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PCB: option footprints for anything uncertain

When a circuit's right value or topology is not proven, the schematic gets a footprint for the alternative, populated or not. Lay out only what we are sure of, but leave room to change it on the bench without a respin.

  • What counts as uncertain: an unverified car/load interface (input polarity, pull-up value, a signal whose function is unknown), a part value picked from a forum post, a route that may need to move (signal ground vs analog ground, harness pin vs pigtail).
  • How:
  • Optional resistor: a real value and a real LCSC number, so fitting it later is a BOM edit, not a redesign.
  • Optional link: a 0R jumper (Hellen Jumper-Pad-Pad, 0805, C17477).
  • Spare MCU pins and rails go to pads, next to a prototype area.
  • Not assembled = DNP in KiCad and a removal line in the board's bom_replace_*.csv (the Hellen pipeline's removal list). Fitting a part means deleting its line. Main-sheet designators reach the JLC BOM unchanged, so the line is just the reference (R38,).
  • Every option is listed with "when to fit" in the board's documents/README.md.

Why: the owner prefers one layout that can be tuned on the bench over guessing and re-spinning the board.

Related: rusefi-sti-2004.