- The author has replaced the logic board of a classic Casio F-91W watch with a new ARM Cortex M0+ powered board from Sensor Watch.
- The new board allows the watch to be programmed and customized, including adding features like TOTP (time-based one-time password) support for two-factor authentication.
- The author was able to set up TOTP support for their Google and GitHub accounts, allowing them to access the OTP codes directly on their wrist.
- The author also created a new "ratemeter" watchface that can be used to measure rates, such as rowing strokes or cadence.
- The document provides detailed instructions on how to add TOTP secrets to the watchface code and how the ratemeter watchface was implemented.
- The Sensor Watch project provides a clean and easy to modify set of watchfaces and complications that can be customized.
- The F-91W watch case, combined with the new programmable board, offers a powerful and hackable platform with long battery life.
- The author highlights the availability of a WASM-based emulator that makes it easy to test and play with the custom builds.
- The document mentions other interesting watchfaces available in the Sensor Watch project, including a pulsometer and orrery.
- The author recommends getting a Sensor Watch from Oddly Specific Objects, though they have no affiliation with the company.

I write type safe generic data structures in C using a technique that I haven't seen elsewhere. It involves unions and typeof, but we'll get to that. My approach works for any type of data structure: maps, arrays, binary trees… but for this article I illustrate the ideas by implementing a basic linked list.