qmk_firmware/keyboards/handwired/splittest/promicro/keyboard.json

{
    "matrix_pins": {
        "cols": ["B6"],
        "rows": ["B3"]
    },
    "diode_direction": "COL2ROW",
    "split": {
        "soft_serial_pin": "D1"
    },
    "ws2812": {
        "pin": "D3"
    },
    "processor": "atmega32u4",
    "bootloader": "caterina"
}