This is a circuit of Power-Fail and Low-Line Signals. This circuit is used to warn the processor when the power failure or a brownout is occurs. The processor will enter a power-down routine, when either of those signals interrupts the processor. Back up important data prior to the POR placing the processor in reset and cease its current activities are things that the processor do when enter the routine. Here is the circuit :
Power-fail comparator is used to create a power-fail signal by monitoring the unregulated DC voltage. The regulator powers both supervisor circuit and the processor by using the the unregulated DC voltage. The regulator’s output capacitor retains its output voltage, so the unregulated voltage drops before the regulator’s voltage. And also, we can know drop in the regulator’s voltage. The processor can enter its power-down routine prior to being reset, if the power supply voltage were to drop low enough because of Detecting the regulator’s voltage drop.
A supervisor provides a low-line signal, which goes active whenever the monitored power supply drops to a level slightly above the reset threshold so the processor can still receive warning of an imminent power failure when there is no access to the unregulated voltage. To cause the POR to issue a reset, the processor warned that the power-supply voltage may decrease enough by the low-line signal. To anticipate the POR generating a reset, the processor backs up important data because of power failure or a brownout. [Source: maxim-ic.com]