This particular package was interesting because it contained:
- built in (albeit small) lipo cell
- small OLED
- IMU, buzzer, IR emitter
- Watch-like form factor including a clip that has watch strap pin holes on it
Other things that were not as interesting, but could be useful include a clip that has Lego-compatible holes (the "brick" clip), and another "wall" clip that just has screw holes. There's also a Grove/HY2.0 JST connector for adding external sensors, as well as a blue LED. There are two buttons broken out on the edges, one is a reset, while another is on a GPIO pin. The current documentation for the system is here. Some broken links and images (as of Apr 2019), but the linked examples work at least.
The first hidden feature, which I found out later was common on the M5stack/stick, is that the rest button has a curious feature: it appears to be connected to the lipo management system as well, and a double press while on battery power will turn off the device! I haven't delved into the schematics to see how its done, but I assume its a feature of the power management chip. For example, this cannot be done when you're on USB power.
Looks like there are a number of variants of the board with different accelerometer and screen combinations. So far the main limit I can see is that the battery is a bit on the small side, but overall it sits nicely between a bare dev board and a fully enclosed solution, and I expect it will come in handy...
