microSD write & read-back

A real 64 MB SD card in SPI mode: the Uno runs the honest init handshake (CMD0, CMD8, ACMD41), writes a message into a raw block through the hardware SPI port, reads it back with CMD17 and prints it on the serial monitor — watch the activity LED blink on the breakout. The card ships FAT16-formatted, so the Code tab’s SD.h sketch (compile & upload) can write real files to it too.

What's on the bench

• Arduino Uno
• Battery
• microSD card

How it's wired

• Battery · pos→Arduino Uno · 5v
• Battery · neg→Arduino Uno · gnd
• Arduino Uno · 3v3→microSD card · vcc
• Arduino Uno · gnd2→microSD card · gnd
• Arduino Uno · d13→microSD card · sck
• Arduino Uno · d11→microSD card · mosi
• Arduino Uno · d12→microSD card · miso
• Arduino Uno · d10→microSD card · cs

The code

This Arduino C++ sketch lives in the Code tab; sign in and press Compile & upload to build real firmware for the emulated board.

// The same card through the Arduino SD library (the card comes
// FAT16-formatted, so SD.begin just works). CS on pin 10; the SD
// library drives the hardware SPI port (11/12/13) itself.
#include <SPI.h>
#include <SD.h>

void setup() {
  Serial.begin(9600);
  if (!SD.begin(10)) {
    Serial.println("card init failed");
    return;
  }
  Serial.println("SD init OK");
  File f = SD.open("log.txt", FILE_WRITE);
  f.println("HELLO FROM SD :)");
  f.close();
  f = SD.open("log.txt");
  Serial.print("log.txt: ");
  while (f.available()) Serial.write(f.read());
  f.close();
}

void loop() {}

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