Embedded Firmware From Hardware

Core Rule

Treat the schematic/PCB as the source of truth for pins and nets, and treat vendor libraries as state machines with required setup, service, restart, and IO-bias contracts. Do not replace a vendor peripheral flow with intuition such as "this should be high impedance" unless the working reference, datasheet, or measured behavior supports it.

Workflow

1. Extract hardware evidence
   • Build a pin map: MCU pin, alternate function, schematic net, series parts, destination component, PCB pad, and uncertainty.
   • Trace each user-visible function end-to-end: input electrode/switch -> resistor/filter -> MCU pin -> firmware symbol -> behavior.
   • Mark polarity, pull state, power domain, level shifting, reset/default state, and "needs measurement" items.
   • For MCU datasheets, app notes, and vendor PDFs, extract text first; if text is sparse or screenshots contain instructions, render pages as images and read the visual content before deciding the implementation. Prefer scripts/pdf_evidence.py for repeatable extraction/rendering. For Chinese filenames on Windows, enumerate PDFs with Python Path.glob() or use a Python launcher/environment that preserves Unicode paths.