Deep Sleep Shield: Persistent low voltage limit

[dbrgn]

The deep sleep shield allows you to set a low voltage limit.

>>> s = DeepSleep()
>>> s.peek(0x41)
159
>>> s.set_min_voltage_limit(2.9)
>>> s.peek(0x41)
181

But the value is lost after a power cycle.

>>> s.DeepSleep()
>>> s.peek(0x41)
159

The downside of that is that if someone removes a battery from a working device and inserts it again, it'll be dead if the voltage is below the default value of 3.3V. With a good LiIon battery and low power draw, there could still be around 300 mAh in the battery between 3.3V and 2.8V.

Is the volatile behavior intended? Does the PIC support persisting these values?