--- title: "The Hardest Bug I Never Solved" url: https://daily.dev/posts/the-hardest-bug-i-never-solved-p0jfllfrf source_url: https://www.embeddedrelated.com/showarticle/1214.php type: article source: "EmbeddedRelated" published: 2026-05-31T07:47:48.962Z updated: 2026-06-04T02:15:38.543Z tags: ["dma", "embedded", "firmware", "stm32"] reading_time: 13 upvotes: 0 comments: 0 language: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # The Hardest Bug I Never Solved **[EmbeddedRelated](https://daily.dev/sources/embeddedrelated)** · 13 min read · 0 upvotes · 0 comments ## Summary 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. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: --- Tags: [#dma](https://daily.dev/tags/dma), [#embedded](https://daily.dev/tags/embedded), [#firmware](https://daily.dev/tags/firmware), [#stm32](https://daily.dev/tags/stm32) [View this post on daily.dev](https://daily.dev/posts/the-hardest-bug-i-never-solved-p0jfllfrf)