Circuit and method of switching between redundant clocks for a p

Details Circuit and method of switching between redundant clocks for a p Circuit and method of switching between redundant clocks for a p

1991-05-28

1993-11-09

Chin, Stephen

Pulse or digital communications

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375120, 331 18, 331 25, 371 221, 371 31, H04L 700

Patent

active

052609790

ABSTRACT:
A phase lock loop monitors the frequency of redundant input clock signals and switches back and forth therebetween should one or the other become invalid. Thus, the PLL may continue normal operation even with a failure of one input clock signal. If both the input clock signals fail, an internal reference signal maintains the PLL at a nominal operating frequency until one of the input clock signals is restored whereby the loop can quickly re-establish phase lock. To determined validity, the input clock signals are sampled and stored by the reference signal in a predetermined manner. The input clock signal is valid if the samples of the input clock signal each have the same logic state after the sampling period; otherwise, the input clock signal is invalid if the samples of the input clock signal have at least one different logic state after the sampling period.

REFERENCES:
patent: 4144448 (1979-03-01), Pisciotta et al.
patent: 4392226 (1983-07-01), Cook
patent: 4718389 (1988-01-01), Honig et al.
patent: 4908841 (1990-03-01), Leis et al.
patent: 4918404 (1990-04-01), Vitiello et al.
patent: 4964117 (1990-10-01), Shier
patent: 4972442 (1990-11-01), Steierman

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