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Method and apparatus for transparently switching clock sources

  • US 5,059,925 A
  • Filed: 09/28/1990
  • Issued: 10/22/1991
  • Est. Priority Date: 09/28/1990
  • Status: Expired due to Term
First Claim
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1. A circuit for maintaining a stable output from a phase-locked loop frequency multiplier circuit, comprising a phase detector, a voltage controlled oscillator, and a divider, when switching an input to said phase-locked loop frequency multiplier circuit from a first input signal to a second input signal, comprising:

  • (a) a load circuit coupled to receive as a first input an output of said divider of said phase-locked loop frequency multiplier circuit, said load circuit changing its output load signal from a first type of state to a second type of state when said second input signal is selected and the output of said divider changes from said first type of state to said second type of state, said divider of said phase-locked loop frequency multiplier circuit coupled to receive said load signal, said divider cleared when said load signal is in said second type of state;

    (b) a first switch coupled to receive said first input signal and said second input signal, said first switch for switching its output from said first input signal to said second input signal when said second input signal is selected, said load circuit coupled to receive as a second input said first switch output signal, said load circuit changing said load signal from said second type of state to said first type of state when said second input signal is selected and said first switch output signal changes from said first type of state to said second type of state, said load signal in said first type of state causing said divider of said phase-locked loop frequency multiplier circuit to continue counting;

    (c) a second switch coupled to receive said first input signal, said second input signal, and said load signal, said second switch for switching its output from said first input signal to said second input signal when said second input signal is selected and said load signal changes from said second type of state to said first type of state;

    (d) a logic circuit coupled to receive said load signal and said second switch output signal, said logic circuit for generating the logical AND of said load signal and said second switch output signal, said logic circuit output coupled to the input of said phase-locked loop frequency multiplier circuit.

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