A21 — Code-Protected Firmware Library

help The problem

In access control roughly 80% of the firmware is the same from board to board — readers, I/O handling, access logic. In this codebase it was all cross-linked, so the reusable majority could not be reused: every new product meant reworking shared code, and the same hardware could not be sold as different products without a rewrite. A second, harder constraint sat on top: the shared IP could not ship as source — not even as a static library, which still links into the customer's app where it can be picked apart.

build What we built

• The reusable core split out as a standalone firmware library distributed as a precompiled binary at a fixed memory location — never source, never a linkable object.
• Two-stage boot: a primary bootloader loads the library; the library in turn hands off to and starts the application.
• A syscall-style ABI — every service, from hardware peripherals (GPIO, I2C, SPI, ADC) to logical modules (inputs, outputs, card readers), exposed through a vector table at a known location, the way an OS exposes a syscall table. The app enters through a software interrupt (SVC); arguments and return codes are marshalled by stack-frame manipulation, so the interface stays stable with no linking between app and library.
• The library owns and arbitrates all resources centrally, so two logical modules cannot collide over the same hardware.
• The application is deliberately trivial — Arduino-simple C holding only business logic. A HAL wrapper beneath the library allows the MCU to be swapped later without touching either side.