Casio F-91W using Statecharts | Hacker News Reader

Casio F-91W using Statecharts

github.com

This is funny, I just ordered a FOSS replacement movement for the F91W, which, with a few calibrations spread out over a few months, can achieve an accuracy of <15s/yr (it also has an accelerometer and a thermometer built in). I was planning on writing some astronomy and navigation apps for it

If anybody is interested:

https://www.crowdsupply.com/oddly-specific-objects/sensor-wa...

Dude this is sick! I wish this supported my F-105W because I really prefer the better backlight. Does anyone know anything similar for this model?

The backlight is integrated into the movement. SWP has a red, green and blue LED, depending on whatever suits your preference.

It's also compatible with the A158WA-1, which is basically an F91W, but with a steel case (the steel bracelet can be replaced with an 18mm leather strap if you prefer).

I haven't put the SWP in yet, so I can't comment on how good it is, but it looks very promising.

BTW, if you want the high accuracy feature, you have to change some settings and flash a new firmware with nanosec enabled.

Does high accuracy reduce battery life?

I don't believe that it should, but tbh I can't say for certain. My understanding is that you set the time using a high accuracy time source, reset it a month later, then a few months after that, etc and nanosec updates the crystal's drift compensation value which applies corrections to the time displayed, after accounting for the temperature and battery voltage. So it shouldn't be making too many writes, and as long as it isn't checking and correcting the drift too often (like every second or even every minute) it should be fairly minimal additional power draw. I'd be more concerned about excessive backlight usage, but again, this all just me speculating.

On the topic of battery usage though, SWP has a low energy mode, which only updates the face every minute. You can also adjust backlight intensity.

Ollee watch [1] is compatible with F-105W:

> Tested: All Casio A168 and F-105W models that use module 3298 (manufactured since ~2012).

[1] https://www.olleewatch.com/shop/p/ollee-watch-one-kit-syp37

These are really fun. I've installed both the original and new versions:

https://blog.jgc.org/2022/10/pimping-my-casio-with-oddly-spe...

https://blog.jgc.org/2025/07/pimping-my-casio-part-deux.html

This is great. did you leave out the alarm ringing and the continuous increment when holding the A button because they were hard to model or just to keep the demo simple?

It renders funny on Firefox mobile for iOS.

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