Delay locked loop circuit

1 . A delay locked loop circuit comprising:

a voltage-controlled delay line that comprises one or more variable components, wherein the voltage-controlled delay line is configured to:

receive a clock-in signal;

receive a control-voltage signal; and

apply a time delay to the clock-in signal in order to provide a first output signal and a last output signal, wherein a magnitude of the time delay is based on the control-voltage signal and a delay code, and wherein the delay code defines values of the one or more variable components in the voltage-controlled delay line;

a phase detector circuit that is configured to process the first output signal and the last output signal in order to provide: i) an up-pulse signal; and ii) a down-pulse signal, wherein the difference between a width of the up-pulse signal and a width of the down-pulse signal represent the phase difference between the first output signal and the last output signal;

a charge pump that is configured to provide a feedback voltage signal based on the up-pulse signal and the down-pulse signal;

a phase signal processor that is configured to process the up-pulse signal and the down-pulse signal in order to provide a delay code locking signal, which is representative of whether or not the first output signal is 2Π radians out of phase with the last output signal;

a delay code setter that is configured to provide a delay code setting signal that represents one of a sequence of different candidate delay codes;

a fixed voltage source that is configured to provide a fixed voltage signal; and

a switching circuit that is configured to put the delay locked loop circuit in either an active mode of operation or a calibration mode of operation, wherein:

when the delay locked loop circuit is in the calibration mode of operation:

the fixed voltage source is configured to provide the fixed voltage signal as the control-voltage signal for the voltage-controlled delay line;

while the delay code locking signal represents the first output signal not being 2Π radians out of phase with the last output signal: the delay code setter is configured to apply the delay code setting signal to the voltage-controlled delay line such that it sequentially applies different candidate delay codes to the voltage-controlled delay line as an active candidate delay code; and

when the delay code locking signal represents the first output signal being 2Π radians out of phase with the last output signal: the delay code setter is configured to assign the active candidate delay code as a selected-delay-code;

when the delay locked loop circuit is in the active mode of operation:

the charge pump is configured to provide the feedback voltage signal as the control-voltage signal for the voltage-controlled delay line; and

the voltage-controlled delay line is configured to use the selected-delay-code as the delay code; and

wherein the voltage-controlled delay line comprises:

a delay line bias circuit, wherein:

the delay line bias circuit includes a resistor bank;

the resistor bank comprises a plurality of resistors that are connected in a series arrangement between a resistor-bank-connection node and a ground terminal, wherein the resistor bank has a resistance value that affects the magnitude of the time delay that is applied by the voltage-controlled delay line;

the delay line bias circuit includes a plurality of bypass switches, each of the plurality of bypass switches being associated with a respective resistor in the resistor bank;

each bypass switch is configured to selectively include or exclude its associated resistor in the series arrangement between the resistor-bank-connection node and the ground terminal in order to set the resistance value of the resistor bank based on a resistive-code signal; and

the delay line bias circuit comprises a controller that provides the resistive-code signals for each of the bypass switches based on the delay code.

2 . The delay locked loop circuit of claim 1 , wherein the delay code setter is configured to apply the delay code setting signal to the voltage-controlled delay line such that it applies a sequence of candidate delay codes through which application the value of the delay code is gradually increased until the last output signal is at least 2Π radians out of phase with the first output signal.

3 . The delay locked loop circuit of claim 1 , wherein the phase signal processor comprises a D-type flip flop which has: a data input terminal, a clock input terminal, and an output terminal, wherein:

the data input terminal is configured to receive one of the up-pulse signal and the down-pulse signal;

the clock input terminal is configured to receive the other one of the up-pulse signal and the down-pulse signal; and

the output terminal is configured to provide the delay code locking signal.

4 . The delay locked loop circuit of claim 3 , wherein the D-type flip flop further comprises an enable input terminal, and wherein:

the enable input terminal is configured to receive an enable input signal when the delay locked loop circuit is in the calibration mode of operation; and

the enable input terminal is configured to receive a disable input signal when the delay locked loop circuit is in the active mode of operation.

5 . The delay locked loop circuit of claim 1 , wherein the phase detector is a proportional phase detector.

6 . The delay locked loop circuit of claim 1 , further comprising:

a reset generator that is configured to ensure that the delay code setter receives a first rising edge of the last output signal before the corresponding rising edge of the first output signal before the delay code setter sequentially applies the different candidate delay codes to the voltage-controlled delay line.

7 . The delay locked loop circuit of claim 1 , wherein:

the voltage-controlled delay line comprises a plurality of delay buffers that are connected in series to successively apply a time delay to the clock-in signal;

the output of a first delay buffer of the plurality of delay buffers is the first output signal;

the output of a last delay buffer of the plurality of delay buffers is the last output signal; and

the magnitude of the time delay that is applied by each delay buffer of the plurality of delay buffers is based on a bias-voltage signal;

the delay line bias circuit further comprises a calibration transistor;

the resistance value of the resistor bank is configured to set a current through the calibration transistor; and

the delay line bias circuit is configured to set the bias-voltage signal based on the current through the calibration transistor.

8 . The delay locked loop circuit of claim 1 , wherein:

the voltage-controlled delay line comprises a plurality of delay buffers that are connected in series to successively apply a time delay to the clock-in signal;

each of the plurality of delay buffers has:

an input terminal;

an output terminal;

an intermediate terminal; and

a duty cycle distortion reduction circuit that is connected in series between the intermediate terminal and the output terminal;

the duty cycle distortion reduction circuit comprises:

an ac-coupled capacitor and an inverter in series with each between the intermediate terminal and the output terminal, wherein the inverter has an inverter-input-terminal and an inverter-output-terminal; and

a feedback resistor in series between the inverter-input-terminal and the inverter-output-terminal.

9 . The delay locked loop circuit of claim 8 , wherein the duty cycle distortion reduction circuit further comprises:

a first enable switch in series with the feedback resistor between the inverter-input-terminal and the inverter-output-terminal; and

a second enable switch in series between the inverter-output-terminal and a ground terminal.