I modified ATOM Motion to add a circuit that can monitor the battery voltage.
Since the battery level of ATOM Motion is usually unknown, it is inconvenient not to know whether the battery is charged or how long it can be used.
The circuit can be placed in ATOM Motion.
It divides the voltage of the positive electrode of the battery in half and inputs it to GPIO G33. It consumes about 0.03mA only while the power is on, but it should be negligible. IO1 on port B cannot be used for any other purpose.
Calibration is required. The calibration value is the ratio between the battery voltage measured with a voltmeter and the AD value of G33 switched to the input mode.
I wrote a program that calculates the remaining usage time according to the voltage and flashes the LED of ATOM's front panel according to the time.