The Hardest Bug I Never Solved
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A firmware engineer recounts a stubborn intermittent bug on an STM32F427-based IoT device where the MCU would repeatedly land in the reset handler during startup. Multiple senior engineers failed to find the cause. A junior engineer, Unni, eventually traced it to a single overlooked sentence in the ARM datasheet: vendor-specific alignment requirements for the ISR vector table. The code enforced 256-byte alignment when the STM32F427 required 512 bytes, causing misaligned interrupts to jump to the reset handler instead of their actual handlers. Fixing the alignment resolved the issue permanently. The story is accompanied by community comments sharing similar tales of elusive hardware and firmware bugs.
