Abstract:
A delay-locked-loop (DLL) that has increased precision and a wide range of operation is formed by utilizing a chain of delay blocks to add or subtract a discreet amount of delay, and a voltage-controlled delay line (VCDL) to add or subtract a smaller amount of delay. The delay blocks allow the delayed clock signal to get close to the reference clock signal, while the VCDL allows the delayed clock signal to lock onto the reference clock signal.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a delay-locked loop and, more particularly, to the delay settings for a delay-locked loop implementation that has increased precision and a wide range of operation. 
   2. Description of the Related Art 
   A delay-locked loop (DLL) is a circuit that outputs a delayed clock signal that is delayed from and in phase with a reference clock signal.  FIG. 1  shows a schematic diagram that illustrates a conventional DLL  100 . As shown in  FIG. 1 , DLL  100  includes a voltage-controlled delay line (VCDL)  110  that receives a reference clock signal VCLK and a control voltage VCNTL, and outputs a delayed clock signal VDCK that is a delayed version of the reference clock signal VCLK. The amount of delay, in turn, is defined by the magnitude of the control voltage VCNTL. 
   As further shown in  FIG. 1 , DLL  100  also includes a phase detector  112  that detects the difference in phase between the reference clock signal VCLK and the delayed clock signal VDCK. When the reference clock signal VCLK leads the delayed clock signal VDCK, phase detector  112  asserts an up signal VUP. 
   On the other hand, when the reference clock signal VCLK lags the delayed clock signal VDCK, phase detector  112  asserts a down signal VDN. When the reference clock signal VCLK and the delayed clock signal VDCK are in phase, phase detector  112  asserts neither the up signal VUP nor the down signal VDN. 
   In addition, DLL  100  also includes a charge pump  114  that outputs a pump voltage VPM. Pump  114  increases the pump voltage VPM when the up signal VUP is asserted, and decreases the pump voltage VPM when the down signal VDN is asserted. The pump voltage VPM is unchanged when both the up signal VUP and the down signal VDN are de-asserted. Further, DLL  100  includes a filter  116  that filters the voltage output from pump  114  to provide the control voltage VCNTL. 
   In operation, phase detector  112  continues to adjust the pump voltage VPM via the up and down signals VUP and VDN, and thereby the control voltage VCNTL, until VCDL  110  adjusts the timing of the delayed clock signal VDCK to be in phase with the reference clock signal VCLK. When the delayed clock signal VDCK is in phase with the reference clock signal VCLK, DLL is locked and phase detector  112  inhibits the up and down signals VUP and VDN until the clock signals VDCK and VCLK fall out of lock. 
   DLLs are typically formed to accommodate a range of signal periods. One problem with DLLs, however, is that it is difficult to form a DLL that can accommodate a wide range of signal periods. For example, it is difficult to track clocks with periods varying from in 1 nS to 20 nS as the DLL should be able to delay the reference clock signal VCLK by a minimum of 1 nS and a maximum of 20 nS. 
   Another problem with DLLs is that it is difficult to obtain high precision (granularity) such that the delay varies evenly with the control voltage VCNTL. Ideally, the minimum delay provided by the DLL corresponds with a control voltage VCNTL equal to the lower supply voltage VSS, and the maximum delay provided by the DLL corresponds with a control voltage VCNTL equal to the upper supply voltage VCC. In addition, intermediate delays ideally vary proportionally as the control voltage VCNTL varies between the lower and upper supply voltages VSS and VCC. 
   In addition, DLLs are typically sensitive to temperature and process variations which, in turn, can prevent a DLL from locking on all frequencies. Thus, there is a need for a delay locked loop with increased precision and range of operation that is ideally insensitive to temperature and process variations. 
   SUMMARY OF THE INVENTION 
   The present invention provides a delay-locked-loop (DLL) that has increased precision and a wide range of operation by utilizing a chain of delay blocks to add or subtract a discreet amount of delay, and a delay line to add or subtract a smaller amount of delay. The delay blocks bring the difference between the delayed clock signal and the reference clock signal to within the locking range of the delay line, while the delay line locks the delayed clock signal to the reference clock signal. 
   The DLL of the present invention includes a voltage-controlled delay line (VCDL) that varies a timing of the delayed clock signal with respect to an intermediate clock signal in response to the magnitude of a control voltage. The DLL also includes a phase detector connected to the VCDL that detects a difference in phase between a reference clock signal and the delayed clock signal. The phase detector asserts an up signal when the reference clock signal leads the delayed clock signal, a down signal when the reference clock signal lags the delayed clock signal, and a synch signal when the reference clock signal and the delayed clock signal are in phase. 
   Further, the DLL includes a charge pump connected to the phase detector that outputs a pump voltage. The charge pump increases the pump voltage when the up signal is asserted, decreases the pump voltage when the down signal is asserted, and leaves the pump voltage unchanged when the synch signal is asserted. In addition, the DLL includes a filter connected to the charge pump and the VCDL that filters the pump voltage to output the control voltage. 
   In accordance with the present invention, the DLL includes a delay circuit that varies a timing of the intermediate clock signal with respect to the reference clock signal by adding or subtracting incremental units of delay in response to the control voltage and the logic states of the up signal, the down signal, and the synch signal. This is a preliminary process of coarse delay adjustments, which can be implemented using digital delay blocks. 
   The present invention also includes a method for locking a delayed clock signal to a reference clock signal such that the delayed clock signal has a delay with respect to the reference clock signal. The method of the present invention includes the step of varying a timing of the delayed clock signal with respect to an intermediate clock signal with a voltage-controlled delay line (VCDL) in response to a magnitude of a control voltage. This process involves fine delay adjustments and is implemented using analog blocks for greater precision. 
   The method also includes the step of detecting with a phase detector a difference in phase between a reference clock signal and the delayed clock signal. The method further includes the step of asserting with the phase detector an up signal when the reference clock signal leads the delayed clock signal, a down signal when the reference clock signal lags the delayed clock signal, and a synch signal when the reference clock signal and the delayed clock signal are in phase. 
   In addition, the method includes the step of outputting with a charge pump a pump voltage. The pump voltage increases when the up signal is asserted, decreases when the down signal is asserted, and is unchanged when the synch signal is asserted. Further, the method includes the step of filtering with a filter the pump voltage to output the control voltage. 
   In accordance with the method of the present invention, the method includes the step of varying a timing of the intermediate clock signal with respect to the reference clock signal with a delay circuit by adding or subtracting incremental units of delay in response to the control voltage and the logic states of the up signal, the down signal, and the synch signal. 
   A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram illustrating a conventional delay-locked-loop (DLL)  100 . 
       FIG. 2  is a schematic diagram illustrating a DLL  200  in accordance with the present invention. 
       FIG. 3  is a schematic diagram illustrating an example of VCDL  210  in accordance with the present invention. 
       FIG. 4  is a schematic diagram illustrating an example of phase detector  212  in accordance with the present invention. 
       FIGS. 5A–5B  are timing diagrams illustrating the operation of circuit  408  in accordance with the present invention. 
       FIG. 5C  is a timing diagram illustrating the operation of circuit  430  in accordance with the present invention. 
       FIG. 6  is a schematic diagram illustrating a conventional implementation of charge pump  214 . 
       FIG. 7  is a schematic diagram illustrating control circuit  222  in accordance with the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 2  shows a schematic diagram that illustrates a delay-locked-loop (DLL)  200  in accordance with the present invention. As shown in  FIG. 2 , DLL  200  includes a voltage-controlled delay line (VCDL)  210  that varies a timing of a delayed clock signal VDCK with respect to an intermediate clock signal VBCK in response to the magnitude of a control voltage VCNTL.  FIG. 3  shows a schematic diagram that illustrates an example of VCDL  210  in accordance with the present invention. 
   As shown in  FIG. 3 , VCDL  210  includes a voltage-controlled current stage  308  that generates a switching current IS that has a magnitude that is defined by the magnitude of the control voltage VCNTL. Stage  308  includes a p-channel diode-connected transistor  310  that has a source connected to a power supply node VCC, and a gate and drain connected to a first diode node ND 1 . VCDL  210  also includes a p-channel transistor  312  that has a source connected to the power supply node VCC, a gate connected to the gate of transistor  310 , and drain connected to a second diode node ND 2 . 
   Stage  308  further includes an n-channel transistor  314  and a n-channel diode-connected transistor  316 . Transistor  314  has a drain connected to the first diode node ND 1 , a gate connected to receive the control voltage VCNTL, and a source connected to ground. Transistor  316  has a drain and a gate connected to the second diode node ND 2 , and a source connected to ground. 
   In operation, the control voltage VCNTL on the gate of transistor  314  sets the drain current of transistor  314  which, in turn, sets the drain current of transistor  310 . The drain current of transistor  312  mirrors (is proportional to) the drain current of transistor  310  which, in turn, sets the drain current of transistor  316 . The drain currents of transistors  312  and  316  are equal, and define the switching current IS. 
   As further shown in  FIG. 3 , VCDL  210  also includes a fine-delay stage  320  that delays the intermediate clock signal VBCK to output the delayed clock signal VDCK. Stage  320  has a propagation delay which is defined by the magnitude of the switching current IS. The delayed clock signal VDCK, in turn, is delayed in time from the intermediate clock signal VBCK by the propagation delay. 
   Stage  320  includes a number of p-channel mirror transistors P 1 –Pr. Each transistor P 1 –Pr has a source connected to the power supply node VCC, a gate connected to the gate of diode-connected transistor  310 , and a drain connected to a corresponding one of a number of nodes NP 1 –NPr. 
   Stage  320  further includes a number of n-channel mirror transistors N 1 –Nr. Each transistor N 1 –Nr has a source connected to ground, a gate connected to the gate of diode-connected transistor  316 , and a drain connected to a corresponding one of a number of nodes NN 1 –NNr. 
   Stage  320  additionally includes a number of inverters INV 1 –INVm (where m equals 2r) which are connected to nodes NP 1 –NPr and NN 1 –NNr so that each odd numbered inverter is connected to a node NP 1 –NPr and a node NN 1 –NNr. In addition, the first inverter INV 1  is connected to receive the intermediate clock signal VBCK, and the last inverter INVm is connected to output the delayed clock signal VDCK. 
   In operation, the drain currents of transistors P 1 –Pr mirror (are proportional to) the drain current of transistor  310 , while the drain currents of transistors N 1 –Nr mirror (are proportional to) the drain current of transistor  316 . Thus, the magnitude of the current flowing through each odd-numbered inverter is proportional to the switching current IS which, in turn, is defined by the magnitude of the control voltage VCNTL. 
   The magnitude of the current flowing through an inverter determines how quickly the inverter can change logic states. Thus, by varying the control voltage VCNTL, which varies the switching current IS, the time required for a change in logic state on the input of the first inverter INV 1  to appear on the output of the last inverter INVm can be varied. The delay provided by the VCDL can be seen to decrease with increase in VCNTL and vice-versa. For dual edge tracking, the duty cycle should not be degraded. The transistors have to be appropriately sized to get equal rise and fall delays. 
   Returning again to  FIG. 2 , DLL  200  also includes a phase detector  212  that detects the difference in phase between the reference clock signal VCLK and the delayed clock signal VDCK. When the reference clock signal VCLK leads the delayed clock signal VDCK, phase detector  212  asserts an up signal VUP. 
   On the other hand, when the reference clock signal VCLK lags the delayed clock signal VDCK, phase detector  212  asserts a down signal VDN. When the reference clock signal VCLK and the delayed clock signal VDCK are in phase (have rising edges at the same time or within a predefined error tolerance), phase detector  212  asserts a phase synchronization signal PYSYNC. 
     FIG. 4  shows a schematic diagram that illustrates an example of phase detector  212  in accordance with the present invention. As shown in  FIG. 4 , phase detector  212  includes an up/down circuit  408  that asserts the up and down signals VUP and VDN in response to the relative timing of the rising edges of the reference clock signal VCLK and the delayed clock signal VDCK. 
   Circuit  408  includes a D-Q flip-flop  410  and an XNOR gate  412 . Flip-flop  410  has a D input connected to receive the delayed clock signal VDCK, and a clock input connected to receive the reference clock signal VCLK. XNOR gate  412  has the delayed clock signal VDCK and the reference clock signal VCLK as its inputs. 
   Circuit  408  further includes a first inverter  414  which has an input connected to the Q output of flip-flop  410 , and a second inverter  416  which has an input connected to the output of first inverter  414 . In addition, circuit  408  includes a first NOR gate  420  and a second NOR gate  422 . 
   NOR gate  420  has a first input connected to the output of gate  412  and a second input connected to the output of the second inverter  416 . NOR gate  420  also has an output that outputs the up signal VUP. NOR gate  422  has a first input connected to the output of gate  412  and a second input connected to the output of the first inverter  414 . NOR gate  422  also has an output that outputs the down signal VDN. 
   In operation, the outputs of NOR gates  420  and  422  (the up and down signals VUP and VDN) are asserted only when both inputs are low. Since the second input of NOR gate  420  is connected to the output of second inverter  416  and the second input of NOR gate  422  is connected to the output of first inverter  414 , which is the input of the second inverter  416 , only one of these second inputs can be low at any one time. Thus, the up and down signals VUP and VDN can not be asserted at the same time. 
   In addition, the first inputs of NOR gates  420  and  422  are connected to the output of XNOR gate  412  which is low only when the logic states of the clock signals VCLK and VDCK are different. Thus, the up and down signals VUP and VDN can not be asserted when the clock signals VCLK and VDCK have the same logic state. 
     FIGS. 5A–5B  show timing diagrams that illustrate the operation of circuit  408  in accordance with the present invention. As shown in  FIG. 5A , when the logic states of both the reference and delayed clock signals VCLK and VDCK are low, the output of XNOR gate  412  is high. This, in turn, causes the outputs of both NOR gates  420  and  422  (the up and down signals VUP and VDN) to be low. When the logic state of the reference clock signal VCLK goes high, flip-flop  410  latches a logic low on the Q output. In addition, the output of XNOR gate  412  goes low. As a result, NOR gate  420  asserts the up signal VUP by outputting a logic high. 
   When the delayed clock signal VDCK goes high, the output of XNOR gate  412  goes high which, in turn, causes the output of NOR gate  420  to go low. This process of latching a logic low and asserting the up signal VUP continues until the delayed clock signal VDCK, which is lagging the reference clock signal VCLK, begins to lead the reference clocks signal VCLK. 
   As shown in  FIG. 5B , when the logic states of both the reference and delayed clock signals VCLK and VDCK are low, the output of XNOR gate  412  is high. This, in turn, causes the outputs of both NOR gates  420  and  422  (the up and down signals VUP and VDN) to be low. When the logic state of the delayed clock signal VDCK goes high, the output of XNOR gate  412  goes low. Assuming that a logic high is on the Q output, NOR gate  422  asserts the down signal VDN by outputting a logic high. 
   When the reference clock signal VCLK strobes the clock input, flip-flop  410  latches a logic high. In addition, the output of XNOR gate  412  goes high which, in turn, causes the output of NOR gate  422  to go low. This process of latching a logic high and asserting the down signal VDN continues until the delayed clock signal VDCK, which is leading the reference clock signal VCLK, begins to lag the reference clocks signal VCLK. 
   As further shown in  FIG. 4 , phase detector  212  includes a synch detecting circuit  430  that asserts a phase synchronization signal PYSYNC when the reference clock signal VCLK and the delayed clock signal VDCK are in phase (have rising edges at the same time or within a predefined error tolerance). 
   Circuit  430  includes a first rising edge detecting circuit  432  that receives the reference clock signal VCLK, and outputs a reference pulse RP having a predefined width in response to the rising edge of the reference clock signal VCLK. Similarly, circuit  430  includes a second rising edge detecting circuit  434  that receives the delayed clock signal VDCK, and outputs a pulse DP having a predefined width in response to the rising edge of the delayed clock signal VDCK. Circuit  430  further includes a logic circuit  436  that asserts the phase synchronization signal PYSYNC when the reference and delayed pulses RP and DP overlap. 
     FIG. 5C  shows a timing diagram that illustrates the operation of circuit  430  in accordance with the present invention. As shown in  FIG. 5C , if there is any overlap between the reference and delayed pulses RP and DP, the synchronization signal PYSYNC, which is otherwise low, is asserted. Further, by varying the width of the reference and delayed pulses RP and DP, the precision to which the reference and delayed clock signals VCLK and VDCK are considered in phase can be set. 
   Returning again to  FIG. 2 , DLL  200  also includes a charge pump  214  that outputs a pump voltage VPM. Pump  214  increases the pump voltage VPM when the up signal VUP is asserted, and decreases the pump voltage VPM when the down signal VDN is asserted. The pump voltage VPM is unchanged when the synchronization signal PYSYNC is asserted. 
     FIG. 6  shows a schematic diagram that illustrates an example of charge pump  214 . As shown in  FIG. 6 , charge pump  214  includes a first current source I 1 , and an up switch  610  (such as a MOS transistor) that is connected to the first current source I 1  and a pump node NPM. In addition, charge pump  214  includes a down switch  612  (such as a MOS transistor) that is connected to the pump node NP, and a second current source I 2  that is connected to down switch  612 . 
   Further, charge pump  214  includes a logic block  614  that passes the logic state of the up signal VUP when the synchronization signal PYSYNC is de-asserted, and outputs a logic low when the synchronization signal PYSYNC is asserted. Charge pump  214  also includes a logic block  616  that passes the logic state of the down signal VDN when the synchronization signal PYSYNC is de-asserted, and outputs a logic low when the synchronization signal PYSYNC is asserted. 
   In operation, when the up signal VUP is asserted and the synchronization signal PYSYNC is de-asserted, up switch  610  is closed and the first current source I 1  sources current into the pump node NP, thereby increasing the voltage on the pump node NP. Similarly, when the down signal VDN is asserted and the synchronization signal PYSYNC is de-asserted, down switch  612  is closed and the second current source I 2  sinks current from the pump node, thereby decreasing the voltage on the pump node NP. 
   Returning to  FIG. 2 , DLL  200  includes a filter  216  that filters the pump voltage VPM output from pump  214  to provide the control voltage VCNTL. Filter  216  can be implemented as a monolithic RC filter by using polysilicon resistors and MOS transistors. 
   In accordance with the present invention, DLL  200  also includes a delay circuit  218  that varies the timing of the intermediate clock signal VBCK with respect to the reference clock signal VCLK by adding or subtracting incremental units of delay. Delay circuit  218  includes a number of delay blocks DEL 0 –DELn which each provide a predetermined delay when turned on or inserted into the signal path, and essentially no delay when turned off or removed from the signal path. 
   Delay circuit  218  further includes a control circuit  222  that defines the amount of delay provided by delay blocks DEL 0 –DELn by controlling the on-off state of the delay blocks DEL 0 –DELn. Delay circuit  218  responds to the control voltage VCNTL and the logic states of the up signal VUP, the down signal VDN, and the phase synchronized signal PYSYNC. 
   In operation, the control voltage VCNTL is divided into three regions; a lower region such as 0 to Vtn where Vtn is the threshold voltage of an n-channel transistor, a middle region such as Vtn to (VCC-|Vtp|) where |Vtp| is the absolute value of the threshold voltage of a p-channel transistor, and an upper region such as (VCC-|Vtp|) to VCC. 
   When the reference clock signal VCLK lags the delayed clock signal VDCK, the down signal VDN is asserted. The assertion of the down signal VDN when the control voltage VCNTL is in the lower region turns on a delay block DEL, which increases the delay added to the delayed clock signal VDCK. This process continues with each cycle of the reference clock signal VCLK until the delay provided by delay blocks DEL 0 –DELn is sufficient to allow VCDL  210  to lock or synchronize the clock signals VCLK and VDCK. 
   When the control voltage VCNTL is in the middle region, the reference clock signal VCLK and the delayed clock signal VDCK are within a range which allows VCDL to lock or synchronize the clock signals VCLK and VDCK. 
   When the reference clock signal VCLK leads the delayed clock signal VDCK, the up signal VUP is asserted. The assertion of the up signal VUP when the control voltage VCNTL is in the upper region turns off a delay block DEL, which decreases the delay added to the delayed clock signal. This process continues with each cycle of the reference clock signal VCLK until the delay provided by delay blocks DEL 0 –DELn is sufficient to allow VCDL  210  to lock or synchronize the clock signals VCLK and VDCK. 
     FIG. 7  shows a schematic diagram that illustrates control circuit  222  in accordance with the present invention. As shown in  FIG. 7 , control circuit  222  includes a shift register  710  that includes a number of registers REG 0 –REGn that output a corresponding number of select signals SEL 0 –SELn. The output of each register REG, in turn, is connected to a corresponding delay block DEL. 
   As further shown in  FIG. 7 , control circuit  222  includes a down stage  712  that asserts a shift left signal VSL in response to the control voltage VCNTL, the down signal VDN, and the phase synchronized signal PYSYNC. Down stage  712  includes a weakly biased p-channel transistor  714  and an n-channel transistor  716 . Transistor  714  has a source connected to the power supply voltage VCC, a gate connected to ground, and a drain connected to a node NLH. Transistor  716  has a source connected to ground, a gate connected to receive the control voltage VCNTL, and a drain connected to the node NLH. 
   Down stage  712  also includes a NAND gate  722  and a NOR gate  724 . NAND gate  722  has a first input connected to the node NLH, a second input connected to receive the down signal VDN, and an output. NOR gate  724  includes a first input connected to receive the output of NAND gate  722 , a second input connected to receive the synchronized signal PYSYNC, and an output that outputs the shift left signal VSL. 
   Control circuit  222  further includes an up stage  730  that asserts a shift right signal VSR in response to the control voltage VCNTL, the up signal VUP, and the phase synchronized signal PYSYNC. Up stage  730  includes a p-channel transistor  732  and a weakly biased n-channel transistor  734 . Transistor  732  has a source connected to the power supply voltage VCC, a gate connected to the control voltage VCNTL, and a drain connected to a node NLL. Transistor  734  has a source connected to ground, a gate connected to the power supply voltage VCC, and a drain connected to the node NLL. 
   Up stage  730  also includes an inverter  736 , a NAND gate  738  and a NOR gate  740 . Inverter  736  has an input connected to the node NLL and an output. NAND gate  738  has a first input connected to the output of inverter  736 , a second input connected to receive the up signal VUP, and an output. NOR gate  740  includes a first input connected to receive the output of NAND gate  738 , a second input connected to receive the synchronized signal PYSYNC, and an output that outputs the shift right signal VSR. 
   In operation, the first input of NAND gate  722  is a logic high only when the control voltage VCNTL is in the lower region (is less than the threshold voltage Vtn of transistor  716 ). Thus, NAND gate  722  outputs a logic low each time the down signal VDN is asserted and the control voltage VCNTL is in the lower region. 
   Further, as long as the synchronized signal PYSYNC is low (clock signal VCLK and VDCK not in synch), NOR gate  724  outputs a logic high each time the output from NAND gate  722  is low, thereby causing the shift left signal VSL to be asserted. When the synchronized signal PYSYNC is high (clock signal VCLK and VDCK in synch), NOR gate  724  outputs a logic low. 
   Similarly, the input of inverter  736  is a logic low only when the control voltage VCNTL is in the upper region (within the range of the threshold voltage Vtp of transistor  732  from the upper supply rail VCC). Thus, the first input of NAND gate  738  is a logic high only when the control voltage VCNTL is in the upper region. 
   NAND gate  738  outputs a logic low each time the up signal VUP is asserted and the control voltage VCNTL is in the upper region. Further, as long as the synchronized signal PYSYNC is low (clock signal VCLK and VDCK not in synch), NOR gate  740  outputs a logic high each time the output from NAND gate  738  is low, thereby causing the shift right signal VSR to be asserted. When the synchronized signal PYSYNC is high (clock signal VCLK and VDCK in synch), NOR gate  740  outputs a logic low. 
   Each time the shift right signal VSR is asserted, a logic zero is shifted right in shift register  710 . For example, assuming a logic one is output by registers REG 0  and REG 1 , and a logic zero is output by the remaining registers. If the shift right signal VSR is asserted once, register REG 1  changes to output a logic zero, while register REG 0  remains the same and other registers still output a logic zero. 
   In addition, each time the shift left signal VSL is asserted, a logic one is shifted left in shift register  710 . For example, assuming a logic one is output by only register REG 0 , and a logic zero is output by the remaining registers. If the shift left signal VSL is asserted once, register REG 1  changes to output a logic one, while register REG 0  remains the same and other registers still output a logic zero. 
   Further, as registers REG 0 –REGn turn off and on (logic zero or logic one respectively), the corresponding delay block DEL is turned off and on. For example, if the delayed clock signal VDCK leads the reference clock signal VCLK, additional delay needs to be added to the delayed clock signal VDCK. As noted above, each time the delayed clock signal VDCK leads the reference clock signal VCLK when the rising edge of the reference clock signal VCLK is detected, phase detector  212  asserts the down signal VDN. The down signal VDN, in turn, causes the shift left signal VSL to be asserted which, in turn, causes an additional delay block DEL to be turned on. This adds delay to the intermediate clock signal VBCK and thus, adds delay to the delayed clock signal VDCK. 
   On the other hand, if the delayed clock signal VDCK lags the reference clock signal VCLK, delay needs to be removed from the delayed clock signal VDCK. As noted above, each time the delayed clock signal VDCK lags the reference clock signal VCLK when the rising edge of the reference clock signal VCLK is detected, phase detector  212  asserts the up signal VUP. The up signal VUP, in turn, causes the shift right signal VSR to be asserted which, in turn, causes a delay block DEL to be turned off. This removes delay from the intermediate clock signal VBCK and thus, removes delay from the delayed clock signal VDCK. 
   Returning again to  FIG. 2 , DLL  200  of the present invention also includes a set/reset circuit  226 . Circuit  226  asserts a reset signal VRST that resets each register REG to a logic low when the output of each register REG is a logic high and the shift left signal VSL is asserted. In addition, circuit  226  also asserts a set signal VST that sets each register REG to a logic high when the output of each register REG is a logic low and the shift right signal VSR is asserted. (When DLL  200  is initially powered on, the registers REG 0 –REGn are set to a logic low to turn off all of the delay blocks DEL 0 –DELn.) 
   DLL  200  may get stuck in a state without locking. In this case, circuit  226  sets/resets the logic state of each register REG to start the feedback process all over again. DLL  200  can get stuck because of improper initial conditions, a random glitch, or any other unexpected condition. 
   Thus, a DLL has been described that has a wide range of operation with a high precision dual-edge synchronization. Since the DLL of the present invention utilizes a feedback mechanism, the DLL is largely insensitive to process and temperature variations, provided that it is within the range of the DLL. 
   It should be understood that various alternatives to the embodiment of the invention described herein may be employed in practicing the invention. Thus, it is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.