Microprocessor reset with power level detection and watchdog timer

Abstract

A combination watchdog timer and input voltage level detector circuit is coupled to a microcomputer (or microprocessor) for insuring proper operation thereof under various conditions. The watchdog timer is coupled to the microcomputer and is responsive to a status signal output by the microcomputer indicating the operating state thereof. Failure of the watchdog circuit to detect the status signal indicates that the microcomputer has become unstable or is in a locked up condition and causes the watchdog circuit to initiate a microcomputer reset by means of a reset trigger circuit. A voltage level detector is coupled to the input voltage source and to the reset trigger circuit for similarly initiating the resetting of the microcomputer in the event the input voltage to the microcomputer drops below a predetermined value. When power is initially applied, a power up detector coupled between the voltage level detector and the watchdog timer ensures that the reset trigger circuit maintains the microcomputer in a reset condition until the input voltage reaches a predetermined level to permit normal microcomputer operation. The present invention thus ensures that the microcomputer is maintained or is placed in a reset condition in the event of input power transients, upon initial application of power to the microcomputer, and upon the occurrence of irregularities in microcomputer program execution.