Unique ID

Each ATmega32U4 has an undocumented 10-byte fabrication ID in its signature row at offsets 0x0E–0x17 (lot number, wafer number, X/Y coordinates). This ID is unique enough in practice to tell boards apart, but there is no manufacturer guarantee on global uniqueness or collision-freedom.

The stock Arduino AVR core ignores the signature row entirely. LUFA exposes these bytes as a 20-character hex USB serial via USE_INTERNAL_SERIAL, but that doesn’t help in the Arduino ecosystem.

Our script patches USBCore.cpp to read the signature row via boot_signature_byte_get() and installs a new board variant “Arduino Leonardo (FabID)” in the Arduino IDE. Selecting that variant automatically produces a per-chip USB serial — no sketch-level changes needed.

[...]
const char* USBCore_GetFabricationId() {
  static bool initialized = false;
  static char id[19];
  if (!initialized) {
    for (uint8_t i = 0; i < 9; i++) {
      uint8_t b = boot_signature_byte_get(0x0E + i + (i > 5 ? 1 : 0));
      id[i * 2]     = "0123456789ABCDEF"[b >> 4];
      id[i * 2 + 1] = "0123456789ABCDEF"[b & 0x0F];
    }
    id[18] = '\0';
    initialized = true;
  }
  return id;
}
[...]

Once patched, you can read the ID in two ways:

• In a sketch — call USBCore_GetFabricationId() (see example below)
• On the host — read the USB serial string, e.g. via PowerShell

#include <Keyboard.h>

extern const char* USBCore_GetFabricationId();

void setup() {
  // --- print serial number ----------
  Serial.begin(9600);
  delay(1000);
  Serial.println(USBCore_GetFabricationId());
  Serial.end();
  // -------------------------
}

void loop() {
}