US 4,228,346 AGranted

Self-calibrating electric clock circuit

AssigneeRobertshaw Controls Company

InventorCharles E. Barker

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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.

Claims7

Dependent claims

A method of self calibrating an electric clock circuit operated by a power source providing constant frequency alternating current and employing a direct current backup electrical power source, a high frequency oscillator for generating timing pulses of greater frequency than said alternating current, a time recorder for registering elapsed time, and by updating said time recorder with the elapse of cyclical periods of said alternating current comprising defining the duration of the periods of said constant frequency alternating current source in terms of a number of pulse counts from said high frequency oscillator by:

(a) repetitively testing for the existence of said constant frequency alternating current,
and upon detecting the presence of said alternating current,
(b) ascertaining and storing the number of oscillator pulses from said high frequency oscillator occuring between successive repetition points in a cycle of said alternating current,
(c) in the absence of detection of the existence of said alternating current,
(i) loading said counter with said number of oscillator pulses that occurred between said successive repetition points,
(ii) strobing said counter with said high frequency oscillator to change the contents thereof in a single direction of advancement incrementally with each oscillator pulse, and
(iii) updating said time recorder upon advancing said counter to a limit, and retesting for the presence of said constant frequency alternating current.

4 more elements

Dependencies are inferred from the claim text.