What is brownout reset?

Brownout reset is a circuit that forces the microprocessor to reset if there is a short interruption of power - one that is long enough to disrupt operation, but not long enough to force a normal power on reset. It is usually a diode, resistor, and capacitor. The diode discharges the capacitor quickly when Vcc goes away, and the resistor charges the capacitor slower when Vcc comes back.