How to use the "ground-referenced" comparator on this microcontroller?

Question

I chose to use an STM8L101F3P6 low power microcontroller for a sensor node. This microcontroller, however, lacks an ADC and instead implements 2 comparator units with a block diagram presented on its reference manual :

I was going to use the comparators and a combination of zeners/voltage-references to implement a basic battery level monitor. However to my surprise, the reference pin (the "-" pin) of comparators are not available on some packages as an IO, and instead, is permanently connected to ground; so basically, as much as I understand, the comparator is reduced to a crude digital pin which activates on a non-zero signal!

And in the register description we can see:

1. Is there any option to use this ground-referenced comparators to measure battery level?
2. Is there any benefit at all to use a comparator with one leg referenced to ground, and the other leg available only within positive voltage range?

Answer 1

• Is there any option to use this ground-referenced comparators to measure battery level?
• Is there any benefit at all to use a comparator with one leg referenced to ground, and the other leg available only within positive voltage range?

Lacking an ADC, it is possible to use this comparator to implement a slope-type ADC to measure voltage or current. A counter, is employed along with software to implement integrating ADCs: some microcontroller software resources will be required.

Comparing against ground adds some complication, since a reference voltage above ground in this particular case is not an option.
A negative-voltage reference could be used with a resistor divider so that the positive voltage can be measured with a ground-referenced comparator. A microcontroller charge-pump circuit can generate a negative reference. A very simple version of this employs an oscillating GPIO pin with diodes:

^{simulate this circuit – Schematic created using CircuitLab}