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Lock detect circuit for detecting a lock condition in a phase locked loop and method therefor

  • US 5,394,444 A
  • Filed: 07/12/1993
  • Issued: 02/28/1995
  • Est. Priority Date: 07/12/1993
  • Status: Expired due to Term
First Claim
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1. A lock detect circuit, comprising:

  • first input means for receiving a reference frequency;

    second input means for receiving a feedback frequency;

    a reference counter coupled to the first input means for receiving the reference frequency, the reference counter counting to a reference count value during a first period of time, the reference counter providing a freeze signal to indicate the reference counter has reached the reference count value;

    a feedback counter coupled to the second input means for receiving the feedback frequency, the feedback counter counting for the first period of time to generate a feedback count value;

    comparison means for comparing the feedback count value to the reference count value, the comparison means asserting a first match signal when the feedback count value is one of a range of lock values determined by the reference count value and a locked signal is asserted to indicate the first match signal is asserted for a second and a third period of time, the third period of time successively following the second period of time, the comparison means being coupled to the feedback counter for receiving the feedback count value; and

    a match detector coupled to the comparison means for providing the locked signal, the match detector asserting the locked signal when the comparison means asserts the first match signal for both the second period of time and the third period of time.

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