Lock detect circuit for detecting a lock condition in a phase locked loop and method therefor
First Claim
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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Accused Products
Abstract
A lock detect circuit (18) determines when a reference frequency and a feedback frequency are frequency locked using a reference counter (32) and a feedback counter (36). The reference counter (32) and the feedback counter (36) are clocked by the reference frequency and the feedback frequency, respectively. After a first period of time, the outputs of the counters are compared. The outputs of the counters are also compared at the end of a second period of time. To be frequency locked, the two count values must be equal at both the end of the first and the second periods of time. A count window is generated from the reference frequency signal to indicate a range of frequencies for which the feedback frequency is locked. Once lock is achieved, the count window is widened such that the feedback frequency is still within a lock range when some aliasing occurs.
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Citations
19 Claims
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1. A lock detect circuit, comprising:
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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. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
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10. A method for detecting lock in a phase lock loop circuit, comprising the steps of:
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i) receiving a reference frequency signal; ii) receiving a feedback frequency signal; iii) enabling a reference counter to count for a first period of time using the reference frequency signal, the reference counter providing a first reference count value; iv) concurrently enabling a feedback counter to count for the first period of time using the feedback frequency signal, the feedback counter providing a first feedback count value; v) comparing the first reference count value and the first feedback count value to provide a first match signal; vi) providing the first match signal to a match detect circuit, the match detect circuit storing the first match signal therein; vii) enabling the reference counter to count for a second period of time using the reference frequency signal, the reference counter providing a second reference count value; viii) concurrently enabling the feedback counter to count for the second period of time using the feedback frequency signal, the feedback counter providing a second feedback count value; ix) comparing the second reference count value and the second feedback count value to provide a second match signal, the second match signal being asserted when the second feedback count value is equal to the second reference count value and the locked signal is not asserted; and x) providing the second match signal to the match detect circuit, the match detect circuit asserting the locked signal when both the first match signal and the second match signal are asserted. - View Dependent Claims (11, 12, 13, 14)
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15. A phase lock loop system, comprising:
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first interface means for receiving a reference frequency signal; second interface means for receiving a feedback frequency signal; a phase comparator for comparing a phase of the reference frequency signal to a phase of the feedback frequency signal, the phase comparator providing a frequency control signal in response to a difference between the phase of the reference frequency signal and the phase of the feedback frequency signal, the phase comparator having a first input coupled to the first interface means for receiving the reference frequency signal, the phase comparator having a second input coupled to the second interface means for receiving the feedback frequency signal; a charge pump coupled to the phase comparator for receiving the frequency control signal, the charge pump providing a voltage value in response to the frequency control signal; a voltage controlled oscillator coupled to the charge pump for receiving the voltage value, the voltage controlled oscillator providing the feedback frequency signal, the feedback frequency signal being determined in response to the voltage value; and a lock detect circuit for detecting when the feedback frequency signal is locked to the reference frequency signal, the lock detect circuit asserting a locked signal to indicate when the feedback frequency signal is locked to the reference frequency signal, wherein the lock detect circuit comprises; a reference counter coupled to the first interface means for receiving the reference frequency signal, 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 interface means for receiving the feedback frequency signal, 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 asserting a match signal when the feedback count value is one of a range of lock values and a locked signal is asserted to indicate the 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; 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 a first and a second match signal, the first and the second match signals being successively asserted; and an auxiliary control generator for generating an auxiliary control signal for enhancing operation of the charge pump, the auxiliary control signal being asserted when the feedback count value is equal to an auxiliary count value. - View Dependent Claims (16, 17, 18, 19)
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Specification