A power-on reset (PoR) is a circuit that provides a predictable, regulated voltage to a microprocessor or microcontroller with the initial application of power. The PoR system ensures that the microprocessor or microcontroller will start in the same condition every time that it is powered up.
Readers' Choice Awards '08: IT Security Products of the Year
Information Security magazine's annual Readers'...(SearchSecurity.com)
A PoR system can be a peripheral, but in sophisticated processors or controllers the PoR is integrated on the main chip. The most basic PoR system can comprise a resistor and capacitor connected together with values tailored so that, when power is first applied, the capacitor takes a predictable and constant time to charge up. For computer use, however, additional components are often required, including a circuit called a Schmitt trigger. When the PoR circuit is designed, the charge-up time should be adjusted by trial and error so that all of the processor or controller circuits can set themselves to the correct initial values before the computer begins to function.
A well-designed PoR circuit can ensure that when power is applied to a computer, it will start up properly every time (or almost every time), and will never (or rarely) freeze up right away. This feature not only saves the user a great deal of frustration, but it offers a last resort in case of a stubborn system crash: the so-called cold boot, where the computer is completely powered-down for a minute or two, and then powered-up all over again.
Continue reading about power-on reset:
Maxim Integrated Products discusses PoR and related functions for processors.
Dr. Howard Johnson outlines common problems that can occur with poorly designed PoR circuits.
HyperPhysics describes how a Schmitt trigger works.