Wow this is incredibly cool and yet very cheap. Tempted to get one and reprogram it :)

Same here, but I wonder what I'd display on it. I guess a companion phone app that can speak BLE with it would be necessary.

We have this already! Supporting many bands and watches, taking care of notifications, alarms, steps and also workouts in some cases. Gadgetbridge - A free and cloudless replacement for your gadget vendors' closed source Android applications. See the list for supported devices and the wiki for more details.

https://gadgetbridge.org

I can vouch for this tool being excellent!

I use it with a Mi Band 5 very successfully.

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Very interested in the design of the SWire interface. I wonder if the inventors were specifically trying to work around some patent issues or if the design focused on being the very bare minimum of transistors to save money.