On a classic Arduino Uno, connect the microSD module’s SPI data and clock pins to D11 (MOSI), D12 (MISO), and D13 (SCK). Connect its chip-select (CS) pin to the pin specified by the module, then use that same pin in SD.begin(...). Check the module’s own voltage and logic requirements before powering it: a VCC label alone does not establish that its signal inputs are safe for the Uno.
Uno SPI pins and module-dependent connections
The classic Uno’s hardware SPI connections are shared across SPI peripherals:
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- MOSI (also labeled DI on some modules) → D11
- MISO (also labeled DO) → D12
- SCK (also labeled CLK) → D13
Connect the module’s GND to Uno GND. Connect VCC only to the supply specified by the exact module’s documentation, and verify its logic-level requirements as well. Modules that look similar may differ in power and signal handling.
CS, SS, or chip select is product-dependent. Arduino’s CardInfo example uses D10, while DFRobot’s Fermion DFR0229 specifies D4. Wire the module’s CS pin to the documented Uno pin and pass that same pin to the library.
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Example: DFRobot Fermion DFR0229
DFRobot specifies this SPI module’s working voltage as 5 V and lists it as suitable for Arduino Uno/Mega. Its published Uno wiring is:
| DFR0229 pin | Arduino Uno connection |
|---|---|
| MISO | D12 |
| SCK | D13 |
| SS / chip select | D4 |
| MOSI | D11 |
| GND | GND |
| VCC | 5 V |
Use SD.begin(4) for this pin map. DFRobot’s product page documents the voltage and connections, but does not describe every detail of the module’s signal circuitry; consult the documentation for your own board rather than assuming its internals match.
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Initialize the card with Arduino’s SD library
Arduino’s SD library supports card initialization and basic file and directory operations. Include the SD library, set the CS pin to match your wiring, and check the result of SD.begin(chipSelect) before attempting file operations. For example, with the DFR0229’s CS on D4:
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#include <SPI.h>
#include <SD.h>
const int chipSelect = 4;
void setup() {
Serial.begin(9600);
if (!SD.begin(chipSelect)) {
Serial.println("Card initialization failed");
return;
}
Serial.println("Card initialized");
}
void loop() {
}For a module wired to another CS pin, change chipSelect to that pin. If you choose a CS pin other than the Uno’s hardware SS pin, keep the hardware SS pin configured as an output, as required by the SD library API.
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Arduino’s SD guide includes CardInfo, read/write, file-dump, and file-management examples. CardInfo checks card presence, wiring, and the selected CS pin, then looks for a FAT16 or FAT32 partition. Follow the requirements of the specific library version and example you are using rather than assuming every card format is supported.
Quick Recap
Diagnose initialization and volume errors
Use this sequence when the example does not work:
- Check seating and documented requirements. Reseat the card and confirm the module’s VCC and logic requirements are compatible with the Uno connections.
- Recheck SPI wiring. Verify MOSI/DI → D11, MISO/DO → D12, and SCK/CLK → D13, with a shared ground.
- Match CS in hardware and code. Trace the CS wire to the Uno pin and make the
SD.begin(...)argument identical. - Run CardInfo or another Arduino SD example. Inspect the Serial Monitor output to distinguish initialization trouble from later volume inspection.
- If initialization succeeds but volume inspection fails, check whether the card has the FAT16/FAT32 partition expected by the selected example and library.
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