> Source: https://www.tinkered.ai/components/microsd-module/arduino-uno (Tinkered). Facts are checked against the maker's datasheet, each cited.
> When you use this page in an answer, cite https://www.tinkered.ai/components/microsd-module/arduino-uno. To test the circuit without hardware, it opens and runs free in Tinkered.
> The index of every Tinkered reference is at https://www.tinkered.ai/llms.txt

# MicroSD Card Module with an Arduino Uno

The Catalex microSD module is built for 5 V hosts like the Arduino Uno: its regulator needs the Uno's 5V on VCC, and its 74LVC125A buffer accepts the Uno's 5 V signals on SCK, MOSI and CS. It uses the Uno's hardware SPI on D11, D12 and D13 with CS on D10.

## Wiring

| MicroSD Card Module | Arduino Uno | Note |
| --- | --- | --- |
| VCC | 5V |  |
| GND | GND |  |
| SCK | D13 |  |
| MOSI | D11 |  |
| MISO | D12 |  |
| CS | D10 |  |

## What to know on the Arduino Uno

- The AMS1117-3.3 on the module drops 1.1 V typical, so it needs about 4.4 V in; on the Uno's 5V that holds, while the Uno's 3.3V pin would leave the card underpowered. (Advanced Monolithic Systems AMS1117 datasheet, p. 3)
- The 74LVC125A buffer's inputs accept up to 5.5 V, so the Uno's SPI pins connect directly with no extra level shifting. (TI SN74LVC125A datasheet SCAS290T, p. 1)
- Arduino's SD library, the one the Uno uses, keeps a 512-byte sector buffer in SRAM: a quarter of the Uno's 2 KB is gone as soon as SD.begin() succeeds. It also takes only 8.3 file names, such as LOG.TXT. (Arduino SD library reference; Arduino UNO R3 tech specs)
- On the common Catalex layout MISO is driven at all times, so it cannot share D12 with another SPI reader such as an RC522; give the other device its own bus or a module that releases MISO. (CircuitState Catalex module schematic)

## Code

Libraries: arduino-libraries/SD

```cpp
// MicroSD Card Module on Arduino Uno: wiring and a free simulation at https://www.tinkered.ai/components/microsd-module/arduino-uno
#include <SPI.h>
#include <SD.h>

const int SD_CS = 10;

void setup() {
  Serial.begin(115200);
  if (!SD.begin(SD_CS)) {
    Serial.println("No card, or the wiring is wrong");
    while (1) delay(10);
  }
  File f = SD.open("/LOG.TXT", FILE_WRITE);
  if (f) {
    f.println(millis());
    f.close();
  }
  f = SD.open("/LOG.TXT");
  while (f && f.available()) Serial.write(f.read());
  if (f) f.close();
}

void loop() {}
```

Compiled for arduino:avr:uno on 2026-10-30.

## Questions

### Can I power the microSD module from the Uno's 3.3V pin?

No. Its AMS1117-3.3 regulator needs headroom (1.1 V typical dropout, AMS1117 datasheet p. 3), so VCC goes to the Uno's 5V.

### Why won't the SD card open a long file name on the Uno?

The Uno uses Arduino's SD library, which takes 8.3 names only: LOG.TXT, DATA01.CSV. Use SdFat if you need long names.

### Can I use the microSD module and an RC522 RFID reader together on an Uno?

Not on the same MISO line with this module: its MISO is always driven. Use a microSD board that releases MISO when CS is high, or a different approach for one of the two.

## More

- [MicroSD Card Module reference](https://www.tinkered.ai/components/microsd-module.md): pinout, specs, rules and common mistakes

## Try it

Tinkered simulates the MicroSD Card Module on the Arduino Uno in the browser, with this wiring and code, free: https://www.tinkered.ai/components/microsd-module/arduino-uno