PLL circuit with self-selecting variable divide ratio
First Claim
1. A phase-locked loop (PLL) circuit with self-selecting variable divide ratio, comprising:
- a voltage-controlled oscillator (VCO) having an input and which produces a clock output having a frequency that varies within a predetermined frequency range with a control voltage applied to said input;
a variable divider having an divide ratio control input and which receives said VCO clock output and divides it by a divide ratio value that varies with a divide ratio control signal applied to said divide ratio control input;
a phase detector which receives a reference clock and said divided-down VCO clock at respective inputs and produces one or more outputs that vary with the difference in phase between said inputs; and
a loop filter which receives at least one of said phase detector outputs and provides said control voltage to said VCO input so as to reduce the difference in phase between said reference clock and said divided-down VCO clock; and
a frequency band select circuit which provides said divide ratio control signal to said variable divider'"'"'s divide ratio control input such that a divide ratio value is selected which enables said phase difference to be driven toward zero with said VCO clock output within said predetermined frequency range.
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Abstract
A PLL circuit provides a self-selecting divide ratio, which is varied as necessary to lock the circuit to a reference clock which may have several possible frequencies, thereby enabling the VCO to employ a type of oscillator having a superior jitter characteristic. The PLL circuit includes a variable divider which divides the VCO output by a divide ratio value provided by a frequency band select circuit, which provides the divide ratio needed to drive the phase difference between the reference and divided clocks toward zero while the VCO clock output operates within a predetermined frequency range. The self-selecting variable divide ratio allows the VCO'"'"'s oscillator to have a narrow output frequency range, thereby allowing the use of an oscillator type with a jitter characteristic which may be low enough to meet the requirements of JEDEC, for example.
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Citations
13 Claims
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1. A phase-locked loop (PLL) circuit with self-selecting variable divide ratio, comprising:
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a voltage-controlled oscillator (VCO) having an input and which produces a clock output having a frequency that varies within a predetermined frequency range with a control voltage applied to said input;
a variable divider having an divide ratio control input and which receives said VCO clock output and divides it by a divide ratio value that varies with a divide ratio control signal applied to said divide ratio control input;
a phase detector which receives a reference clock and said divided-down VCO clock at respective inputs and produces one or more outputs that vary with the difference in phase between said inputs; and
a loop filter which receives at least one of said phase detector outputs and provides said control voltage to said VCO input so as to reduce the difference in phase between said reference clock and said divided-down VCO clock; and
a frequency band select circuit which provides said divide ratio control signal to said variable divider'"'"'s divide ratio control input such that a divide ratio value is selected which enables said phase difference to be driven toward zero with said VCO clock output within said predetermined frequency range. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
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13. A method of setting a divide ratio for a phase-locked loop circuit which includes a voltage controlled oscillator (VCO) that produces an output clock signal (VCOClk), comprising:
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monitoring a reference clock (RefClk) having a frequency fref and a clock (DivClk) having a frequency fdiv produced by dividing VCOClk by a divide ratio;
detecting a “
cycle slip”
, defined as when a plot of the time difference between the rising edge of RefClk and the rising edge of DivClk versus time passes through zero, said cycle slip being an “
up-slip”
when said plot passes through zero due to fdiv>
fref and a “
down-slip”
when said plot passes through zero due to fref>
fdiv;
setting said divide ratio to a mid-ratio value when the number of detected cycle slips exceeds a predetermined minimum number;
incrementing said divide ratio when a predetermined number of consecutive up-slips are detected and decrementing said divide ratio when a predetermined number of consecutive down-slips are detected;
detecting when said divide ratio is being toggled between adjacent values;
detecting when said divide ratio has toggled between adjacent values a predetermined number of times;
selecting the higher of the two divide ratios when said divide ratio has toggled between adjacent values said predetermined number of times and the time that between consecutive up-slips is greater than the time between consecutive down-slips; and
selecting the lower of the two divide ratios when said divide ratio has toggled between adjacent values said predetermined number of times and the time that between consecutive up-slips is less than the time between consecutive down-slips.
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Specification