Arduino Uno R3 (SPI), with the SD library.
// VCC → 5V (module has a regulator + level shifter), GND → GND,
// MISO → D12, MOSI → D11, SCK → D13, CS → D10.
// Card: FAT32 (or FAT16), 32 GB or smaller. File names: 8.3 format, e.g. LOG.TXT.
#include <SPI.h>
#include <SD.h>

const uint8_t CS_PIN = 10;

void setup() {
  Serial.begin(9600);
  Serial.print(F("Initialising SD card... "));
  if (!SD.begin(CS_PIN)) {
    Serial.println(F("failed - check the card is FAT32, inserted, and CS is D10"));
    for (;;) {}
  }
  Serial.println(F("OK"));
}

void loop() {
  File f = SD.open("LOG.TXT", FILE_WRITE);  // appends to the end
  if (!f) {
    Serial.println(F("Could not open LOG.TXT"));
  } else {
    f.print(millis() / 1000);
    f.print(',');
    f.println(analogRead(A0));
    unsigned long size = f.size();
    f.close();  // close (or flush) so the data is really on the card
    Serial.print(F("Logged - LOG.TXT is now "));
    Serial.print(size);
    Serial.println(F(" bytes"));
  }
  delay(2000);
}

// microSD card module on an Arduino Uno R3 (SPI), with the SD library.
// VCC → 5V (module has a regulator + level shifter), GND → GND,
// MISO → D12, MOSI → D11, SCK → D13, CS → D10.
// Card: FAT32 (or FAT16), 32 GB or smaller. File names: 8.3 format, e.g. LOG.TXT.
#include <SPI.h>
#include <SD.h>

const uint8_t CS_PIN = 10;

void setup() {
  Serial.begin(9600);
  Serial.print(F("Initialising SD card... "));
  if (!SD.begin(CS_PIN)) {
    Serial.println(F("failed - check the card is FAT32, inserted, and CS is D10"));
    for (;;) {}
  }
  Serial.println(F("OK"));
}

void loop() {
  File f = SD.open("LOG.TXT", FILE_WRITE);  // appends to the end
  if (!f) {
    Serial.println(F("Could not open LOG.TXT"));
  } else {
    f.print(millis() / 1000);
    f.print(',');
    f.println(analogRead(A0));
    unsigned long size = f.size();
    f.close();  // close (or flush) so the data is really on the card
    Serial.print(F("Logged - LOG.TXT is now "));
    Serial.print(size);
    Serial.println(F(" bytes"));
  }
  delay(2000);
}

Library: SD (Arduino, SparkFun (ESP32 and Pico cores ship their own SD library)) — compiled with SD 1.3.0.

Expected serial output

Initialising SD card... OK
Logged - LOG.TXT is now 8 bytes
Logged - LOG.TXT is now 16 bytes
Logged - LOG.TXT is now 24 bytes

Initialising SD card... OK
Logged - LOG.TXT is now 8 bytes
Logged - LOG.TXT is now 16 bytes
Logged - LOG.TXT is now 24 bytes

Common errors