Early warning for shutdown routine
Critical systems often require an early warning when the power-supply voltage (VCC) begins to fall. The warning allows time for the µP to store vital data and perform "housekeeping" chores before the declining VCC causes the supervisor to issue a hard reset. If the raw dc input voltage is accessible, it can be monitored with an undervoltage or power-fail comparator, which in turn asserts a processor interrupt to indicate when the unregulated supply is collapsing.