Abstract:
A delay-locked loop (DLL), including frequency detection logic and a phase detector, is described having an operating range as wide as a conventional charge pump phase locked loop. The frequency detector logic counts the number of rising edges of the multi-phase clocks generated from a reference clock during one period of the reference clock. A loop filter is used to adjust the frequency of each multi-phase clock until frequency lock is obtained by comparing the number of rising edges. After frequency lock, phase detection logic is used to finely tune out the remaining phase error.

Description:
This application claims priority under 35 U.S.C. 119(e) from U.S. provisional patent application Ser. No. 60/136,640 having a filing date of Jun. 27, 1999 which is entitled “Unlimited Frequency Range Delay-Locked Loop Circuit,” and which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     This invention relates to delayed-locked loops. 
     2. Description of the Related Art 
     Skew reduction techniques using a phase-locked loop (PLL) or delay-locked loop (DLL) have become increasingly important as the required system bandwidth increases. Especially, the DLL has become more popular as a zero delay buffer because of its better stability and better jitter characteristics than the PLL. However, the conventional DLL does not offer a frequency range as wide as the PLL does because of its inherent limitation on the frequency range and the problem of false locking. PLLs and DLLs are typically used in synchronous systems wherein the integrated circuits in the system are synchronized to a common reference clock. 
     In the phase-locked loop, a voltage-controlled oscillator produces a local clock. The phases of the local clock and a reference clock are compared by a phase-frequency detector, with the resulting error signal used to drive the voltage-controlled oscillator via a loop filter. The feedback via the loop filter phase locks the local clock to the reference clock. Stability of the feedback loop, however, depends in part on the loop filter. The electronic characteristics of the loop filter, in turn, often depend significantly on manufacturing parameters. As a result, the same loop filter design may result in a stable feedback loop when manufactured with one process but an unstable loop when manufactured by another. It is difficult to produce a single loop filter design for use with all manufacturing processes, and the design of the loop filter typically must be optimized on a process by process basis. 
     The delay-locked loop generates a synchronized local clock by delaying the incoming reference clock by an integer number of periods. This approach avoids the stability problem inherent in the phase-locked loop approach. Delay-locked loops, however, have a disadvantage of narrow frequency range. The delay-locked loop adjusts the amount of additional delay in order to achieve the desired synchronization, but this adjustment is essentially a phase adjustment. The conventional delay-locked loop lacks any significant frequency adjustment, thus limiting the overall frequency range of conventional delay-locked loops. Furthermore, delay-locked loops may falsely lock on a frequency. 
     Accordingly, it is desirable to achieve a delay-locked loop that can operate over a wide frequency range and which can provide protection against false locking. 
     SUMMARY 
     The present invention provides a DLL that is operable over a wide frequency range, and that provides protection against false locking. 
     The DLL in accordance with the present invention generates a set of multiphase clocks whose delays are locked to an input reference signal. In one embodiment, the DLL includes a plurality of delay elements adapted to incrementally delaying the input reference clock to generate a set of multi-phase clocks, frequency detector logic adapted to counting the number of rising edges occurring on the set of multi-phase clocks in one period of the input reference clock and a loop filter adapted to generating a control signal to adjust the delay amount of each delay element when the number of rising edges is different from a predetermined number. The predetermined number can be set by the number of the delay elements minus one. The process of locking to the frequency of the input reference clock by comparing the number of rising edges with the predetermined number prevents false locking that occurs when the delay time through the delay chain is a multiple of the reference clock period, in which case the numbers would not match. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates one embodiment of a DLL in accordance with one embodiment of the present invention. 
     FIG. 2 shows one embodiment of frequency detection logic in accordance with the present invention. 
     FIG. 3 is an example of a timing diagram for the embodiment of the frequency detection logic shown in FIG.  2 . 
     FIG. 4 shows one embodiment of a phase detector in accordance with the present invention. 
     FIG. 5 illustrates embodiments of charge pumps and a loop filter that may be used in the DLL in accordance with the present invention. 
     FIG. 6 is a plot showing an example of simulated gain of overall phase detection. 
     FIG.  7 ( a ) shows an example of a simulated waveform of a delay control voltage. 
     FIG.  7 ( b ) shows an example of a measured DLL jitter histogram. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates an embodiment of a DLL in accordance with the present invention. The DLL  10  comprises a delay chain  11  having a plurality of delay elements  18 ′, a frequency detection logic  12 , a phase detector  13 , two charge pumps  14 ,  15 , and a loop filter  16 . A delay cell  19 ′ comprising two inverters  6 ,  7  whose outputs are controlled by a delay control signal which activates switches  8 ,  9  is an example of a delay element that may be used in accordance with the present invention. The plurality of delay elements  18 ′ is adapted to generating a multi-phase clock. In this embodiment, the delay chain  11  comprises seven delay cells to generate a seven-phase clock (CK[ 1 : 7 ]). 
     The frequency detection logic  12  receives an input reference clock (REF_CK) and a seven-phase clock (CK[ 1 : 7 ]). The logic  12  continuously counts the number of rising edges of CK[ 1 : 7 ] within one period of the input reference clock to decide if the phase of each delayed edge lags or leads the reference clock, or is in a locked state. This embodiment detects the situation of false locking to a different frequency, which arises when the delay time through the chain is a multiple of the reference clock period. 
     The charge pump  14 , charges or discharges the loop filter according to the frequency detection logic signals shown as FUP and FDOWN. During the time when frequency lock is being obtained, the phase detector  13  is disabled, and thus the charge pump  15  doesn&#39;t contribute to the loop operation. 
     When frequency lock is obtained, the frequency detection logic  12  asserts a frequency lock signal to the phase detector  13  before being decoupled from the loop. The charge pump  15  then can take over the loop control. The phase detector  13  and charge pump  2   15  finely tune out the remaining phase error between the input reference clock (REF_CK) and, in this embodiment, CK[ 7 ]. 
     FIG. 2 illustrates one embodiment of the frequency detection logic  12 . The frequency detection logic comprises a frequency divider  21 , seven frequency detection cells (FD CELL [N])  22 ′, decision logic  23 , and two pulse generators  24 ,  25 . 
     FD CELL[N]  22 ′ receives CK[N] as a trigger pulse and moves the output (EDGE[N]) from 0 to 1 on the rising edge of CK[N]. An embodiment  26 ′ of a frequency detection cell is shown comprising a logical combination of inverters  27 ,  29 , and  30  and switches  31 - 37 , an example of a switch being a field effect transistor, for outputting EDGE[N] as a “1” in response to a rising edge of CK[N] during one period of the reference clock signal. 
     The decision logic  23  counts the number of 1&#39;s in EDGE[ 1 : 7 ] within one period of the input reference clock. The decision logic asserts the frequency lock signal when the rising edge of the input clock propagates and arrives at the sixth delay cell within one period (EDGE[ 1 : 7 ] 1111110). In one embodiment, the decision logic may be implemented using Boolean logic. For example, the decision logic may include a counter whose output is tied to logic gates that generate a signal indicating frequency lock or the direction in which frequency needs to be adjusted. 
     FIG. 3 illustrates a timing diagram of the embodiment of frequency detection logic  12  shown in FIG.  2 . Case (a) shows an example of a frequency lag. After a reset, the rising edge of the input clock propagates and arrives for this example, at the fourth delay cell within one reference clock period, resulting in EDGE[ 1 : 7 ]=1111000. This means that the delay chain is too slow to acquire a phase lock and the pulse generator  24  generates an FUP signal accordingly. 
     Case (b) illustrates an example of a frequency lock. In this embodiment where there are seven delay cells, each delay cell, when locked to the input reference frequency, should delay the input reference clock by an increment of one seventh ({fraction (1/7)}) of one clock period. In this case, the first through sixth instances of the delayed input clock occur within one clock period, whereas the seventh instance occurs after one clock period. This is illustrated by the figure where the rising edge of the input clock propagates and arrives up to the sixth delay cell resulting in EDGE[ 1 : 7 ]=1111110, a pattern that can distinguish the case of frequency lock from the case of frequency lead or frequency lag. A false locking possibility is avoided because, in the case where the delay time through the delay chain is a multiple of the input clock period, the number of rising edges would not be equal to six, the number of delay cells minus  1 . A frequency lock signal then can be asserted to indicate that the phase detector can take over the loop control to tune out the remaining phase error. 
     Case (c) illustrates an example of frequency lead. The rising edge of the input clock propagates and passes beyond the seventh delay cell in less than one input clock period, resulting in EDGE[ 1 : 7 ]=1111111. The result indicates that the delay chain is too fast to acquire a phase lock and the pulse generator  25  generates an FDOWN signal. 
     FIG. 4 illustrates an embodiment of a phase detector  13  for accurate phase tuning. Resettable D-type flip-flops (DFF&#39;s)  41 ,  42  are used as main function blocks. Dummy delay elements  43  are inserted in the signal paths to reduce the dead zone of the detector gain curve. The frequency lock signal from the frequency detection logic  12  enables the phase detector  13  after a frequency lock is obtained. 
     FIG. 5 illustrates an example of how the two charge pumps  14 ,  15 , one for frequency detection and the other for phase detection, and the common loop filter  16  may be embodied. Because the active side of the charge pumps shuts out the inactive side, the charge pumps do not suffer from the problem of charge sharing and control signal feed-through, which can induce undesirable phase noise. 
     In one embodiment, the DLL of the present invention has been fabricated using a 0.35 μm CMOS process. The area occupied by the DLL is 390 μm×500 μm. It draws 5.12 mA with 3.3 V supply, at 150 MHz. 
     FIG. 6 illustrates an example of a simulated gain of the overall phase detection. It illustrates that the dead zone of the phase detection can be reduced to 5 picoseconds. The simulation is based on a circuit simulation using a device model. 
     FIG.  7 ( a ) illustrates the simulated waveform of the delay control voltage. The linear portion of the curve indicates the frequency detection stage, whose slope is controlled by the current source I 1  for the charge pump as embodied in FIG.  5 . The nonlinear portion indicates the fine phase tuning on the phase detection stage. 
     FIG.  7 ( b ) illustrates an example of a measured DLL jitter histogram with the root mean square (rms) value of 13 picoseconds in the 150 MHz operation. The measured frequency range is from 9.5 MHz to 203 MHz, which is limited only by the minimum delay time of the delay chain. 
     While the invention has been described with reference to various embodiments, it is not intended to be limited to only these embodiments. It will be appreciated by those of ordinary skill in the art that many modifications can be made to the structure and form of the described embodiments without departing from the spirit and scope of this invention.