Abstract:
A reference circuit and method for mitigating switching jitter and delay-locked loop (DLL) using same are provided. The reference circuit and method determine a number of steps of a fine delay line (FDL) that are equivalent to a step of a coarse delay line (CDL). Switching jitter of the DLL is reduced since the delay of the step of the CDL that is switched when on an underflow or overflow condition of the FDL is detected is equivalent to the delay of the provided number of steps of the FDL.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is the first application filed for the present invention. 
     FIELD OF THE INVENTION 
     The present invention relates generally to delay-locked loops (DLL). More specifically, the present invention relates to an apparatus and method for mitigating switching jitter in a DLL. 
     BACKGROUND OF THE INVENTION 
     A digital delay-locked loop (DLL) generally includes a phase detector which detects the phase difference between a system clock and a feedback clock, and causes adjustment of a time delay circuit in a loop which causes a DLL output clock to be adjusted to lock with the system clock. The time delay is generally provided by an adjustable delay line. 
     Since the adjustable delay line is typically adjusted in steps, the finest delay resolution depends on the delay line step increments. In order to hold the locked condition, the adjustable delay line is continuously increased and decreased in step increments around a lock point, which results in inherent tracking jitter. In order to reduce the jitter, the adjustable delay line includes a plurality of coarse delay elements (CDE), forming a coarse delay line (CDL), in series with a plurality of fine delay elements (FDE) forming a fine delay line (FDL). After power-up of the circuit, the CDL is adjusted, and once a lock point has almost been determined, the FDL is adjusted, which narrows the window or eye around the lock point, which represents a nominal amount of jitter in a typical application. 
     The FDL preferably includes enough steps for providing a maximum time delay which is equal to or slightly greater than a time delay of a step of the CDL. Once the DLL has stabilized to the lock point, the adjustable delay line will automatically compensate for variations in delay caused by changing temperature and voltage conditions, by varying the FDL. 
     In case of major drift, adjustments in the FDL will underflow/overflow its minimum/maximum delay. In that case, another CDE is switched out/in series, and at the same time the FDL is adjusted to compensate for the CDL decrease/increase to provide the same total delay as before. However, now the FDL can be used again to compensate changes without immediate danger of underflow/overflow. 
     It is assumed in the prior art that exchanging (or switching) a predetermined number FDL steps for a CDL step provides an equivalent delay. However, any differences between the two appear as switching jitter on the DLL output. 
     DLL jitter includes factors such as inherent tracking jitter, power supply noise, and substrate noise induced jitter. The inherent tracking jitter is caused by the up and down adjustments to the fine delay while the DLL is in the locked condition, and as described above, is a variation equivalent to the delay achieved through a single step in the FDL. The jitter caused by switching between the CDL and FDL elements caused by the mismatch between the elements is referred to as switching jitter. This mismatch is highly dependent on the manufacturing process, and thus is hard to predict in the design stage. As operating frequencies continue to increase, the switching jitter can undesirably reduce data eye significantly. In addition, since this switching occurs only infrequently, it is inherently difficult to detect during testing and can cause apparently randomly dropped bits when the DLL is in use in the field. 
     Analog techniques can be used to achieve a wide range of fine resolution tracking for various applications. In particular DLLs based on phase mixers have been shown to achieve high fine resolution tracking range through quadrature mixing. However, most analog based DLL designs employ some form of charge pumps for voltage controlled delay lines and as such they suffer from a limited resolution of the delay steps since the controlling element affects an entire delay line. In addition such DLLs often require a large acquisition time due to loop bandwidths being limited to a small fraction of the clock frequency to ensure stability of the loop. This effect also causes a poor jitter performance in analog DLLs. 
     Furthermore, analog DLL designs are inherently more susceptible to all sources of noise as their control variables (usually voltage) are reduced to achieve finer resolutions. In particular, synchronous dynamic random access memories (SDRAM) provide a very noisy environment for analog blocks in form of supply and substrate noise, which when combined with area restrictions in SDRAMs, sometimes preventing adequate implementation of noise prevention techniques through layout, can result in unreliable DLLs in noisy field environments. 
     Clearly, there is a need for an improved DLL having reduced switching jitter compared to conventional DLLs. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a method for determining a number of steps of a fine delay line (FDL) which are substantially equivalent to a step of a coarse delay line (CDL), the method including steps of: providing a clock signal; delaying the clock signal by a first delay substantially equivalent to a predetermined delay plus an adjustable number of steps of the FDL to provide a first delayed clock signal; delaying the clock signal by a second delay substantially equivalent to the predetermined delay plus a step of the CDL to provide a second delayed clock signal; and adjusting the number of adjustable steps of the FDL so that the first delay is substantially equal to the second delay to provide the number of steps of the FDL that are substantially equivalent to the step of the CDL. 
     According to another aspect of the present invention there is provided a method for determining a number of steps of a fine delay line (FDL) that are substantially equivalent to a step of a coarse delay line (CDL), the method including steps of: providing a clock signal; delaying the clock signal by a first delay substantially equivalent to a first predetermined delay plus an adjustable number of steps of the FDL; delaying the clock signal by a second delay substantially equivalent to a second predetermined delay; adjusting the number of adjustable steps of the FDL so that the first delay is substantially equal to the second delay and providing a first number of adjustable steps of the FDL; delaying the clock signal by a third delay substantially equal to the second predetermined delay plus a step of the CDL; adjusting the number of adjustable steps of the FDL so that the first delay is substantially equal to the third delay and providing a second number of adjustable steps of the FDL; subtracting the first number from the second number of adjustable steps of the FDL to provide the number of steps of the FDL that are substantially equivalent to a step of a CDL. 
     According to still another aspect of the present invention there is provided a reference circuit for determining a number of steps of a fine delay line (FDL) that are substantially equivalent to a step of a coarse delay line (CDL), the reference circuit including: a first path for receiving a clock signal including: a first CDL for providing a first predetermined delay; and a first FDL for providing an adjustable number of delay steps plus a second predetermined delay, a second path for receiving the clock signal including: a second CDL for providing a third predetermined delay substantially equal to the first predetermined delay plus a step of the CDL; and a second FDL for providing a fourth predetermined delay that is substantially equal to the second predetermined delay, a phase detector for receiving outputs from the first and second paths and providing a phase difference of the outputs from the first and second paths, and a controller for: receiving the phase difference from the phase detector; providing a plurality of control signals for adjusting the number of steps of the first FDL so that a total delay of the first path is substantially equal to a total delay of the second path; and providing the number of steps of the FDL that are substantially equivalent a step of the CDL. 
     According to still another aspect of the present invention there is provided a delay-locked loop (DLL) including: a main coarse delay line (CDL) for delaying a main clock signal by zero or more steps of the main CDL; a main fine delay line (FDL) for further delaying the main clock signal by zero or more steps of the main FDL; and a reference circuit for determining a number of steps of the main FDL that are substantially equivalent to one step of the CDL, the reference circuit including: a first path for receiving a divided clock signal including: a first CDL for providing a first predetermined delay; and a first FDL for providing an adjustable number of delay steps plus a second predetermined delay, wherein one step of the first FDL is substantially equivalent one step of the main FDL, a second path for receiving the divided clock signal including: a second CDL for providing a third predetermined delay that is substantially equal to the first predetermined delay plus a step of the main CDL greater the first predetermined delay; and a second FDL for providing a fourth predetermined delay that is substantially equal to the second predetermined delay, a phase detector for receiving outputs from the first and second paths and providing a phase difference of the outputs from the first and second paths, and a controller for: receiving the phase difference from the phase detector; providing a plurality of control signals for adjusting the number of steps of the first FDL so that a total delay of the first path is substantially equal to a total delay of the second path; and providing the number of steps of the FDL that are substantially equivalent a step of the CDL. 
     A reference circuit for determining a number of steps of a fine delay line (FDL) that are substantially equivalent to a step of a coarse delay line (CDL), the reference circuit including: a FDL for receiving a clock signal, and for providing a first predetermined delay plus an adjustable number of delay steps; a CDL for receiving the clock signal, and for providing a second predetermined delay plus an adjustable number of delay steps; a phase detector for receiving outputs from the first and second paths and providing a phase difference of the outputs from the first and second paths; a controller for: receiving the phase difference from the phase detector, providing a control signal to the CDL for setting a first number of steps of the CDL, providing a plurality of control signals for adjusting a first number of steps of the FDL so that a total delay of FDL is substantially equal to a total delay of the CDL, providing the control signal to the CDL for setting a second number of steps of the CDL wherein the second number of delay steps is equal to the first number of delay steps plus one, providing the plurality of control signals for adjusting a second number of steps of the FDL so that the total delay of FDL is substantially equal to the total delay of the CDL, and subtracting the first number of steps of the FDL from the second number of steps of the FDL, and providing the number of steps of the FDL that are substantially equivalent a step of the CDL. 
     According to another aspect of the invention there is provided a delay-locked loop (DLL) including: a main coarse delay line (CDL) for delaying a main clock signal by zero or more steps of the coarse delay line; a main fine delay line (FDL) for further delaying the main clock signal by zero or more steps of the FDL; and a reference circuit for determining a number of steps of a fine delay line (FDL) that are substantially equivalent to a step of a coarse delay line (CDL), the reference circuit including: a FDL for receiving a clock signal, and for providing a first predetermined delay plus an adjustable number of delay steps; a CDL for receiving the clock signal, and for providing a second predetermined delay plus an adjustable number of delay steps; a phase detector for receiving outputs from the first and second paths and providing a phase difference of the outputs from the first and second paths; a controller for: receiving the phase difference from the phase detector, providing a control signal to the CDL for setting a first number of steps of the CDL, providing a plurality of control signals for adjusting a first number of steps of the FDL so that a total delay of FDL is substantially equal to a total delay of the CDL, providing the control signal to the CDL for setting a second number of steps of the CDL wherein the second number of delay steps is equal to the first number of delay steps plus one, providing the plurality of control signals for adjusting a second number of steps of the FDL so that the total delay of FDL is substantially equal to the total delay of the CDL, and subtracting the first number of steps of the FDL from the second number of steps of the FDL, and providing the number of steps of the FDL that are substantially equivalent a step of the CDL. 
     Advantageously, the present invention therefore provides a reference circuit and method for mitigating switching jitter and a DLL having reduced switching jitter compared to conventional DLLs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
         FIG. 1  is a block diagram an embodiment of a delay-locked loop (DLL) in accordance with the present invention; 
         FIG. 2  is a schematic diagram of an embodiment of a main coarse delay line (CDL) shown in  FIG. 1 ; 
         FIG. 3  is a schematic diagram of an embodiment a main fine delay line (FDL) shown in  FIG. 1 ; 
         FIG. 4  is a block diagram of a first embodiment of a reference circuit shown in  FIG. 1 ; 
         FIG. 5  is a block diagram of a second embodiment of the reference circuit shown in  FIG. 1 ; 
         FIGS. 6 to 9  are flowcharts of a first method of determining a number of steps of a FDL that are equivalent to a step of a CDL; and 
         FIGS. 10 to 15  are flowcharts of a second method of determining a number of steps of the FDL that are equivalent to a step of the CDL. 
     
    
    
     It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  illustrates a delay-locked loop (DLL)  100  in accordance with an embodiment of the present invention. A main phase detector  102  receives a main clock (CLK) signal  104  and a feedback clock (F_CLK) signal  106 , compares a phase of the F_CLK signal  106  with a phase of the CLK signal  104 , and issues up  108  and down  110  count control signals to a coarse adjust state machine  112 , and fine adjust state machine  114 . The up and down signals  108 , 110  are also monitored by a main controller  116 , which controls the state machines  112 , 114 . 
     The main coarse adjust state machine  112  preferably includes a one state per flip-flop type state machine for providing a fully decoded output  125  to a main coarse delay line (CDL)  122 . Alternatively, the main coarse adjust state machine  112  may include an up/down counter and thermometer decoder for providing a fully decoded output  125  to the main CDL  122 . 
     The main fine adjust state machine  114  also preferably includes a one state per flip-flop type state machine for providing a fully decoded output to a main fine delay line (FDL)  124 . Alternatively, the main fine adjust state machine  114  may include an up/down counter and thermometer decoder for providing a fully decoded output  126  to the main FDL  124 . 
     The outputs  125 , 126  of the main coarse adjust state machine  112  and fine adjust state machine  114  are preferably tri-state logic signals. A low impedance output enables a respective coarse delay element (CDE) or fine delay element (FDE) (described herein below). A high impedance output disables a respective CDE  206  or FDE  306  thereby reducing a number of steps of the CDL  122  or FDL  124 . 
     The CLK signal  104  is provided to an input of the main CDL  122 , and an output  123  of the main CDL  122  is provided to the input of the main FDL  124 . The main FDL  124  provides the F_CLK signal  106  to the main phase detector  102 . The F_CLK signal  106  is also provided as an output of the DLL  100  having substantially zero delay from the CLK signal  104 . 
     Referring to  FIG. 2 , the main CDL  122  includes a plurality of CDEs  206 , each CDE is preferably a substantially equal valued capacitor based RC delay element. A buffer driver  202  receives the CLK signal  104  and drives a series resistor  204  followed by a plurality of substantially equal valued capacitors  206  which are selectable by tri-state logic signals  125  output from the coarse adjust state machine  112 . A step of the CDL  122  is defined as a incremental delay provided by enabling a CDE  206 . 
     Referring to  FIG. 3 , the main FDL  124  includes a plurality of FDEs  306 , each FDE is preferably a substantially equal valued capacitor based RC delay element. A buffer driver  302  receives an output  123  from the main CDL  122  and drives a series resistor  304  followed by a plurality of substantially equal valued capacitors  306  which are selectable by tri-state logic signals  126  output from the fine adjust state machine  114 . A step of the FDL  124  is defined as a incremental delay provided by enabling a FDE  306 . 
     The embodiments of the CDL  122  and FDL  124  shown in  FIGS. 2 and 3  are simplified for clarity. Those skilled in the art will appreciate that the CDL  122  and FDL  124  may include more buffers, resistors, and transistors than those shown in order to provide specified maximum delays of the CDL  122  and FDL  124 . For example, U.S. Pat. No. 7,190,202, “TRIM UNIT HAVING LESS JITTER”, to O H, issued Mar. 13, 2007, which is hereby incorporated by reference, provides a delay line wherein each delay element includes a select transistor and a load capacitor coupled in series between the delay line and ground potential, and includes a filter circuit having an input to receive an enable signal and having an output coupled to a gate of the select transistor. 
     Referring again to  FIG. 1 , the main controller  116  controls the coarse adjust state machine  112  and fine adjust state machine  114  to adjust a number of steps of the main CDL  122  and main FDL  124  in order to lock the phases the CLK  104  and F_CLK  106  signals together as closely as possible. 
     The main controller  116  senses overflow of the main fine adjust state machine  114 . Overflow is defined as a number of the signals  126  to the main FDL  124  in the low impedance state being greater than a predefined upper limit. Thereupon the main controller  116  controls the coarse adjust state machine  112  to increase the number of coarse delay elements enabled by one by increasing the number of the signals  125  to the main coarse adjust line  122  in low impedance state by one, and controls the fine adjust state machine  114  to lower the number of fine delay elements enabled by M  128  by reducing the number of signals  126  to the main fine adjust delay line  124  in the low impedance state by M  128 , where M  128  is substantially equal to a number of steps of the main FDL  124  needed to provide a delay substantially equal to one step of the main CDL  122 . A value of M  128  is provided by a reference circuit  130  (described herein below). 
     The main controller  116  also senses underflow of the main fine adjust state machine  114 . Underflow is defined as a number of the signals  126  to the main fine adjust line  120  in the low impedance state being less than a predefined lower limit. Thereupon the main controller  116  controls the coarse adjust state machine  112  to decrease the number of coarse delay elements  206  enabled by one by decreasing the number of the signals  125  to the main CDL  122  in low impedance state by one, and controls the fine adjust state machine  114  to increase the number of fine delay elements enabled by M  128  by increasing the number of signals  126  to the main FDL  124  in the low impedance state by M  128 . A range of the main FDL  124 , defined as a difference between the predefined upper limit and the predefined lower limit, preferably chosen to be greater than or equal to a step of the main CDL  122  over all specified operating conditions. 
     A DIV_CLK signal  120  is provided to the coarse adjust state machine  112 , main controller  116 , fine adjust state machine  114 , and reference circuit  130 . A frequency the DIV_CLK signal  120  is preferably a submultiple (that is, divided by N) of a frequency of the main clock  104  for reducing power requirements. 
     Referring to  FIG. 4 , a block diagram of a first embodiment of the reference circuit  130  is shown. A first delay path  402  receives the DIV_CLK signal  120 . A first CDL  406  provides a first predetermined delay. The first CDL  406  is similar (that is, having substantially equal delay steps but preferably having a fewer number of CDEs than the main CDL  406  for reducing circuit area requirements) to the main CDL  122  but having its inputs  407  preferably set to “0” (that is, all inputs are hardwired a high impedance state). Alternatively, a small number (X) compared to a total number of CDEs of the main CDL  122 , of the inputs  407  of the first CDL  406  may be set to a low impedance state. 
     The first delay path  402  also includes a first FDL  408  that is similar (that is, having substantially equal delay steps and preferably a substantially equal number of FDEs) to the main FDL  124 . The first FDL  408  receives a plurality of signals  418  from a reference circuit controller  416  for adjusting a number of steps of the first FDL  408 . 
     A total delay of the first path  402  is substantially equal to a delay of the first CDL  406  plus a delay of the first FDL  408 . An output of the first path  402  is provided to a reference circuit phase detector  414 . It should be noted that the order of the first CDL  406  and the first FDL  408  may be reversed from that shown in  FIG. 4  and still be within the present invention. 
     A second delay path  404  also receives the DIV_CLK signal  120 . A second CDL  410  provides a second predetermined delay. The second CDL  410  is similar (that is, having a substantially equal intrinsic delay and having substantially equal delay steps) to the first CDL  406  but having its inputs  411  preferably set to “1” (meaning that all but one inputs are set to a high impedance state, the other set to a low impedance state). Generally, a number of steps of the second CDL  410  is chosen to be one greater (X+1) than the first CDL  406 . 
     The second delay path  404  also includes a second fine delay line  412  that is substantially similar (that is, having a substantially equal intrinsic delay and having substantially equal delay steps) to the first FDL  408  but having all of its inputs  413  set to “0” (that is, all inputs  413  are set to a high impedance state). 
     A total delay of the second delay path  404  is substantially equal to a delay of the second CDL  410  plus a delay of the second FDL  412 . An output of the second path  408  is provided to the reference circuit phase detector  414 . It should be noted that the order of the second CDL  410  and the second FDL  412  may be reversed from that shown in  FIG. 4  and still be within the present invention. 
     Since the delay lines  406 , 408 , 410 , 412  in the reference circuit  130  are preferably manufactured simultaneously with the main CDL  122  and main FDL  124 , and are preferably located on the same integrated circuit in close proximity and same orientation, they will exhibit substantially the same characteristics over time, temperature, and process variation. 
     Outputs of the first delay path  402  and the second delay path  404  are connected to inputs of a phase detector  414  that is preferably substantially identical to the main phase detector  102 . The phase detector  414  provides a phase difference  415  preferably as up count and down count signals to the reference circuit controller  416 . 
     The reference circuit controller  416  provides a fully decoded set of control signals  418  to the first FDL  408 . The reference circuit controller  416  may include a one-state per flip-flop type state machine wherein outputs of the flip-flops directly provide the control signals to the first FDL  408 . Alternatively, the reference circuit controller  416  may include an up/down counter and a thermometer decoder for providing the control signals  418  to the first FDL  408 . 
     The reference circuit controller  416  adjusts the control signals  418  provided to the first FDL  408  so that the phase difference  415  is substantially zero and therefore the total delay of the first delay path  402  is substantially equal to the total delay of the second delay path  404 . M  128  is continually updated as the temperature and voltage conditions change, thereby providing an accurate count of the FDEs that ensures a minimum mismatch between the steps of the main CDL  122  and the steps of the main FDL  124  across process parameters and temperature and voltage drifts. 
     Referring to  FIG. 5 , a block diagram of a second embodiment of the reference circuit  130  is shown. The reference circuit  130  includes a reference FDL  508  that is similar (that is, having substantially equal delay steps and preferably a substantially equal number of delay steps) to the main FDL  124  and first FDL  408 . The reference FDL  508  receives a plurality of signals  418  from a reference circuit controller  516  for adjusting a number of steps of the reference FDL  508 . 
     A reference CDL  510  also receives the DIV_CLK signal  120 . The reference CDL  510  is similar (that is, having a substantially equal intrinsic delay and having substantially equal delay steps) to the main CDL  122 . The second CDL  410  also receives a signal  504  from the reference circuit controller  516  for adjusting a number of steps of the reference circuit CDL  510 . 
     Outputs of the reference circuit FDL  508  and the reference circuit CDL  404  are connected to inputs of a phase detector  414  that is preferably substantially identical to the main phase detector  102 . The phase detector  414  provides a phase difference  415  preferably as up count and down count signals to the reference circuit controller  516 . 
     The reference circuit controller  516  provides a fully decoded set of control signals  418  to the reference circuit FDL  508 . The reference circuit controller  516  may include a one-state per flip-flop type state machine wherein outputs of the flip-flops directly provide the control signals to the first FDL  508 . Alternatively, the reference circuit controller  516  may include an up/down counter and a thermometer decoder for providing the control signals  418  to the reference circuit FDL  508 . 
     Firstly, a number of steps of the reference circuit CDL  510  is set to “0” (that is, the input  504  is set to a high impedance state). A reference circuit controller  516  adjusts the control signals  518  provided to the reference circuit FDL  508  so that a delay of the reference circuit FDL  508  is substantially equal to a delay of the reference circuit CDL  510  by setting a first number of control lines  518  to a low impedance state and the rest to the low impedance state. 
     Secondly, a number of steps of the reference circuit CDL  510  is set to “1” (that is, the input  504  is set to a low impedance state). The reference circuit controller  516  adjusts the control signals  518  provided to the reference circuit FDL  508  so that a delay of the reference circuit FDL  508  is substantially equal to a delay of the reference circuit CDL  510  by setting a second number of control lines  518  to a low impedance state and the rest to the low impedance state. 
     Thirdly, the reference circuit controller  516  subtracts the first number from the second number thereby providing a third number M  128  which is substantially equal the number of steps of the main FDL  124  that are equivalent to a steps of the main CDL  122 . It should be noted that the first and second number can be determined in any order and still be within the present invention. 
     M  128  is continually updated as the temperature and voltage conditions change, thereby providing an accurate number of the FDEs that ensures a minimum mismatch between the CDL  122  and the FDL  124  across process parameters and temperature and voltage drifts. 
     Referring to  FIG. 6 , a method  600  for determining a number of steps of a FDL that are substantially equivalent to a step of a CDL according to the present invention is provided. 
     The method  600  includes steps of i)  602  delaying a clock signal  120  by a first delay  402  substantially equivalent to a predetermined delay plus an adjustable number of steps of the FDL thereby providing a first delayed clock signal, ii)  604  delaying the clock signal by a second delay  404  substantially equivalent to the predetermined delay plus a step of the CDL thereby providing a second delayed clock signal, and iii)  606  adjusting the number of adjustable steps of the FDL so that the first delay is substantially equal to the second delay thereby providing the number of steps  128  of the FDL that are substantially equivalent to the step of the CDL. 
     In step i)  602  ( FIG. 7 ), the clock signal  120  is preferably delayed by a delay substantially equal to an intrinsic delay of the CDL  702 , plus a delay substantially equal to an intrinsic delay of the FDL, plus the adjustable number of steps of the FDL  704 . 
     In step ii)  604  ( FIG. 8 ), the clock signal  120  is preferably delayed by delay substantially equal to an intrinsic delay of the CDL plus the step of the CDL  802 , plus a delay substantially equal to an intrinsic delay of the FDL  804 . 
     In step iii)  606  ( FIG. 9 ), if the first delay is less than the second delay then the number of adjustable steps of the FDL is preferably adjusted up  902  and if the first delay is greater than the second delay the number of adjustable steps of the FDL is preferably adjusted down  904 . 
     Referring to  FIG. 10 , another method  1000  for determining a number of steps of a FDL that are substantially equivalent to a step of a CDL according to the present invention is provided. 
     The method  1000  includes steps of i)  1002  delaying the clock signal  120  by a first delay substantially equivalent to a first predetermined delay plus an adjustable number of steps of the FDL, ii)  1004  delaying the clock signal by a second delay substantially equivalent to a second predetermined delay, iii)  1006  adjusting the number of adjustable steps of the FDL so that the first delay is substantially equal to the second delay and providing a first number of adjustable steps of the FDL, iv)  1008  delaying the clock signal by a third delay substantially equal to the second predetermined delay plus a step of the CDL, v)  1010  adjusting the number of adjustable steps of the FDL so that the first delay is substantially equal to the third delay and providing a second number of adjustable steps of the FDL, and vi)  1012  subtracting the first number from the second number of adjustable steps of the FDL thereby providing the number of steps of the FDL that are substantially equivalent to a step of a CDL. 
     In step i)  1002  ( FIG. 11 ) the clock signal is preferably delayed by a delay substantially equal to in intrinsic delay of the FDL plus the adjustable number of steps of the FDL. 
     In step ii)  1004  ( FIG. 12 ) the clock signal is preferably delayed by a delay substantially equal to an intrinsic delay of the CDL. 
     In step iii)  1006  ( FIG. 13 ) if the first delay is less than the second delay the number of steps is preferably adjusted up, and if the first delay is greater than the second delay the number of steps is preferably adjusted down. 
     In step iv)  1008  ( FIG. 14 ) the clock signal is preferably delayed by a delay substantially equal to an intrinsic delay of the CDL plus the step of the CDL. 
     In step v)  1010  ( FIG. 15 ) if the first delay is less than the third delay the number of steps is preferably adjusted up, and if the first delay is greater than the third delay the number of steps is preferably adjusted down. 
     While the above embodiments have been described using the DLL as the circuit to which they are applied in order to reduce switching jitter, the concepts can be used in other applications that involve tracking delays with respect to any reference delay path. For example, the invention can be used in clock recovery circuits, pin timing tuners used in integrated circuit testers, etc. 
     The DLL  100  provided is especially useful for clock tree management in field programmable gate arrays (FPGAs) and application specific integrated circuits (ASICs). Also, synchronous integrated circuits such as synchronous dynamic random access memories (SDRAMs), synchronous static random access memories (SSRAMs), serially connected memories such as FLASH may benefit using the DLL  100  for synchronizing an external clock signal to internal operations. 
     The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 Table of Elements 
               
             
          
           
               
                   
                 Element Name 
                 Reference Number 
               
               
                   
                   
               
               
                   
                 delay-locked loop (DLL) 
                 100 
               
               
                   
                 main phase detector 
                 102 
               
               
                   
                 main clock (CLK) 
                 104 
               
               
                   
                 feedback clock (F_CLK) 
                 106 
               
               
                   
                 up count control line 
                 108 
               
               
                   
                 down count control line 
                 110 
               
               
                   
                 coarse adjust state 
                 112 
               
               
                   
                 machine 
               
               
                   
                 fine adjust state machine 
                 114 
               
               
                   
                 main controller 
                 116 
               
               
                   
                 divided clock (DIV_CLK) 
                 120 
               
               
                   
                 main coarse delay line 
                 122 
               
               
                   
                 (CDL) 
               
               
                   
                 main CDL output/main FDL 
                 123 
               
               
                   
                 input 
               
               
                   
                 main fine delay line (FDL) 
                 124 
               
               
                   
                 coarse adjust state 
                 125 
               
               
                   
                 machine outputs/main CDL 
               
               
                   
                 inputs 
               
               
                   
                 fine adjust state machine 
                 126 
               
               
                   
                 outputs/main FDL inputs 
               
               
                   
                 reference circuit output 
                 128 
               
               
                   
                 (M) 
               
               
                   
                 reference circuit 
                 130 
               
               
                   
                 main CDL input buffer 
                 202 
               
               
                   
                 main CDL resistor 
                 204 
               
               
                   
                 main CDL delay elements 
                 206 
               
               
                   
                 (CDE) 
               
               
                   
                 main CDL output buffer 
                 208 
               
               
                   
                 main FDL input buffer 
                 302 
               
               
                   
                 main FDL resistor 
                 304 
               
               
                   
                 main FDL delay elements 
                 306 
               
               
                   
                 (FDE) 
               
               
                   
                 main FDL output buffer 
                 308 
               
               
                   
                 first delay path 
                 402 
               
               
                   
                 second delay path 
                 404 
               
               
                   
                 first CDL 
                 406 
               
               
                   
                 first CDL input 
                 407 
               
               
                   
                 first FDL 
                 408 
               
               
                   
                 second CDL 
                 410 
               
               
                   
                 second CDL input 
                 411 
               
               
                   
                 second FDL 
                 412 
               
               
                   
                 second FDL input 
                 413 
               
               
                   
                 reference circuit phase 
                 414 
               
               
                   
                 detector 
               
               
                   
                 reference circuit phase 
                 415 
               
               
                   
                 difference 
               
               
                   
                 reference circuit 
                 416 
               
               
                   
                 controller 
               
               
                   
                 reference circuit 
                 418 
               
               
                   
                 controller output/first 
               
               
                   
                 FDL input 
               
               
                   
                 reference circuit CDL 
                 504 
               
               
                   
                 control signal 
               
               
                   
                 reference circuit FDL 
                 508 
               
               
                   
                 reference circuit CDL 
                 510 
               
               
                   
                 reference circuit FDL 
                 518 
               
               
                   
                 control signal 
               
               
                   
                 first method for 
                 600-904 
               
               
                   
                 determining number of 
               
               
                   
                 steps 
               
               
                   
                 second method for 
                 1000-1504 
               
               
                   
                 determining number of 
               
               
                   
                 steps