Anatomy of a board spec

Every board in PCB Studio begins as a SPEC: a JSON document that describes the board you want. Not a drawing, not code that runs — a structured description. The engine reads it, validates it against the design rules you will meet in Lesson 4, and only then generates the schematic. This lesson teaches you to read and write that document by hand, because the palette can only ever click together what you already understand.

A complete, valid spec — the smallest interesting board, one LED on GP15:

{
  "name": "First Light",
  "board": { "width": "100mm", "height": "80mm" },
  "mcu": { "part": "pico2w_socket", "refdes": "U1" },
  "peripherals": [
    {
      "id": "led_1",
      "part": "led_5mm",
      "refdes": "D1",
      "color": "red",
      "pin": "GP15",
      "series_resistor": "330ohm"
    }
  ]
}

Five fields are REQUIRED, and the validator says so in plain language if you omit one: "spec.name is required", "spec.board.width and spec.board.height are required", "spec.mcu.refdes is required", and "spec.peripherals must be an array". The array may be empty — a bare Pico on a board is a valid design.

What does the engine BUILD from the spec above? Three components: the Pico socket, the LED, and a 330 ohm resistor you never declared. The library knows an LED must not hang directly on a GPIO, so the generator inserts the series resistor by construction — your series_resistor field sets its value. Count the traces: GPIO to resistor, resistor to LED anode, LED cathode to ground — three — plus one trace each for the two power rails the board always carries, 3V3 and GND. Five traces, three components. When you generate this spec in the Studio, the status line will read exactly that: 3 components, 5 traces, 0 errors.

One more constraint hides in board: physical room. The Pico socket is a 40-pin device roughly 50 mm long, and the design-rule check runs whether or not you ask for a PCB layout. A 50mm x 40mm board fails with more than a dozen clearance errors BEFORE you add a single peripheral — the socket's plated holes collide with the board edge. Measured against the engine: 60mm x 50mm still fails, with the end pins a hair inside the edge; 65mm x 55mm is the first round size that passes; 70mm x 60mm leaves a little margin; and 100mm x 80mm, the Bitsy Board's size, is a comfortable default with room for peripherals.