(1) Field of the Invention
This invention is in the field of methods of synchronizing a digital timer with the frequency of a source of A.C. electric power such as is provided by an electric utility.
(2) Description of the Prior Art
Digital timers which maintain current, or real time, time utilizing clock signals produced by crystal controlled oscillators, or clocks, are well know. Relatively low cost clocks of reasonable accuracy of .+-.0.05%, for example, are satisfactory for digital timers which are required to maintain real time over shorter periods of time, or where precise accuracy is not a requirement. Longer term stability of the clock signals applied to a digital timer can be achieved by using clocks which are more accurate, but the cost of achieving a significantly higher degree of accuracy over long periods of time, measured in weeks, months, or years, as is required in process control systems is significantly high. Thus, there is a need for a lower cost more reliable way to achieve long term stability with the desired degree of accuracy for digital timers using conventional reatively low cost digital clocks.
A very reliable source of real time timing information which is gnerally available is the frequency of A.C. electric power from public utilities. The utilities, over long periods of time maintain, or control, the accuracy of the frequency of the A.C. power such that it normally does not deviate by more than one cycle per second over long periods. Thus, the frequency of such an A.C. source is available as a timing reference at essentially no cost. However, there are two standard frequencies at which A.C. power is supplied, 60 cycle and 50 cycle. Thus a digital timer, or timing subsystem, that is to be used in equipment essentially worldwide must be able to synchronize itself with a source operating at either frequency if it is to use the frequency of such sources as a timing reference to obtain long term stablility with the desired degree of accuracy.