Abstract:
A delay locked loop for generating a replica clock signal synchronized to an externally generated clock signal comprises a succession of separately controlled delay lines. Each of the delay lines has different delay resolution.

Description:
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   BACKGROUND OF THE INVENTION 
   This invention relates to the field of semiconductor integrated circuits, particularly to circuits useful for generating local clock signals synchronized to an external reference clock, and to semiconductor memory devices with such circuits. 
   System specifications for double data rate dynamic random access memory(DDR DRAM) require the device in a read cycle to switch its data lines(DQ) coincident with transitions of an externally generated reference clock. The fact that the frequency of the reference clock is not known precisely presents an obstacle to meeting the above requirement. The fact that the DQ line drivers have significant delays presents another obstacle. 
   To meet the above requirement, a DDR DRAM device typically employs a delay locked loop(DLL) to create a delayed replica clock that exactly matches the system reference clock in both frequency and phase. The DLL creates its replica clock by making a delayed copy of the reference clock. The copy has the same frequency as the reference clock because delaying a signal does not change its frequency. The DLL adjusts the delay of the copy until the copy is delayed by one or more full clock cycles from the reference clock. At this point, the reference clock and the delayed copy of the reference clock are synchronized, and the copy is then a replica of the reference clock. 
   The following discussion describes the DLL as locked when its replica clock matches the frequency and phase of the reference clock, and describes the smallest available delay increment by which the DLL may be adjusted as the resolution of the DLL. 
   The DLL typically taps a DQ clock signal from its delay signal path at a point preceding the replica clock by one clock input buffer delay, plus a DQ line driver delay(buffer delay). The phase of the DQ clock therefore leads the phase of the replica clock by one buffer delay. When the DLL is locked, the phase of the DQ clock signal also leads the reference clock by one buffer delay. The DQ clock may be used to trigger the DQ line drivers so that the DQ lines switch coincident with the reference clock transitions, thus meeting the above requirement. 
   The speed with which a DLL can achieve the locked condition is an important aspect of DLL performance. At system startup, after every self-refresh, and after exiting low power modes, a DLL which locks more quickly than other designs can perform its first read sooner, improving overall performance of the device. 
   The stability of the DLL locking in the presence of normal electrical disturbances is another important aspect of DLL performance. A design which loses lock due to a change in supply voltage or temperature cannot function until lock is regained. 
     FIG. 1  shows prior art in delay locked loops as summarized in Keeth and Baker, ‘DRAM Circuit Design, A Tutorial,’ IEEE Press, New York, 2001, page 143. In  FIG. 1 , a shift register  120  selects the number of delay increments applied by a delay line  110  to an external reference clock  102 . Delayed clock  130  emerging from the delay line triggers output data  160 , data strobe  170 , and feeds back to phase detector  150  through delay block  140 . Delay block  140  replicates the delay of input buffer  104 . Phase detector  150  controls shift register  120  to remove any phase difference between its input signals  106  and  146  that are larger than the incremental delays controlled by its counter, as known by one skilled in the art. 
   The  FIG. 1  design uses a single delay line having the same unit delay in each stage. This approach has the disadvantage of requiring many delay stages to do its job. As a typical example, if the maximum clock period is 10 nanoseconds, and the minimum clock period is 5 nanoseconds, and the delay resolution is 100 picoseconds, the  FIG. 1  design requires 50 delay stages to meet the requirements with no margin. Implementing such a large number of stages requires more device size and power consumption than necessary. 
   The large number of stages also causes the disadvantage of slow locking. The phase detector must wait at least two clock cycles before making each decision, and each decision can only adjust total delay by a single unit delay. Thus the design of  FIG. 1  moves toward locking by taking small steps, often over many steps. 
   The design of  FIG. 1  has the further disadvantage of making delay adjustments by varying the location where the clock signal enters the delay line, so the adjustments propagate through all active delay elements before evaluation at the phase detector. Adjusting at the beginning end, rather than the trailing end, of the delay line slows evaluation of the adjustment, because the design has to wait before evaluating until each adjustment propagates to the phase detector. The design must address the worst case, and pause before evaluating for the full length of the delay line. 
   The  FIG. 1  design has a further disadvantage of requiring flip-flops in the shift register outputs that control the delay line. Because the design needs flip-flops with near-zero setup time, the design operates the flip-flops close to the region where metastable operation can occur. For reliable flip-flop performance, the design must add either extra filtering circuitry, or extra setup delay. The use of flip-flops causes the device to suffer extra size or reduced performance. 
     FIG. 2  shows further prior art, U.S. Pat. No. 6,438,067, issued Aug. 20, 2002 to Kuge et al. This patent teaches a DLL having an adjustable delay buffer and an adjustable delay line in series. Delay elements in the delay buffer provide delay increments that are smaller than those in the delay line. Reference clock  202  enters a delay buffer  204  where the delay is controlled by selectively connecting capacitive loads  222  responsive to the low-order bits of a count in counter  224 . The clock signal then enters delay line  206 , where its delay is further adjusted by passing through an adjustable number of delay elements  210 , set by the high-order bits of the same counter. Decoders  215 - 1 , . . .  215 -n select which one of delay elements  210 - 1 , . . .  210 -n admits buffered clock signal  208  into the delay line. 
   The Kuge patent further controls the delay of each delay element  210  by varying its supply voltage on node  255 , so that a fairly small number of delay elements will suffice. At circuit startup, DLL control circuit  250  enables reference potential generating circuit  212  to adjust the supply voltage of the delay line, responsive to the output signal  240  until the remaining adjustment is within a predetermined range. Then the DLL control circuit uses gates  252  and  254  to disable voltage supply variation and enable delay line variation to further adjust DLL delay. 
   The Kuge patent has the disadvantage of using analog voltage controls for initial steps toward lock, causing speed of locking to be less than optimal. As is known to one skilled in the art, supply voltage controls must operate more slowly than digital controls, to avoid underdamped oscillations (ringing). This approach gives a wide frequency range with a small number of delay line elements, but will have a slow initial lock. 
   The Kuge patent has the further disadvantage of having variable size delay steps in its delay line, while the delay steps in its delay buffer are a fixed size. With both the delay buffer and delay line driven from the same counter, each counter change needs to change the total delay of both stages in a uniform fashion to be able to smoothly adjust the total delay. This is impossible since the delay of each delay line element changes with node  255  voltage, while the delay of the delay buffer step does not change. This problem will cause the DLL to have a variable locking resolution. The total delay will not change in a uniform manner as the counter is incremented and decremented. 
   For example, when the low-order three bits of the counter contain all ones, then all seven units of capacitance in the delay buffer are switched on. When the counter increments, the capacitances are all switched out of the circuit, and one delay-line increment is added to the total delay. The delay-line increment should equal eight of the buffer capacitance units, but the delay-line increments vary significantly due to the voltage controls. When the delay line increment is less than seven of the buffer capacitance units, and the count increments across the boundary between the buffer and the delay line to call for more delay, the line delay decreases instead of increasing. When the delay line increment is more than eight of the buffer capacitance units, and the count crosses the boundary between the delay buffer and the delay line there is a large change in total delay. A gap appears every eighth count, at this boundary, in total delay values which the delay line can provide, due to the variable-voltage controls. 
   Large temperature variations are common between quiescent conditions and full speed operation. Temperature variations cause changes in circuit delay, requiring the DLL to make small delay adjustments after the initial lock. The margin between external clock and the DQ as the temperature varies will be larger than other prior art, and the current invention, because the total delay of the delay buffer and delay line do not increment in a uniform manner over temperature changes. 
   BRIEF SUMMARY OF THE INVENTION 
   The current invention provides a DLL which overcomes the disadvantages of prior art circuits by providing multiple adjustable delay lines, each having separate controls and different delay resolutions. The delay lines are arranged in series so that total delay from the reference clock to the replica clock is the sum of delays imparted by each delay line. The reference clock feeds into a first delay line providing relatively coarse delay increments. Delay lines following the first delay line provide smaller delay increments, and more precise delay control. However, the first delay could have a small delay increment with the following delay lines providing a larger delay increment and still fall within the scope of the current invention. 
   Each delay line has separate delay controls including a counter that sets the number of incremental delays applied by the delay line to the clock signal. When the delay of a delay line is adjusted, the counter of the next, higher resolution delay line is typically set at midrange, to maximize its available range of locking. 
   A DLL typically waits two or more clock cycles between each adjustment, to allow the clock signal to stabilize, and to allow the adjusted clock timing to propagate through the delay line and back to the phase detector inputs. By having coarse delays, my DLL design can match its replica clock to the external clock more quickly than a DLL without coarse delay steps, because each initial adjustment may take larger steps toward lock. My design also locks more quickly than prior art which uses analog voltage controls, because my design does not vary the supply voltage, so it has no need to move slowly enough to avoid supply voltage oscillation. 
   The coarse delays also permit the DLL to operate over the required frequency range using far fewer delay elements than designs having a only single delay line, giving the advantages of smaller layout and lower power consumption compared to prior art. 
   Since the coarse delay line length is only changed during the initial lock, my DLL has a uniform delay line step for temperature transients. The uniform delay line steps enable the phase detector to keep the maximum difference between reference clock transitions and DQ transitions(margin) within the resolution of the smallest delay line element. 
   In the current invention, each counter is typically a conventional up/down counter which increments and decrements its count responsive to pulses on UP and DN input nodes, respectively. An alternate implementation may use a counter with a single COUNT input node, in which the counter keeps track of the direction of counting, and reverses its direction at end-of-range. 
   A separate phase detector for each delay line typically controls the counter of the delay line. The phase detector compares the reference clock with the replica clock, with resolution roughly equal to the delay elements of its delay line. Based on the phase comparison, the phase detector directs the counter to increment its count when more delay is needed, to decrement its count when less delay is needed, and to make no change when the phase difference is less than the resolution of the delay line. In the preferred embodiment, a wrap control replaces the phase detector in one of the delay lines as described more fully below. 
   Therefore, objects and advantages of the current invention include:
         (a) Separate delay lines for coarse and fine resolution, each having separate controls,   (b) Large range of available delay with a smaller number of delay line elements via coarse adjustments,   (c) Rapid locking via coarse adjustment capability,   (d) Small locking resolution via fine adjustment capability,   (e) Minimized layout size and power consumption,   (f) Small resolution, uniform delay step sizes over all operating conditions,   (g) All-digital controls for rapid convergence and stable, robust operation over full range of temperature and manufacturing process conditions,   (h) Adjusting delays at the trailing end of the delay line, obtaining the shortest signal path for each adjustment to propagate to the phase detector, and the fastest possible evaluation of each adjustment, and a constant amount time after each adjustment until the adjustment may be evaluated at the phase detector,   (i) No flip flops, therefore no problem of flip flop metastability, and   (j) No variable voltage supply, therefore no problem of underdamped oscillation.       

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       FIG. 1  shows a prior-art DLL design described in Keeth and Baker, ‘DRAM Circuit Design, A Tutorial,’ IEEE Press, New York, 2001, page 143. 
       FIG. 2  shows a prior art DLL circuit taught by U.S. Pat. No. 6,438,067, issued Aug. 20, 2002 to Kuge. 
       FIG. 3  shows a DLL according to the current invention, having two delay lines. 
       FIG. 4  shows a DLL according to the current invention, having three delay lines. 
       FIG. 5  shows the preferred embodiment of the current invention. 
       FIG. 6  details the logic of the one-input delay block shown in  FIG. 3  and  FIG. 4 , and the two-input delay block shown in  FIG. 5 . 
       FIG. 7  shows a DDR SDRAM device. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIG. 3 , a buffered reference clock  302  is the input signal. For brevity,  FIG. 3  does not show the reference clock input buffer. Buffered reference clock  302  connects to a first delay element in first delay line  300 , to a first input of phase detector  312  in delay line  300 , and to a first input of phase detector  332  in delay line  320 . DQ clock  350  is the output signal from the DLL. DQ clock is driven by delay line  320 , and connects further to buffer delay model  342 . A replica clock signal  340 , driven by the buffer delay model, is the feedback control signal for the loop. The replica clock connects to a second input of phase detector  312 , and to a second input of phase detector  332 . 
   In  FIG. 3  delay line  300 , the clock signal passes through delay elements  304  in series. Each delay element has an input node for receiving a clock signal, and an output node for conveying a copy of the clock signal delayed by a first delay time. The input node of each delay element in the delay line, except the first element, connects to the output of the preceding element in the delay line. The input node of the first element in the delay line connects to reference clock  302 . The output node of each delay element in the delay line, except the last, connects as described to the input node of the succeeding element. The output node of each delay element in the delay line connects further to a first input node of a separate tap gate  308  for each delay element. 
   In  FIG. 3 , a conventional phase detector  312 , and counter  314  control the total delay of the clock signal by delay line  300  as follows. Phase detector  312  compares replica clock  340  to reference clock  302 . When the phase of replica clock  340  leads the phase of reference clock  302  by more than a delay of a delay element  304 , phase detector  312  issues a pulse on its UP 1  output, incrementing counter  314 , effectively increasing the delay of delay line  300  by the delay of one delay element  304 . When the phase of replica clock  340  follows the phase of reference clock  302  by more than a delay of a delay element  304 , phase detector  312  issues a pulse on its DN 1  output, decrementing counter  314  to effectively decrease the delay of delay line  300  by the delay of one delay element  304 . Nodes UP 1  and DN 1  couple to counter  314  and further to counter  334 . A pulse on either UP 1  or DN 1  sets the count of counter  334  to its midrange value. When neither action is needed, phase detector  312  issues neither signal and coarse delay line  300  is locked. After pulsing UP 1  or DN 1 , phase detector  312  waits long enough for the adjustment to propagate through the DLL and back to the phase detector inputs before repeating its compare/adjust operation. 
   Counter  314  in  FIG. 3  comprises UP 1  and DN 1  input nodes, driven as described by phase detector  312 . Counter  314  typically increments or decrements its count from a predetermined nominal delay value at system startup, but it may also begin from a random count at that time. 
   Multi-wire bus  316  routes all bits of the count of counter  314  in parallel to a separate decoder  306  for each delay element in the first delay line. Each decoder has an output connected to a second input of tap gate  308  following its delay element. The second input of each tap gate enables and disables the tap gate for transmitting the delayed clock signal of its first input. Decoders  306  each enable a tap gate  308  only when bus  316  conveys a count that matches its position in the delay line. One of the decoders  306  enables its tap gate for each count held by counter  314 . 
   In  FIG. 3 , the output signal from each tap gate  308  in the first delay line drives a separate input of first delay line output gate  310 . Decoders  306  and tap gates  308  disable all but one of the signals driving gate  310 . The enabled tap gate  308  drives its clock signal onto an input of gate  310 . Gate  310  then drives a clock signal on node  322  having a delay substantially equal to the output of the delay element driving the enabled tap gate. 
   First delay line output clock signal  322  in  FIG. 3  enters second delay line  320 , where it passes through delay elements  324  in series. Each delay element  324  has an input node for receiving a clock signal, and an output node for conveying a delayed replica of the clock signal. The input node of each delay element in the delay line, except the first element, connects to the output of the preceding element in the delay line. The input node of the first element in the delay line connects to node  322 . The output node of each delay element in the delay line, except the last, connects as described to the input node of the succeeding delay element. The output node of each delay element in the delay line connects further to a first input node of a separate tap gate  328  for each delay element. Each of the delay elements in the second delay line causes further delay by a substantially equal second delay amount, which is less than the first delay of each delay element of the first delay line. 
   Every clock cycle, phase detector  332  compares replica clock  340  to reference clock  302 . When the phase of replica clock  340  leads the phase of reference clock  302  by more than a delay of a delay element  324 , phase detector  332  issues a pulse on its UP 2  output, incrementing counter  334 , effectively increasing the delay of delay line  320  by the delay of one delay element  324 . When the phase of replica clock  340  follows the phase of reference clock  302  by more than a delay of a delay element  324 , phase detector  332  issues a pulse on its DN 2  output, decrementing counter  334  to effectively decrease the delay of delay line  320  by the delay of one delay element  324 . When the phase of replica clock  340  is closer to reference clock  302  than the margin of phase detector  332 , phase detector  332  issues neither signal, and the loop is locked. Nodes UP 1  and DN 1  also couple to counter  334 . A pulse on either UP 1  or DN 1  initializes the count of counter  334  to its midrange value whenever delay line  300  is adjusted. Pulses on UP 2  and DN 2  then adjust the count of counter  334  to minimize the phase difference between replica clock  340  and reference clock  302  as described. The initializing of counter  334  to a mid-point value is desirable but not necessary. After pulsing UP 2  or DN 2 , phase detector  332  waits long enough for the adjustment to propagate through the DLL and back to the phase detector inputs before repeating its compare/adjust operation. 
   All bits of the count of counter  334  are passed via multi-wire bus  336  to a separate decoder  326  for each delay element of the second delay line. Each decoder  326  has an output coupled to a second input of tap gate  328  for its delay element. The count on bus  336  causes one decoder  326  corresponding to the value of the count to activate its tap gate  328 . The output node of each tap gate  328  connects to a separate input of second delay line output gate  330 . The activated tap gate provides a path for the clock signal to exit the second delay line and drive one input of second delay line output gate  330  with a clock signal having the particular delay of the enabled tap gate of delay line  320 . Gate  330  then drives the DQ clock output signal on node  350  with substantially this same delay. The logic function of gates  308  and  310 , or gates  328  and  330 , could be implemented in many different ways without affecting the scope of this invention. 
     FIG. 4  shows a second implementation of the current invention, in which the delay locked loop comprises three delay lines  400 ,  420  and  440  connected in series. Buffered reference clock  402  is the input signal. The reference clock connects to the input node of first delay element  404  of delay line  400 , and to a first input of conventional phase detectors  412 ,  432 , and  452 . Output gate  450  of third delay line  440  drives the DQ clock output on node  470 . Node  470  further couples to the input node of buffer delay model  462 , which drives replica clock  460 . The replica clock connects as a second input to each of three phase detectors  412 ,  432 , and  452 . 
   In  FIG. 4 , the reference clock enters delay line  400  and traverses delay elements  404  in succession, being delayed by a first delay value in traversing each element. The first delay value represents a relatively coarse fraction of the period of the input reference clock signal. Each delay element has an input node for receiving a clock signal, and an output node for conveying a delayed copy of the clock signal. The input node of each delay element in the delay line, except the first element, connects to the output of the preceding element in the delay line. The input node of the first element in the delay line connects to reference clock  402 . The output node of each delay element in the delay line, except the last, connects as described to the input node of the succeeding element. The output node of each delay element in the delay line connects further to a first input node of a separate tap gate  408  for each delay element. 
   In  FIG. 4 , a conventional phase detector  412 , and counter  414  control the delay of the clock signal by the first delay line as follows. Every clock cycle, phase detector  412  compares replica clock  460  to reference clock  402 . When the phase of replica clock  460  leads the phase of reference clock  402  by more than the delay of a delay element  404 , phase detector  412  issues a pulse on its UP 1  output to increment counter  414 , thereby increasing the delay of delay line  400  by the delay of one delay element  404 . When the phase of replica clock  460  follows the phase of reference clock  402  by more that the delay of a delay element  404 , phase detector  412  issues a pulse on its DN 1  output to cause counter  414  to count down, decreasing the delay of delay line  400  by the delay of one delay element  404 . Nodes UP 1  and DN 1  couple phase detector  412  to counter  414 , and further couple to counter  434  and counter  454 . A pulse on either UP 1  or DN 1  sets counters  434  and  454  to their midrange count values. The initialization of counters  434  and  454  to a mid-point value is desirable but not necessary. After pulsing UP 1  or DN 1 , phase detector  412  waits long enough for the adjustment to propagate through the delay lines and back to the phase detector inputs before repeating its compare/adjust operation. When the phase of replica clock  460  is closer to the reference clock  402  than the phase detector margin, phase detector  412  issues neither signal and the first delay line is locked. If the reference clock has constant frequency, no further change is needed in the first delay line. 
   Counters  414 ,  434 , and  454  in  FIG. 4  are conventional up/down counters, with UP 1  and DN 1  input nodes driven as described by phase detector  412 . 
   Each bit of the count in counter  414  couples to a separate wire of counter bus  416 . Bus  416  routes all bits of the count of counter  414  in parallel to a separate decoder  406  for the output of each delay element in the first delay line. Each decoder has an output connected to a second input of tap gate  408  for its delay element. The second input of each tap gate enables and disables the tap gate for transmitting the clock signal on its first input. A decoder  406  enables a tap gate  408  only when bus  416  conveys a count that matches its position in the delay line. One decoder  406  enables its tap gate  408  for each count held by the counter. 
   In  FIG. 4 , an output signal from each tap gate  408  in the first delay line connects to a separate input of first output gate  410 . Decoders  406  and tap gates  408  disable all but one of the signals driving gate  410 . The single enabled tap gate drives its delayed clock signal onto an input of gate  410 . Gate  410  then drives a clock signal on node  422  having a delay substantially equal to the output of delay element  404  driving the enabled tap gate. 
   First delayed clock  422  connects as an input to a second delay line  420  in  FIG. 4 . Second delay line  420  operates in similar fashion to first delay line  400 . However, the second delay of each delay element in the second delay line is substantially less than the first delay of each delay element in the first delay line. Thus the second delay line provides a delay with finer resolution than that given by the first delay line. When the first delay line is adjusted, counter  434  of the second delay line is initialized at its mid-range value by a pulse on either the UP 1  or the DN 1  node. 
   Delay lines  400 ,  420 , and  440  are arranged in series so that the total delay from reference clock  402  to replica clock  460  is the sum of the delays from the individual delay lines, plus the delay from buffer delay model  462 . The loop is locked when all three delay lines are locked and no further changes are needed to match the phase of the replica clock with that of the reference clock. 
   In  FIG. 4 , first delay line output  422  enters delay line  420  and traverses delay elements  424  in succession, being delayed by a second delay value in traversing each element. The second delay value represents a smaller fraction of the reference clock cycle time than does the first delay value. Each delay element  424  has an input node for receiving a clock signal, and an output node for conveying a delayed copy of the clock signal. The input node of each delay element in delay line  420 , except the first element, connects to the output of the preceding element in the delay line. The input node of the first element in the delay line connects to first delayed clock  422 . The output node of each delay element in the delay line, except the last, connects as described to the input node of the succeeding delay element. The output node of each delay element in the delay line connects further to a first input node of a separate tap gate  428  for that delay element. 
   In  FIG. 4 , a conventional phase detector  432 , and counter  434  control the total delay of the clock signal by the second delay line as follows. Every clock cycle, phase detector  432  compares replica clock  460  to reference clock  402 . When the phase of replica clock  460  leads the phase of reference clock  402  by more than a delay of delay element  424 , phase detector  432  issues a pulse on its UP 2  output to cause counter  434  to increment its count, increasing the delay of delay line  420  by the delay of one delay element  424 . When the phase of replica clock  460  follows the phase of reference clock  402  by more than a delay of delay element  424 , phase detector  432  issues a pulse on its DN 2  output to cause counter  434  to decrement counter  434 , decreasing the delay of delay line  420  by the delay of one delay element  424 . After pulsing UP 2  or DN 2 , phase detector  432  waits long enough for the adjustment to propagate through the delay lines and back to the phase detector inputs before repeating its compare/adjust operation. When the phase of replica clock  460  is closer to reference clock  402  than the phase detector margin, phase detector  432  issues neither signal and delay line  420  is locked. If the reference clock has constant frequency, no further change is needed in delay line  420 . Whenever an adjustment of the delay line occurs, all phase detectors should be disabled, or the clocking of all counters inhibited, until the adjustment has propagated to the replica clock node. 
   Counter  434  in  FIG. 4  is a conventional up/down binary counter having UP 2  and DN 2  input nodes, UP 1  and DN 1  input nodes, and an output count bus  436 . Each bit of the count in counter  434  couples to a separate wire of count bus  436 . 
   Bus  436  routes all bits of the count of counter  434  in parallel to a separate decoder  426  for each delay element in the second delay line. Each decoder  426  has an output connected to a second input of the tap gate  428  for its delay stage. The second input of each tap gate enables and disables the tap gate for transmitting the delayed clock signal on its first input. Each decoder  426  enables its tap gate  428  only when bus  436  conveys a count that matches its position in the second delay line. One decoder  426  enables its tap gate for each count held by the counter. The one enabled tap gate routes the delayed clock signal out of the delay line with a cumulative delay corresponding to its location in the delay line. 
   In  FIG. 4 , an output node from each tap gate  428  in the second delay line connects to a separate input of second delay line output gate  430 . The single enabled tap gate drives its clock signal onto an input of gate  430 . Gate  430  then drives a delayed clock signal on node  442 , having delay substantially equal to the output of delay element  424  driving the enabled tap gate. 
   Second delayed clock  442  connects as an input to third delay line  440  in  FIG. 4 . The amount of delay from each delay element in the third delay line is substantially less than the delay of each delay element in the first and second delay lines. Phase detector  452  has a resolution approximately the same as the delays in the third delay line. Thus the third delay line provides delay control with finer resolution than that given by the first and second delay lines alone. 
   In  FIG. 4 , second delayed clock  442  enters third delay line  440  and passes through delay elements  444  in succession, being delayed by a substantially equal delay time in traversing each element. Each delay element  444  has an input node for receiving a clock signal, and an output node for conveying a delayed copy of the clock signal. The input node of each delay element in delay line  440 , except the first element, connects to the output of the preceding element in the delay line. The input node of the first element in the delay line connects to second delayed clock  442 . The output node of each delay element in the delay line, except the last, connects as described to the input node of the succeeding element. The output node of each delay element in the delay line connects further to a first input node of a separate tap gate  448  for each delay element. 
   In  FIG. 4 , a conventional phase detector  452 , and counter  454  control the total delay of the clock signal by the third delay line as follows. Counter  454  is initialized to its midrange count by a pulse on UP 1  or DN 1  whenever coarse delay line  400  is adjusted. Another embodiment of the invention will initialize counter  454  to its midrange count whenever a pulse on UP 2  or DN 2  occurs. Every clock cycle, phase detector  452  compares replica clock  460  to reference clock  402 . When the phase of replica clock  460  leads the phase of reference clock  402  by more than the delay of a delay element  444 , phase detector  452  issues a pulse on its UP 3  output to increment counter  454 , to increase the delay of delay line  440  by the delay of one delay element  444 . When the phase of replica clock  460  follows the phase of reference clock  402  by more than the delay of a delay element  444 , phase detector  452  issues a pulse on its DN 3  output to decrement counter  454 , decreasing the delay of delay line  440  by the delay of one delay element  444 . After pulsing UP 3  or DN 3 , phase detector  452  waits long enough for the adjustment to propagate through the delay lines and back to the phase detector inputs before repeating its compare/adjust operation. When the phase of replica clock  460  is closer to reference clock  402  than the phase detector margin, the phase detector issues neither signal and the loop is locked. 
   Each bit of the count in counter  454  couples to a separate wire of count bus  456 . Bus  456  routes all bits of the count of counter  454  in parallel to a separate decoder  446  for each delay element in the second delay line. Each decoder  446  has an output connected to a second input of tap gate  448  for the delay stage of the decoder. The second input of each tap gate  448  enables and disables the tap gate for transmitting the delayed clock signal on its first input. Each decoder  446  enables its tap gate only when bus  456  conveys a count that matches its position in delay line  440 . One decoder  446  enables its tap gate for each count held by the counter. The enabled tap gate routes the delayed clock signal out of delay line  440  at a location in the delay line where the clock signal has passed through a number of delay elements  444  equal to the count in counter  454 . 
   In  FIG. 4 , an output node of each tap gate  448  in the third delay line connects to a separate input of third delay line output gate  450 . The single enabled tap gate drives its clock signal onto an input of gate  450 . Gate  450  then drives the DQ clock output signal on node  470 , the DQ clock output having a total delay substantially equal to the output of delay element  444  driving the enabled tap gate. 
   Node  470  further drives the input node of buffer delay model  462 . Buffer delay model  462  then drives the replica clock on node  460 . 
     FIG. 5  shows the preferred implementation of the current invention. A buffered reference clock on node  502  is the input signal. A DQ clock on node  570  is the output signal. Three delay lines,  500 ,  520 , and  540  coupled in series provide a path for conveying a delayed copy of the reference clock from node  502  to node  570 . A replica clock driven on node  560  by output buffer delay model  562  is the feedback signal. Reference clock  502  connects to an input node of the first delay element of delay line  500 , and also to a first input node of conventional phase detectors  512  and  552 . Replica clock  560  connects to a second input node of phase detectors  512  and  552 . 
   Delay lines  500 ,  520 , and  540  of  FIG. 5  use delay elements having two inputs. The logic design of two-input delay element  544  is shown in more detail by  FIG. 6 , delay element  620 . Delay element  610  corresponds to delay elements  324  or  444 . Delay elements  504  and  524  will use the NOR function shown in delay element  620  on their input structure of the delay circuit being used. These delay elements have a longer delay than delay element  544  of  FIG. 5 , and to one skilled in logic design can be implemented in many different ways. 
   In  FIG. 5 , second delay line  520  uses a wrap control circuit  532  instead of a phase detector for controlling its counter. The wrap control steers counter  534  in the intermediate delay line via pulses on UP 2  and DN 2  lines between the wrap control and its counter, responsive to the digital count in the counter controlling the last, highest resolution delay line in the loop. The wrap control makes its decision to count up, count down, or do nothing so as to prevent counter  554  of the smallest resolution delay line from wrapping around after its count reaches either end of its range. When counter  554  reaches the minimum end of its range, the wrap control decrements medium range counter  534  by pulsing its DN 2  output, to provide less delay from the intermediate delay line  520  and increments counter  554  to increase the delay of delay line  540  by a delay of delay element  524 . This moves counter  554  away from the minimum value with no delay change in signal  560 . When counter  554  reaches the maximum end of its range, the wrap control increments counter  534  by pulsing its UP 2  output, to increase the delay from the second delay line  520 , and decrements counter  554  to decrease the delay of delay line  540  by a delay of delay element  524 . Counter  554  thus moves away from its maximum value with no delay change in signal  560 . After adjusting counter  534  and counter  554 , wrap control  532  and phase detector  552  wait long enough for the adjustment to propagate through the delay lines and back to phase detector  552  before repeating its compare/adjust operation. When counter  554  is not at the end of its numerical range, the wrap control makes no change to counter  534 . 
   In delay line  500 , the delays of every element  504  are a relatively coarse, substantially equal fraction of the reference clock cycle time. Each delay element  504  has a first input node for receiving a clock signal, a second input node for receiving an active-low enable signal, and an output node for conveying a delayed copy of the clock signal. 
   The first input node of each delay element in delay line  500 , except the first element, connects to the output of the preceding element in the delay line. The first input node of the first delay element in delay line  500  connects to reference clock node  502 . The second input node of each delay line  500 , except the first element, connects to the output of decoder  506  of the preceding delay stage. The second input node of the first delay element of delay line  500  is tied logically low. The output node of each delay element in the delay line, except the last, connects as described to the input node of the succeeding delay element in delay line  500 . The output node of each delay element in delay line  500  connects further to a first input node of a separate tap gate  508  for each delay element. 
   A conventional phase detector  512 , and counter  514  control the total delay of the first delay line as follows. Phase detector  512  directly compares reference clock  502  with delayed replica clock  560 . When the phase of replica clock  560  leads the phase of reference clock  502  by more than the delay of a delay element  504 , phase detector  512  increments counter  514  by sending a pulse on its UP 1  output node to counter  514 . When the phase of replica clock  560  follows the phase of reference clock  502  by more than the delay of a delay element  504 , phase detector  512  decrements counter  514  by sending a pulse on its DN 1  output to counter  514 . A pulse on either UP 1  or DN 1  also sets the counts of counters  534  and  554  to their midrange values. After pulsing UP 1  or DN 1 , phase detector  512  waits long enough for the adjustment to propagate through the delay lines and back to the phase detector inputs before repeating its compare/adjust operation. When the delay of the first delay line is correct within the delay of a delay element  504 , phase detector  512  pulses neither UP 1  nor DN 1 , and the first delay line is locked. 
   Counter  514  may start from a preset count at system startup, or from a random count at startup. Counter  514  may also be initialized to a preset count whenever the DLL is re-enabled. In response to the directional commands from phase detector  512 , counter  514  accumulates a digital count enumerating the number of delays  504  to be applied by delay line  500  to the reference clock. Each bit of counter  514  couples to a separate wire in count bus  516 . Bus  516  routes all bits of the count in counter  514  in parallel to each decoder  506  of first delay line  500 . Each decoder  506  has an output node  507  coupled to a second input of tap gate  508  for the delay element associated with the decoder. Decoder output node  507  further couples to the second input of the delay element of the next succeeding delay stage in delay line  500 . Each decoder  506  disables the following delay element  504  when bus  516  conveys a count equal to its sequential position in the delay line, and enables the following delay element  504  otherwise. Disabling the following delay element at the tap point disables the clock signal from propagating down the remainder of the delay line, effectively powering down all delay elements following the delay line tap point and saving power. 
   In  FIG. 5 , the output node of each tap gate  508  in the first delay line connects to a separate input of first delay line output gate  510 . Decoders  506  and tap gates  508  disable all but one of the signals driving gate  510 . The single enabled tap gate drives its clock signal onto an input of gate  510 , with a delay substantially equal to the output of the delay element  504  driving the enabled tap gate. Gate  510  then drives this clock signal on node  522 . 
   In  FIG. 5 , first delayed clock  522  enters second delay line  520  and passes through successive delay elements  524 , each of which imparts a fixed, substantially equal, second delay to the clock signal. The second delay represents a smaller fraction of the reference clock cycle time than does the first delay value. Each delay element  524  has a first input node for receiving a clock signal, a second input node for receiving an active-low enable signal, and an output node for conveying a delayed copy of the clock signal. 
   The first input node of each delay element, except the first, in delay line  520  connects to the output of the preceding element in the delay line. The first input node of the first element in delay line  520  connects to first delayed clock  522 . The second input node of each delay element in delay line  520 , except the first element, couples to the output node of the decoder of the preceding delay stage in the delay line. The second input node of the first delay element in delay line  520  is coupled logically low. The output node of each delay element in the delay line, except the last, connects as described to the input node of the succeeding element. The output node of each delay element in the delay line connects further to a first input node of a separate tap gate  528  for each delay element. 
   In delay line  520  of  FIG. 5 , counter  534  has input nodes UP 2  and DN 2 , driven as described by wrap control  532 . Counter  534  also has input nodes UP 1  and DN 1 , driven as described by phase detector  512 . Each bit of counter  534  couples to a separate wire of count bus  536 . Bus  536  conveys each bit of the count from counter  534  in parallel to a separate decoder  526  for each delay element of second delay line  520 . Each decoder  526  has an output node coupled to a second input node of tap gate  528  for its delay element. The second input node of each tap gate enables and disables the tap gate for transmitting the delayed clock signal on its first input node. Each decoder  526  enables its tap gate only when the count on bus  536  equals the sequential position of the decoder and its delay element in delay line  520 . Decoder output node  527  further couples to the second input of the delay element of the next succeeding delay stage in delay line  520 . Each decoder  526  disables the following delay element  524  when bus  536  conveys a count equal to the sequential position of the decoder in the delay line, and enables its delay element  524  otherwise. Disabling the following delay element prevents the clock signal from propagating down the remainder of the delay line following the tap point, saving power. For every count held by the counter, a single tap gate  528  is enabled. The output node of each tap gate  528  drives a separate input of output gate  530  of the second delay line. Decoders  526  and tap gates  528  disable all but one of the signals driving gate  530 . The single enabled tap gate drives its clock signal onto an input of gate  530 , with a delay substantially equal to the output of delay element  524  driving the enabled tap gate. The output of gate  530  then drives a clock signal having the particular delay of the enabled tap gate  528  onto node  542 . 
   Delayed clock  542  enters third delay line  540  where it passes through successive delay elements  544 . Each delay element  544  delays the clock signal by a substantially equal amount, which is less than the delays of delay elements  524 . Delay lines  500 ,  520 , and  540  can be sequentially placed in any order without affecting the functionality of the present invention. 
   Each delay element  544  has a first input node for receiving a clock signal, a second input node for receiving an active-low enable signal, and an output node for conveying a delayed copy of the clock signal. The first input node of each delay element in delay line  540 , except the first element, connects to the output of the preceding element in the delay line. The first input node of the first element in the delay line connects to second delayed clock  542 . The second input node of each delay element in delay line  540 , except the first element, connects to the output of decoder  546  of the preceding delay stage in the delay line. The second input node of the first delay element in delay line  540  connects logically low. The output node of each delay element in the delay line, except the last, connects as described to the input node of the succeeding element. The output node of each delay element in the delay line connects further to a first input node of a separate tap gate  548  for that delay element. 
   Delay line  540  of  FIG. 5  is controlled by a conventional phase detector  552 , and counter  554 . Phase detector  552  compares input reference clock  502  to delayed replica clock  560 , and sends commands to counter  554  by pulsing the UP 3  and DN 3  lines connecting these two blocks. When the phase of replica clock  560  leads the phase of reference clock  502  by more than a delay of delay element  544 , phase detector  552  pulses the UP 3  line to increment counter  554 , increasing the delay provided by the third delay line by the delay of one delay element  544 . When the phase of replica clock  560  trails the phase of reference clock  502  by more than a delay of delay element  544 , phase detector  552  pulses the DN 3  line to decrement counter  554 , decreasing the delay of the third delay line by effectively removing the delay of one delay element  544 . After pulsing UP 3  or DN 3  to adjust counter  554 , phase detector  552  waits long enough for the adjustment to propagate through the delay lines and back to the inputs of phase detector  552  before repeating its compare/adjust operation. When no change is needed, phase detector  552  sends no pulses and the loop is locked. 
   Counter  554  in  FIG. 5  starts, after an adjustment of delay line  500 , with a preset count in the middle of its numerical range, driven as described by phase detector  512  via nodes UP 1  and DN 1 . Counter  554  accumulates a count as directed by the pulses on its UP 3  and DN 3  input lines. Should this count reach either end of its numerical range, wrap control  532  adjusts counter  554  away from the end of its numerical range and moves counter  534  one step in the opposite direction with the sum of the delays of delay lines  520  and  540  remaining the same. Each bit of the count in counter  554  couples to a separate wire of count bus  556 . Bus  556  routes each bit of the count in parallel to each decoder  546  in the third delay line, and to wrap control  532  of the second delay line. Each decoder  546  has a single output which drives a second input of a separate tap gate  548  coupled to the decoder and to the output of its associated delay element. Each decoder  546  enables its tap gate  548  only when the count on bus  556  is equal to its sequential position in the third delay line. Each value of the count in counter  554  activates one tap gate  548 , to route the clock signal out from delay line  540  after the clock signal has passed through the number of delay elements  544  indicated by the count in counter  554 . 
   Each output from tap gates  548  drives a separate input of output gate  550  of delay line  540 . Since only one of the tap gates  548  is active, the clock signal passing through the active tap gate  548  drives output gate  550  with the particularly delayed copy of the reference clock signal at the active tap gate  548 . The output of gate  550  drives this delayed clock signal onto node  570 , the DQ output clock. 
   Node  570  further connects to the input of output buffer delay model  562 , which delays the clock signal by an amount equal to the input and DQ buffer delay. Buffer delay model  562  then drives the replica clock on node  560 . The DQ clock output thus precedes the replica clock by one buffer delay. When the loop is locked and the delayed replica clock on node  560  has the same phase as input reference clock  502 , then the DQ outputs transition coincident with the clock, as required. 
   A DDR SDRAM device  700  as shown in  FIG. 7  comprises at least one array of memory cells  720  for retaining data, with support circuitry for reading, writing, and testing. DDR SDRAM device  700  has a write cycle for writing data from data ports  776  to memory array  720 , and a read cycle for reading data from the array to the data ports. Address latches  710  receive signals from address ports  702 , comprising row address, column address, and command data. Address latch signals couple to row decoders  712 , column select  716 , and controls  714 , respectively. Controls  714  obtain a reference clock and control signals such as chip-select, read/write, and test from external ports  704 , and commands from the address latches to operate row decoders  712 , column select/sense amp  716 , and data path circuits  718 . Controls  714  typically contain a DLL to set the timing of local clock signals on the device. Row decoders  712  select a row  722  of memory cells for access, responsive to the row address and section signals from the controls. Column-select and sense amplifier  716  selects bitlines  724  for access, and performs read/write operations via the selected bitlines. Data buffers  728  receive write data signals from data ports  776 , and convey the write data to the memory array via a data bus  726 , data path logic  718 , column-select/sense amp  716 , and bitlines  724 . Bitlines  724  convey read data signals from the memory array to column-select/sense amp block  716 , and thence to data path logic  718 , the data bus  726 , data buffers  728 , and data ports  776 . 
   The current invention overcomes the disadvantages of prior art circuits by providing a hierarchy of adjustable, all-digital delay lines having unique controls. Control of delay lines is performed by counter controls, counter, and decoders disposed for each delay line. The counter controls may comprise a phase detector or wrap-control as described. This topology and the design elements that render it stable and small are unique aspects of the current invention providing a novel improvement to speed, size, and stability in delay locked loop design. 
   Though the above description discloses many details, these details should not be understood to limit the current invention. Obvious changes, such as implementing the tap gates and output gate of each delay line using NOR gates instead of NAND gates, or switching the coarse/fine order of adjustable delay lines, while retaining the structure, function, and/or methods of the current invention, would fall within the scope of the patent rights claimed by the inventor. Therefore the scope of the current invention should be limited only by the appended claims and their legal equivalents.