Building a smartwatch firmware from scratch in three days using Claude Code

A developer used Claude Code (Opus 5.5) to build an open-source firmware named Kronograf for the LilyGO T-Watch 2020. By following a structured phased approach, validating with simulator screenshots, and maintaining decision and blocker logs, they successfully implemented watch faces, apps, Bluetooth notifications, and more.

Reddit r/ClaudeAI/u/cyberdinn

Claude CodeOpus 5.5LilyGO T-Watch 2020 V1ESP32

Added 2026-10-31 · originally published 2026-10-24

Steps

1Provide a structured brief that includes the North Star goal, hardware specs, and development phases 0 through 9.
2Adopt a phased development workflow: build, flash, and test only one phase at a time, moving to the next only after confirming it works.
3Set up a UI simulator that runs on Mac and use Claude for headless screenshots (--shot) to inspect the interface.
4Maintain two running logs in the codebase: DECISIONS.md (recording choices and reasoning) and BLOCKERS.md (recording failures and attempts).
5Write data-handling logic in pure C and test it on the host machine (e.g., ANCS notification protocol, protobuf parsers, time zones, sun/moon calculations).
6Handle physical and hardware-related issues manually (such as flashing connection glitches, wake/sleep bugs on battery power, and wrist-raise threshold tuning).

Copy-ready prompt

Help me develop an open-source firmware project named [Project Name] targeting the hardware model [Hardware Model] with a screen resolution of [Resolution]. We need to build this in phases: first, create a structured brief containing the North Star goal and phases 0-9, then write code strictly one phase at a time. Help me reason through the logic during development, and write the data-handling parts in pure C so they can be tested on the host. Please reference the following requirements: [Detailed Functional Requirements]

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#AI Programming#Claude Code#Firmware Development#Open Source

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