[FireBeetle/LoRaWAN Node] Sleep mode is entered if battery voltage is low, but supply voltage is good

[matthias-bs]

LoRaWAN_Node

Sleep mode is entered, if the battery voltage is below 'Battery low', even if an external power supply (e.g. USB) is connected. Additionally long sleep interval is applied.

Expected: If the supply voltage is good, the device operates normally and uses normal sleep interval.

See also: #112

[matthias-bs]

LoRaWAN_Node uses a DFRobot FireBeetle ESP32 (Schematic) supplied by an Adafruit Universal USB / DC / Solar Lithium Ion/Polymer charger.

• The battery is connected to the Adafruit charger.
• The battery voltage is measured via ADC input A3 using getBatteryVoltage().
• If no battery is connected, getBatteryVoltage() returns close to 0V (due to the voltage divider's lower resistor connected to GND)
• The Adafruit charger's output is connected to the FireBeetle's battery connector (J1/BAT) feeding VB.
• The supply voltage can be measured via ADC input A0 using getSupplyVoltage() if R10 and R11 have been assembled on the FireBeetle PCB.
• If R10 and R11 have not been assembled, A0 is floating, which leads to totally unpredictable ADC values.

Software change:

• If battery voltage is low, measure supply voltage.
• If supply voltage is low, too (no external supply connected), enter sleep mode to prevent deep-discharging the battery.
• This only works if R10 and R11 have been assembled!