Reads an SNES controller from an Arduino and streams the pressed buttons over USB serial at 9600 baud.

The SNES controller contains two cascaded CD4021 parallel-in/serial-out shift registers that together sample 16 digital inputs. Twelve are wired to the button contacts; the remaining four are unused. Every line is active-LOW: a pressed button pulls that input to ground, an unpressed button reads HIGH.

The Arduino drives a strict three-wire protocol to read those 16 bits:

1. Latch — the Arduino pulses LATCH HIGH for ~12 µs. On the rising edge, the shift registers snapshot the state of all 16 inputs in parallel.
2. Clock + Read — for each of 16 iterations, the Arduino drives CLOCK LOW, reads DATA, then drives CLOCK HIGH. Each falling edge of CLOCK shifts the next bit onto DATA. The first bit is already presented on DATA right after the latch, so we read before clocking.

The whole read takes roughly 200 µs and loop() repeats it every 50 ms (~20 Hz polling), which is plenty for a 60 Hz game input.

When no controller is plugged in, the DATA line is not driven by anything and reads LOW for all 16 clocks on this wiring, (0x0000). A real controller cannot produce that value — it would require all 12 buttons and the 4 grounded signature lines to be active simultaneously. The code treats 0x0000 as "disconnected" and prints a one-shot transition message when the state changes.

This project uses PlatformIO. Install the PlatformIO Core CLI (or the VS Code extension) first.

From the project root:

# Build
pio run

# Upload to the Arduino (auto-detects the serial port)
pio run --target upload

# Open the serial monitor at 9600 baud to see button output
pio device monitor -b 9600

To target a specific port, pass --upload-port /dev/ttyACM0 (or the equivalent on your system) to the upload command.