Hawk - Under Voltage Lockout

When power to the device is insufficient to power the device correctly, (i.e. the battery goes flat), the device will enter an Under Voltage Lock Out (UVLO) state to prevent reboot loops, and damage to any components. 

The UVLO behaviour has been designed to cater for different power setups e.g. 

• Battery + Solar Panel
• Battery + stable mains power
• Battery Only

Summary of behaviour:

1. When the battery goes flat, the device resets and enters the UVLO state.
2. The device will then wait for external power to be detected
3. Once detected, it will then take 30sec to determine power stability and does the below
   1. If external power is stable, exit UVLO and increment UVLO count. This is shown in the logs as below
      
      
      
      
   2. If unstable, charge for 15 mins
   3. Check battery again. If voltage is over 3.7V, exit UVLO.
   4. If not repeat from point 2.
4. The UVLO count will be shown in the logs when the device exits UVLO state and begins regular operation. This is a safety measure in case we falsely determine that the external power is stable and exit UVLO when we shouldn't have, which may lead to another reset. When the UVLO count reaches 5, even if the external power is determined as stable, the device will charge for 15 mins before exiting UVLO.
5. If no battery is detected, the power stability check is skipped and the device exits UVLO, but the UVLO count is still incremented. Once the count reaches 5, the device will sleep for 5 mins before exiting.

The above ensures that on top of avoiding reboot loops, we also avoid entering/exiting UVLO repeatedly in the case of a solar panel outputting a low current trying to charge a flat battery. 

Some points to note when bench testing/provisioning the Hawk

1. To avoid getting stuck in the UVLO state use a well charged battery (3.8V+), or connect a stable external power source (e.g. 12V bench power supply)
2. You can exit the UVLO state by pressing the button on the board for around 200ms. The LED will flash to indicate success.