Importance

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Abstract

An adjunct chip, usable as a peripheral to a microprocessor, which detects power failure, and puts the microprocessor into a known state upon power down. In order to reliably and stably put the microprocessor into a known state, several clocks are generated after the reset signal. However, since the power supply is failing, it is possible that the crystal-controlled oscillator may already have become unreliable. Therefore, a simple logic circuit (a ring oscillator, in the presently preferred embodiment) is used to generate the needed additional clocks at power-down. In the presently preferred embodiment, the switch from crystal-controlled oscillator to ring oscillator is stabilized by using a nonlinear filter circuit (driven by both the ring oscillator and the crystal oscillator) to detect when the crystal oscillator actually begins to fail. A transmission gate is then disabled, and the state frozen for long enough to allow any changes to propagate through.

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First Claim