A system which includes a microprocessor (or microcontroller) and an auxiliary chip which monitors the system power supply voltage and performs related functions for the microprocessor, and also provides a watchdog function to monitor the microprocessor's activity. The auxiliary chip itself can put to sleep by the microprocessor to minimize power consumption. The sleep mode of the auxiliary chip saves power by shutting down many of the input-sensing circuits, and the watchdog function. The sleep command is not accepted unless it stands in the proper timing relationship to a signal on the strobe pin. This permits the power savings of the sleep mode to be realized, without any risk of the system being placed in the sleep mode due to an out-of-control system condition.