Hardware abstraction layers are where embedded code often becomes difficult to test. I design traits around capabilities, then keep business logic independent from concrete drivers.
Unit tests run against mock implementations that simulate timing, failures, and edge values. This catches most logic regressions without hardware-in-the-loop.
I still maintain contract tests on real devices for integration guarantees. Those tests validate assumptions the mocks cannot represent, such as startup timing and peripheral quirks.
With this split, fast local feedback and high-confidence deployment can coexist.