Abstract:
A method and circuitry for a Delay Locked Loop (DLL) or a phase Locked Loop (PLL) is disclosed, which improves the loop stability at high frequencies and allows maximum tracking bandwidth, regardless of process, voltage, or temperature variations. Central to the technique is to effectively operate the loop at a lower frequency close to its own intrinsic bandwidth (1/tLoop) instead of at the higher frequency of the clock signal (1/tCK). To do so, in one embodiment, the loop delay, tLoop, is measured or estimated prior to operation of the loop. The phase detector is then enabled to operate close to the loop frequency, 1/tLoop. In short, the phase detector is made not to see activity during useless delay times, which prevents the loop from overreacting and becoming unstable.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation of U.S. patent application Ser. No. 11/124,743, filed May 9, 2005, which is incorporated herein by reference in its entirety and to which priority is claimed. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Embodiments of this invention relate to delay- or phase-locked loops, and particularly to circuits of that type with increased stability. 
       BACKGROUND 
       [0003]    It is often desired in an integrated circuit to delay a signal. In the context of a periodic signal like a clock signal, adjustment of delay can be understood as an adjustment of the phase of the signal. Such phase shifting of a clock signal can be achieved by use of delay lock loops (DLLs) or phase lock loops (PLLs) that are used to generate internal clock signals for an integrated circuit from a master clock signal. Because of the complexity of modern-day integrated circuits, the ability to finely shift the phase of clock signal is particularly important to ensure proper timing within the circuit. For example, DLLs or PLLs are used to set the data output timing in high speed Dynamic Random Access Memories (DRAMs). 
         [0004]    A typical analog DLL  10  is shown in  FIG. 1 . As shown, the DLL  10  derives an output clock signal (ClkOut) from an input clock signal (ClkIn), in which the phase between the two can be tightly controlled. The DLL comprises a variable delay line (VDL) whose delay (tVDL) is controllable given the analog value of a control signal (VDLctrl), and a fixed delay circuit, namely Delay Module (or DM). The output of the delay module (ClkOut_DM) and the ClkIn signals are compared at a phase detector (PD), which essentially determines whether one of the two input signals (ClkIn; ClkOut_DM) are lagging or leading the other, and seeks to bring these two phases into alignment. For example, if ClkOut_DM leads ClkIn, then the phase detector outputs a “down” signal (DN) to reduce the value of VDLctrl, which increases tVDL; if ClkOut_DM lags ClkIn, then the phase detectors outputs an “up” signal (UP) to increase the value of VDLctrl, which decreases tVDL. The bandwidth of the loop is determined in accordance with a loop filter (LF), which in an analog circuit can comprise resistor-capacitor circuits (e.g., an R-C filter). Moreover, and although not shown, the loop filter may comprise a charge pump. In any event, by virtue of the delay module, the output clock signal ClkOut will precedes the input clock signal ClkIn by its delay (tDM). Of course, the DLL circuit  10 , may also be digital in nature, with the loop filter being replaced by a digital control, and wherein VDLctrl comprises digital outputs to the VDL (not shown). 
         [0005]    In general, and assuming the period of ClkIn is tCK, the loop in DLL circuit  10  establishes a relationship of tVDL+tDM=N*tCK, where N equals the smallest possible integer. Because tVDL is usually not larger than tCK, N is primarily determined by tDM, i.e., the delay through the delay module. Though tDM is a fixed value at given conditions, N is still variable inversely proportional to tCK. 
         [0006]    When the delay of the delay module, tDM, is larger than the clock period, tCK, the transfer function of the loop increases in complexity, and instability can result, as will be shown below. Moreover, such problems are worse as the clock frequency increases (i.e., tCK decreases), or as tDM increases. Furthermore, because tDM can vary as a result of process, temperature, or voltage variations, such instability can be particularly hard to control from device to device. 
         [0007]      FIGS. 2 and 3  shows Z-domain modeling of analog DLL circuit  10  with ( FIG. 2 ) and without ( FIG. 3 ) a delay module (DM). Kd is the gain for the VDL, and L(z) is the transfer function of charge pump and loop filter. The z −1  block represents the fact that the phase detector compares the current input clock edge with the VDL output derived from the previous input clock edge. In other words, the z −1  block represents one clock cycle delay (tCK). The DM, by contrast, is represented by a z −m  block ( FIG. 3 ), meaning that the DM delays by m cycles. 
         [0008]    As seen in the figure, the transfer function of  FIG. 2  (without the delay module) is a first order system of z, and optimal parameters (Kd, R, C, etc.) for the circuit can be solved with ease. However, the addition of the delay module, as shown in  FIG. 3 , causes a significant difference. Here, the resulting transfer function is represented by the (m+1)th order of z. It is very difficult to achieve an analytic solution such a high-order system. 
         [0009]    As a result, and as alluded to above, a DLL circuit  10  with a relatively high tDM can be unstable, as shown in  FIG. 4 . As shown, the total delay time of propagation through the loop, tLooP, is about four cycles for an example, and equals the sum of tF and tB, where tF equals the propagation delay though the phase detector (tPD) plus the propagation delay through the loop filter (tLF), and tB=the propagation delay through the VDL (tVDL) plus the propagation delay through the delay module (tDM) (i.e., tF=tPD+tLF; tB=tVDL+tDM). tVDL is generally not larger than tCK, and tF is usually negligible for an analog DLL design, and is shown exaggerated in  FIG. 4 . (tF may however not be negligible for digital filters). In short, it is largely due to tDM that the delay through the loop can be longer than a single clock cycle. 
         [0010]    In  FIG. 4 , a timing error (tER) between ClkIn and ClkOut_DM is shown. Because initially ClkIn leads ClkOut_DM, UP pulses are needed to try and bring them into alignment. Each UP pulse increases an analog value of the VDL&#39;s analog control signal, VDLctrl, which decreases tVDL; each down pulse (DN) achieves the opposite effect. (Fixed pulse widths for UP and DN are assumed for simplicity of explanation). 
         [0011]    However, notice that it takes significant time (i.e., tLooP) for the output of the phase detector (UP; DN) to take effect through the loop so as to update the phase at the input of the phase detector. In the mean time, before this change in phase is effected, the phase detector continues to generate the same signals (initially, UP in  FIG. 4 ) and does so at every clock period, regardless of whether they are needed or not, and despite that the fact that any phase shift wrought by earlier signals is not yet known. Thus, in the example of  FIG. 4 , four UP pulses are output before any change in phase (tER) is registered. This discrepancy in frequency between the clock frequency (1/tCK) and the loop frequency (1/tLooP) causes the loop to overreact and become unstable. Specifically, the timing error, tER, does not converge but oscillates. The amplitude and period of the oscillation depends on the loop gain and the loop delay (tLooP). 
         [0012]    The conventional solution to this problem involved decreasing the loop&#39;s gain and/or reducing the loop&#39;s bandwidth. This can be accomplished by increasing loop filter&#39;s resistance-capacitance values (assuming an analog circuit), reducing the charge pumping current, or increasing the size of the loop filter. But these solutions can consumes larger layout areas and can considerably decreases the tracking bandwidth (i.e., loop gain divided by loop delay), resulting in a longer time to achieve a phase “lock.” In short, such previous approaches involved undesirable trade offs between maximum frequency performance, stability, tracking bandwidth, and layout area. A better solution is therefore needed. 
       SUMMARY 
       [0013]    A method and circuitry for a Delay Locked Loop (DLL) or a phase Locked Loop (PLL) is disclosed, which improves the loop stability at high frequencies and allows maximum tracking bandwidth, regardless of process, voltage, or temperature variations. Central to the technique is to effectively operate the loop at a lower frequency close to its own intrinsic bandwidth (1/tLoop) instead of at the higher frequency of the clock signal (1/tCK). To do so, in one embodiment, the loop delay, tLoop, is measured or estimated prior to operation of the loop. The phase detector is then enabled to operate close to the loop frequency, 1/tLoop. In short, the phase detector is made not to see activity during useless delay times, which prevents the loop from overreacting and becoming unstable. Thus, a loop with the proposed method can operate stably at any frequency, and without increasing the loop filter resistance-capacitance values or decreasing loop bandwidth, such that tracking bandwidth and layout area are not sacrificed. In short, use of the disclosed technique requires no trade off between maximum frequency, stability, and tracking bandwidth. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Embodiments of the inventive aspects of this disclosure will be best understood with reference to the following detailed description, when read in conjunction with the accompanying drawings, in which: 
           [0015]      FIG. 1  illustrates a prior art delay locked loop (DLL) with a delay module among other components. 
           [0016]      FIGS. 2 and 3  illustrate the transfer functions of the DLL of  FIG. 1  with and without consideration of the delay module. 
           [0017]      FIG. 4  illustrates instability and oscillation that can result in use of the DLL of  FIG. 1  when long delays are used in the delay module. 
           [0018]      FIG. 5  illustrates an embodiment of the improved DLL, including use of a phase detector controller. 
           [0019]      FIG. 6  illustrates timing diagrams used during a measuring period, in which the loop delay for the improved DLL circuit of  FIG. 5  is measured or estimated. 
           [0020]      FIG. 7  illustrate timing diagrams for the DLL circuit of  FIG. 5 , showing selective enablement of the phase detector in accordance with the loop frequency, and no oscillations or instabilities. 
           [0021]      FIG. 8  illustrates an embodiment of the invention in the context of a phase-locked loop (PLL). 
           [0022]      FIG. 9  illustrates exemplary circuitry for selectively enabling the phase detector using an enable signal (PDen). 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    The disclosed scheme uses smart filtering to remove the discrepancy between the loop frequency (1/tLoop; delay in propagation through the loop) and the higher clock frequency (1/tCK) by activating the phase detector to work at a rate closer to the loop frequency.  FIG. 5  shows one embodiment for achieving this goal in the context of an improved analog DLL circuit  100 . However, the improvements are equally applicable when applied to digital DLL, or to PLL  100 ′ ( FIG. 8 ), which uses a variable oscillator (VCO) to generate a clock signal whose phase and frequency are locked to those of the input clock, ClkIn. 
         [0024]    As shown in  FIG. 5 , a phase detector control block, PDctrl  105 , has been added to the DLL circuit. It includes a counter  112 , register  114 , and a controller  116  which together are useful in measuring the loop frequency and ultimately in controlling the phase detector in accordance with the loop frequency. 
         [0025]    In a preferred embodiment, the loop frequency, 1/tLoop, is measured prior to operation of the DLL circuit  100 . This is preferred, because the loop frequency can vary with process, voltage, and temperature variations, and can also vary in accordance with the input frequency. Thus, by measuring the loop frequency, a reliable value is acquired which is tailored to the unique environment in which the DLL circuit  100  is used. However, it is not strictly necessary in all useful embodiments to first measure the loop frequency prior to using the same to control the DLL circuit. Instead, if the loop frequency is known or otherwise ascertainable, it can merely be used without the measuring step. 
         [0026]      FIG. 6  shows the timing diagrams used during the loop frequency measuring step. Basically, this step measures the time it takes for an input pulse (ClkIn) to pass through the loop. As shown, a measure signal is used to enable the measuring function. It is preferred during measuring that the phase detector and loop filter circuits ( FIG. 5 ) should be rendered transparent such that input signals received are merely passed to the outputs of these blocks. However, if this is not easy or practical, then the blocks can be by-passed altogether, such as with the use of transmission gates  110 . While by-passing the phase detector and loop filter will cause the measured delay through the loop to be slightly smaller than normal, such small skew in the measurement is satisfactory as the delays in the phase detector and loop filter (i.e., tF=tPD+tLF) are generally negligible (see  FIG. 6 ). The measure signal can be self-generated by the PDctrl  105  block, or can be provided by another logic circuit such as a microcontroller which would normally be on the integrated circuit as the DLL circuit  100 . 
         [0027]    Referring again to  FIG. 6 , when the measure signal goes high, and after detecting a first ClkIn pulse, counter  112  start counting the number of subsequent ClkIn pulses up until the time that a pulse is detected at ClkOut_DM. After such detection takes place, the circuit waits for the next ClkIn pulse, taking this “last” ClkIn pulse as the end of the measuring period. Therefore, the measure signal can be disabled. As shown, the measured delay, tML, spans between the first and last ClkIn pulses in the measuring period, which is slightly longer than the actual loop delay, tLoop, but still comprises a useful measure of the loop delay (i.e., tML˜tLoop). In any event, the measured period can be viewed as a number of cycles of the input clock, m, which in the example of  FIG. 6  equals 4 (i.e., m*tCk=tML˜tLoop). This measured value of m—the approximate ratio between the clock and loop frequencies—is stored in the register  114  for use in controlling the phase detector during normal operation of the DLL  100 , as is explained next with reference to  FIG. 7 . 
         [0028]    During normal operation, the phase detector is only enabled once during each period of the measured (or otherwise provided) loop frequency, i.e., once every tML. Specifically, 1/m controller  116  is used to process the input signal, ClkIn, by frequency dividing that signal by m, i.e., to remove all but every m-th pulse in the train to produce a phase detector enable signal PDen. (The controller  116  may also change the width of the ClkIn signal or its duty cycle). Thus, because m was measured to be four in  FIG. 6 , it is seen in  FIG. 7  that the PDen is high every fourth input clock pulse. 
         [0029]    In any event, because the phase detector is only enabled at the times when PDen is high, an assessment of phase between ClkIn and ClkOut_DM, and subsequent output of an UP or DN signal, is affected only during those limited times, e.g., during windows  150 . Again, these windows  150  are assessed in accordance with the loop frequency (i.e., 1/tML˜1/tLoop), and not in accordance with the clock frequency (1/tCK) as in the prior art. This keeps the loop from overreacting, such as in  FIG. 4 , where several UP signals were generated, and VDLctrl continually modified, before it was even assessed whether such phase adjustment control signals were warranted. As a result, and as shown in  FIG. 7 , using the disclosed technique, the timing error, tER, will converge and not oscillate. (This assumes that the gain in the loop is not too large. The loop gain can be optimized, as one skilled in the art will appreciate, and in any event can be made higher than conventional loops not using embodiments of the disclosed invention). Thus, the effect of long delay through the delay module, tDM, are overcome by in effect measuring that delay as part of the loop delay, and taking that measured delay into account when generating control signals at the phase detector. 
         [0030]    Selective enablement of the phase detector via the PDen signal can be achieved in several different ways, as one skilled in the art will appreciate. In one simple way, shown in  FIG. 9 , the UP and DN signal outputs are grounded (via N-channel transistor  132 ) during those periods when PDen is not low, corresponding to a command that tVDL not be adjusted. During such time, connection of the circuits in the phase detector to the power supply voltage, Vdd, are disconnected (via P-channel transistors  130 ) to ensure no power-to-ground shorts. 
         [0031]    It should be understood from this disclosure that frequency with which the phase detector is activated need not exactly match the loop delay. Thus, as shown in  FIG. 6 , the loop delay, tLoop, is smaller than the measured value, tML, ultimately used to adjust the frequency of phase detector. This results due to the convenience of counting input clock pulses as an estimation of loop delay. In this regard, it should be understood that the frequency of the phase detector need (1/tML) only substantially correspond to the loop frequency (1/tLoop) in a preferred embodiment. In a less-preferred, but still beneficial embodiment, the phase detector is operated at a frequency which is somewhere between the clock frequency and the loop frequency. For example, suppose in  FIG. 7  that the phase detector is enabled (via PDen) every other clock pulse (instead of every fourth as shown). Even thought this would amount to some, amount of overreaction of the loop—because not all phase adjustment commands will have had a chance to percolate through the loop to have effect before new commands are entered—the effect and stability of the loop will still be improved when compared with the prior art. 
         [0032]    The measuring step can occur in an integrated circuit in which the DLL is used upon chip reset or initialization, or can be measured periodically during operation of the integrated circuit to ensure that the measured loop frequency is still optimal. 
         [0033]    Although the disclosed phase detector has been shown separate from the loop filter, it should be understood that use of the term “phase detector” can comprise the loop filter aspects of the circuitry as well (if any). 
         [0034]    While a preferred embodiment of the invention has been disclosed, it should be understood the circuitry used to affect the frequency conversion of the enablement of the phase detector can be achieved in many different ways. In short, it should be understood that the inventive concepts disclosed herein are capable of many modifications. To the extent such modifications fall within the scope of the appended claims and their equivalents, they are intended to be covered by this patent.