Pins, rails, and nets

Forty pins leave the Pico socket, but you may only ASSIGN some of them. Ground pins, 3V3, VBUS, VSYS — the power plumbing — are not yours to hand to a peripheral; the generator wires power by construction. Try, and the validator is blunt: a made-up name earns "is not a Pico pin name", and a real power pin earns "is a GND/3V3/power pin, not an assignable GPIO". One detail catches people: the ground pins are named GND1 through GND8, so a bare GND is, to the validator, a made-up name.

Every GPIO may serve exactly ONE peripheral. The pin is a physical piece of silicon; two components soldered to it would fight. The validator tracks every assignment across the whole spec and reports a collision with both names: "SW1: GPIO GP15 already used by D1 — no pin may serve two peripherals". Fixing a conflict sometimes reveals a second one — the validator reports what it can prove against components that exist.

Three pins are special: GP26, GP27 and GP28 carry the analog-to-digital converter. A part that MEASURES a voltage — the light sensor's divider, the pot's wiper — must land on one of them. The message has the same shape for every analog part — "RV1: potentiometer needs an analog pin — GP16 is not an ADC pin (GP26/GP27/GP28)" — naming the part, the pin you offered, and the three that would work. The digital pins cannot read a divider; they see only high and low.

Behind the pins live NETS: named electrical islands. The two you get for free on every board are the rails, 3V3 and GND. When the generator wires an LED's cathode "to ground" it is joining the GND net, and every other ground connection on the board is the same island of copper. This is why the empty board of Lesson 1 already had two traces — the rails exist before any peripheral does.