Self-calibrating electric clock circuit

Abstract

A clock circuit designed to keep time based upon a known frequency of an alternating current power supply is provided with a d.c. power backup system to operate the clock circuit during periods of a.c. power failure. The clock circuit includes a high frequency oscillator which generates a large number of timing pulses for each alternating current power cycle. During periods of normal operation on alternating current power, the number of oscillator timing pulses occurring between successive null points in the alternating current power supply waveform are counted and stored. When the clock circuit is operated from the d.c. power supply during periods of a.c. power failure, the last count of timing pulses between successive null points in the a.c. cycle is loaded as an initial count into a counter. The counter is decremented down to zero, whereupon the counter registers the passage of a period of time corresponding to the time interval between two null points in the a.c. power cycle, at that time no longer present. This count is used to update a time recorder to maintain accurate time even during periods of power failure.