Power supply control apparatus using last power state memory

Abstract

A last power state apparatus including a power supply controller having a battery-backed memory bit for retaining the power state of an electronic device during a failure of a primary power source. The battery is preferably a lithium type battery and the power supply preferably provides a flea power signal when primary power is available to maintain the state of the memory bit even if the power supply is off. The controller asserts a power status signal to the power supply based on the state of the memory bit, so that the power supply remains off or powers up as appropriate when primary power is next available. A momentary power switch is used to manually turn on and off the power supply and the electronic device by toggling the memory bit. The electronic device, which is typically a computer system, provides signals to the power supply controller to turn off the device. Further, the computer can enable an interrupt to prevent the user from turning off the device until after vital functions are completed. Also, other events, such as a real time clock (RTC) alarm, or the activation of a modem are monitored to toggle the memory bit to turn on the device. The RTC also monitors the battery voltage and provides a standard battery valid signal, where the controller resets the memory bit to a default off state if the battery voltage falls below a predetermined minimum value.