Patent Publication Number: US-2006001494-A1

Title: Cascaded locked-loop circuits deriving high-frequency, low noise clock signals from a jittery, low-frequency reference

Description:
FIELD OF THE INVENTION  
      The present invention relates generally to the field of communications, and more particularly to high speed electronic signaling within and between integrated circuit devices.  
     BACKGROUND  
      Integrated circuits (ICs) experience process, voltage, and temperature variation that render difficult the task of integrating a sufficiently clean and stable clock source to support high-speed communication. ICs therefore rely on specialized external clock sources to reference timing. One such clock source is often shared by components in a system, such as by a number of ICs on a printed circuit board, or number of printed circuit boards in a backplane communication system. Precision external clock sources that employ crystal oscillators provide relatively stable clock frequencies. Temperature compensation circuitry is typically added to improve frequency stability over a range of temperatures. The frequency stability of temperature compensated crystal oscillators (TCXOs) may approach 0.1 PPM.  
      The speed at which high-performance ICs transmit and receive data is ever increasing. Unfortunately, distributed reference clock sources are not keeping pace with the circuits that use them. Part of the problem is a legacy issue, as system designers grow accustomed to using well characterized, stable, and relatively inexpensive clock sources. Routing constraints and system noise exacerbate the problem.  
      ICs that require higher clock frequencies than are provided by external oscillators can be adapted to multiply a reference clock signal to create an internal reference clock signal of the desired frequency. However, in the process of multiplying the reference clock signal, its jitter (phase noise) can be passed along as ever higher frequencies (multiplication factors) are required, so the jitter of the resulting multiplied clock passed from the reference clock becomes a greater percentage of the system unit interval. There is therefore a need for a clocking architecture that derives high-frequency, low-jitter clock signals from relatively low frequency reference clock sources. Such clock signals could be used as transmit and receive clock signals, for example. Ideally, such a clocking architecture would produce an output clock signal that exhibits a considerable tuning range to provide compatibility with communication schemes that employ different clock rates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:  
       FIG. 1  depicts an integrated circuit (IC)  100  in accordance with one embodiment. IC  100  includes cascaded first and second phase-locked loops (PLLs)  105  and  110 .  
       FIG. 2  depicts an integrated circuit (IC)  200  that includes clock synthesis and distribution circuitry in accordance with one embodiment that delivers low-noise transmit and receive clock signals that can be tuned over a broad range of output frequencies.  
       FIG. 3  depicts an IC  300  in accordance with another embodiment. IC  300  is similar to IC  200  of  FIG. 2 .  
       FIG. 4  depicts an IC  400  in accordance with an embodiment that employs a two-stage clocking architecture of the type described above in connection with  FIGS. 2 and 3 .  
       FIG. 5  depicts an IC  500  in accordance with an embodiment extended to derive 750 MHz, 1.5 GHz, and 3 GHz clock signals for SATA interfaces. 
    
    
     DETAILED DESCRIPTION  
       FIG. 1  depicts an integrated circuit (IC)  100  in accordance with one embodiment. IC  100  includes cascaded first and second phase-locked loops (PLLs)  105  and  110 . PLL  105  includes a phase detector  115 , a loop filter  117 , and a voltage-controlled oscillator (VCO)  120 ; PLL  110  includes a phase detector  125 , a loop filter  127 , and a VCO  130 . PLL  105  derives a low-jitter intermediate reference clock signal IRClk from what may be a relatively noisy, low-frequency external reference clock signal RClk using VCO  120 , a high-Q oscillator. PLL  105  has a low loop bandwidth, and thus acts as a low-pass filter (LPF) to remove jitter from reference clock signal RClk. In this example, the loop bandwidth of PLL  105  is substantially less than ten percent of the reference clock frequency. PLL  105  may be adapted to multiply the reference clock frequency by some factor so that the relatively low jitter intermediate clock signal IRClk is of a higher frequency than the reference clock signal (i.e., F IRClk  may be greater than F RClk ).  
      PLL  110  derives an output clock signal (e.g., a transmit and/or receive clock signal) from intermediate reference clock signal IRClk. In comparison with VCO  120 , VCO  130  is a relatively low-Q oscillator that exhibits a considerable tuning range to support a number of output clock frequencies. PLL  110  is adapted to provide high loop bandwidth to minimize phase noise introduced by low-Q VCO  130 .  
      Reference clock RClk and various components of PLLs  105  and  110  introduce undesirable phase noise. The clock multiplication and distribution system of IC  100  addresses these noise sources so that output clock signal ClkO exhibits low jitter over a broad frequency range. The first PLL  105  exhibits a narrow loop bandwidth and employs a high-Q VCO. PLL  105  thus acts as a low-pass filter to reject input phase noise, while the high-Q VCO introduces little additional noise. The second-stage PLL  110  exhibits much higher loop bandwidth than PLL  105  and employs a relatively low-Q VCO that can be tuned over a relatively broad frequency range. The wide loop bandwidth of PLL  110  enables it to reject much of its own VCO-induced noise. Also helpful, prior multiplication of the reference clock RClk by PLL  105  limits the degree to which PLL  110  is required to multiply the intermediate reference clock, and consequently the amount of VCO noise introduced by PLL  110 . The characteristics of PLLs  105  and  110  (e.g., the loop bandwidth, VCO quality, and frequency control) may be tuned to achieve a preferred balance of reference-clock and VCO noise rejection to obtain an optimal noise transfer function.  
      PLLs  105  and  110  are illustrated in connection with basis elements of PLLs that are well known to those of skill in the art. Many implementations, variations, and combinations of those elements may be used, some of which are detailed below.  
       FIG. 2  depicts an integrated circuit (IC)  200  that includes clock synthesis and distribution circuitry in accordance with one embodiment that delivers low-noise transmit and receive clock signals that can be tuned over a broad range of output frequencies. IC  200  includes a clock synthesizer  205  that filters and multiplies an externally generated reference clock signal RClk to produce a low-jitter intermediate reference clock signal IRClk for distribution on IC  200 . Additional clock synthesis circuitry within one or more transceiver blocks  210  then multiply intermediate reference clock signal IRClk to develop transmit and receive clock signals.  
      Clock synthesizer  205  includes a first stage phase-locked loop (PLL)  207  that receives and multiplies reference clock signal RClk to produce intermediate reference clock signal IRClk. Each transceiver block  210  includes a second stage PLL  215  to multiply intermediate reference clock signal IRClk to produce an output clock signal FClk. As compared with the second stage PLLs  215 , PLL  207  of synthesizer  205  exhibits a relatively narrow tuning range and is adapted to reject input phase noise from reference clock signal RClk. The second stage PLLs  215  are adapted to operate over a wider frequency range then PLL  207  in terms of the percentage of the center frequency of each PLL&#39;s respective VCO.  
      IC  200  is a multi-channel transceiver in the depicted embodiment, each transceiver block  210  supporting multiple channels. PLL  207  includes a VCO, as does each of PLLs  215 . The VCO of PLL  207  differs from those of PLLs  215 , however, in that it exhibits a relatively high-Q and a relatively narrow tuning range. As a result, PLL  207  is designed with a relatively narrow (i.e., low) loop bandwidth to produce an intermediate reference clock signal IRClk. In producing signal IRClk, PLL  207  removes much of the phase noise from external reference clock RClk without introducing considerable VCO phase noise. In contrast, each VCO in PLLs  215  is a relatively low-Q oscillator that exhibits a considerable tuning range to support a number of high-speed transmit and receive data rates. PLLs  215  use relatively higher (i.e., wider) loop bandwidths to avoid introducing considerable VCO noise. The following Table 1 correlates a number of common interface standards with the data transfer rates that may be achieved using output clock frequencies provided by the clock multiplication and routing infrastructure of  FIG. 2 . In Table 1, PCIE stands for PCI Express™, XAUI stands for the 10 Gigabit Attachment Unit Interface. DR Mode in Table 1 refers to whether the data is transferred on both edges of the clock (DDR) or on only one edge (rising or falling) of the clock (SDR).  
                                       TABLE 1                                                   Data Transfer           Standard   DR Mode       FClk Freq   Rate                                                                    PCIE   DDR   1.25   GHz   2.5   Gbps           PCIE   SDR   2.5   GHz   2.5   Gbps           Turbo PCIE   DDR   2.5   GHz   5.0   Gbps           Turbo PCIE   DDR   3.125   GHz   6.25   Gbps           XAUI   SDR   3.125   GHz   3.125   Gbps           2 × XAUI   DDR   3.125   GHz   6.25   Gbps                      
 
      The predominant noise sources in the clock multiplication and distribution system of  FIG. 2  are the phase noise of reference clock RClk and the noise introduced by the oscillators in PLLs  215 . (In ring-oscillator-based PLLs, as are typically used in PLL  215 , the phase noise and supply noise of the VCO is typically much more significant than the noise contributed by the other PLL components.) The clock multiplication and distribution system of IC  200  addresses these noise sources so that output clock signals exhibit low jitter over a broad frequency range. The first PLL  207  exhibits a narrow loop bandwidth and employs a high-Q VCO. PLL  207  thus acts as a low-pass filter to reject input phase noise, while the high-Q VCO introduces little additional noise. The second-stage PLLs  215  exhibit much higher loop bandwidth than PLL  207  and employ relatively low-Q VCOs that can be tuned over a relatively broad frequency range. The wide loop bandwidth of PLLs  215  enables them to reject much of their own VCO-induced noise. Also helpful, the prior multiplication of the reference clock RClk by the PLL  207  limits the degree to which PLLs  215  are required to multiply the intermediate reference clock, and consequently the amount of VCO noise introduced by PLLs  215  by enabling the use of a high loop bandwidth. The characteristics of PLLs  207  and  215  (e.g., the loop bandwidth, VCO quality, and frequency control) may be tuned to achieve a preferred balance of reference-clock and VCO noise rejection to obtain an optimal noise transfer function.  
      Clock synthesizer  205  includes a reference clock terminal  220 , a processor clock node  225 , and an intermediate clock node  230 . Clock terminal  220  receives external reference clock signal RClk, which is typically supplied via an external oscillator. Depending upon the value provided on a select port PClkSel, PLL  207  may convey a processor clock signal PClk to other circuits (not shown) via clock node  225 . In this example, PLL  207  provides one of three available PClk frequencies, 125 MHz, 250 MHz, and 312.5 MHz, to some core logic. Irrespective of the value on select port PClkSel, in this embodiment PLL  207  provides a clean, stable intermediate clock signal IRClk of 625 MHz on node  230  for distribution to each link PLL  215 . A second select value provided on select port RateSel determines whether PLLs  215  derive 1.25 GHz, 2.5 GHz, or 3.125 GHz output clock signals FClk from intermediate clock signal IRClk. Each transceiver block  210  employs the selected FClk frequency to establish transmit and receive timing for one or more corresponding transceivers  235 . Depending upon the selected mode, transceivers  235  communicate at 2.5 Gbps, 3.125 Gbps, 5.0 Gbps, or 6.25 Gbps. A standby signal Stby to clock synthesizer  205  is asserted in a standby mode to pause distribution of intermediate clock signal IRClk without disabling processor clock PClk. Turning off the intermediate clock signal deactivates each link to save power when the links are not in use. Processor clock PClk can likewise be gated in response to standby signal Stby, or a separate control signal, to allow the processor clock to be paused. These features may be used to support various power management modes, such as those employed in the PCI Express architecture. In other embodiments, one or more of PLLs  215  can be adapted to provide processor clock PClk, in which case different PLLs can deliver different processor-clock frequencies.  
       FIG. 3  depicts an IC  300  in accordance with another embodiment. IC  300  is similar to IC  200  of  FIG. 2 . IC  300  includes a clock synthesizer that derives three clock signals—a transmit clock signal TXClk, a receive edge clock signal RXEClk, and a receive data clock signal RXDClk—from an external reference clock signal RClk. IC  300  accomplishes this derivation using two cascaded PLL stages, including a first PLL  305  and a second PLL  310 . Additional second PLLs  310  can be provided to support additional transceiver blocks, but these are omitted here for brevity. Although depicted as single lines, the reference clock paths and the forward and feedback paths of both PLL  305  and PLL  310  may be fully differential to achieve high rejection of supply and substrate noise. Advantageously, the circuitry depicted in  FIG. 3  may be formed using standard CMOS processes, and so may be easily integrated.  
      PLL  305  generates a 625 MHz intermediate clock signal IRClk from any of three potential reference clock frequencies: 100 MHz, 125 MHz, and 250 MHz. To provide this flexibility, PLL  305  includes a frequency divider  312  that can be set to divide reference clock signal RClk by a factor R of four, five, or ten to achieve an effective phase detector input clock frequency FPDI of 25 MHz. The following Table 2 shows the correlation between reference clock frequencies and R, the divisor applied by divider  312 . The divisor may be selected via control signals or control registers. Some embodiments detect the reference clock frequency and adjust divider  312  as needed to produce the 25 MHz frequency.  
                           TABLE 2                                   RClk   R                                                    100 MHz   4           125 MHz   5           250 MHz   10                      
 
      PLL  305  can be adapted to generate intermediate clock signal IRClk from more, fewer, or different reference clock frequencies. For example, one embodiment generates a 625 MHz intermediate clock from the reference clock frequencies of Table 2 and from a 25 MHz reference clock frequency (e.g., R=1). The 25 MHz reference clock can be used, for example, during low-speed testing, such as for wafer sort or burn-in. IC  300  can be adapted to support more, fewer, or different frequencies. One embodiment, for example, supports a 312.5 MHz reference clock RClk, in which case divider  312  may be adapted to divide intermediate clock signal IRClk by 12.5. The basic architecture can be extended to other frequencies by e.g. changing the VCO frequencies and selecting appropriate clock-divider ratios.  
      The 25 MHz version of reference clock signal RClk is conveyed to a phase detector (or phase-frequency detector)  314  via an optional clock buffer  316 . Phase detector  314  compares the phase of the signal from buffer  316  with the phase of a 25 MHz feedback signal from a second optional clock buffer  318  to generate a pair of up and down phase-error signals UP and DN. A charge pump  320  generates a correction current CC in response to the phase-error signals, while a loop filter  322  shapes or filters the correction current CC into a VCO-control signal Sf, which can be e.g. a control voltage, current, or logic stage. A VCO  324  produces a 2.5 GHz clock signal VClk that varies in response to changes in control signal Sf. A pair of frequency dividers  328  and  330  divide clock signal VClk by four and twenty-five, respectively, to provide the 25 MHz feedback signal to buffer  318 . As is conventional in PLLs, phase detector  314 , charge pump  320 , and filter  322  adjusts the frequency and phase of VCO  324  to maintain a fixed phase relationship between the two input signals to phase detector  314 , and consequently maintains a fixed frequency and phase relationship between reference clock RClk and intermediate clock IRClk. In one embodiment, the loop bandwidth of PLL  305  is typically set between about 1 and 2 MHz. For a more detailed discussion of PLLs and suitable components for use therein, see “Design of Monolithic Phase-Locked Loops and Clock Recovery Circuits—A Tutorial,” by Behzad Razavi (IEEE Press, 1996), which is incorporated herein by reference. In particular, a widely accepted design principle is to keep the PLL&#39;s bandwidth less than or equal to one tenth of the reference frequency at the phase detector. Thus, in this embodiment, this corresponds to a maximum loop bandwidth of 2.5 MHz.  
      The two most influential noise sources in PLL  305  are the reference clock RClk and VCO  324 . Due to the narrow loop bandwidth, PLL  305  acts as a narrow-band tracking filter that blocks most of the phase noise on reference clock signal RClk. (In the present disclosure, “loop bandwidth” may be otherwise defined as the frequency at which the closed-loop gain drops to about 0.707 times the gain at low-frequency.) To minimize VCO noise, VCO  324  is a high-Q oscillator that may be based upon an LC tank circuit (not shown), and thus does not introduce considerable phase noise. VCO  324  has a 2.5 GHz center frequency, a Q of about three to ten, and a tuning range of about five percent. By combining a narrow loop bandwidth with a high-Q VCO, PLL  305  produces a relatively “clean” 625 MHz intermediate clock IRClk.  
      Frequency divider  332  divides the 2.5 GHz clock signal VClk by 20, 10, or 8 in response to a pair of select signals PCIkSel 1  and PClkSel 2  to produce 125 MHz, 250 MHz, and 312.5 MHz processor clock signals PClk. The following Table 3 shows the combinations of signals PClkSel 1  and PClkSel 2  that produce the three available processor-clock frequencies.  
                                   TABLE 3                                   PClkSel1   PClkSel2   Div1   PClk                                                                0   0   1/20   125   MHz           1   0   1/10   250   MHz           1   1   1/8    312.5   MHz                      
 
 As explained below, the frequency of processor clock signal PClk is adjusted for compatibility with the transmit and receive clock frequencies. 
 
      Turning to PLL  310 , the 625 MHz intermediate reference clock signal IRClk is conveyed to a phase detector (or phase-frequency detector)  340  via an optional clock buffer  342 . Phase detector  340  compares the phase of the signal from buffer  342  with the phase a 625 MHz feedback signal from a second optional clock buffer  344  to generate a pair of up and down phase-error signals UP and DN. A charge pump  346  generates a correction current CC 2  in response to the phase-error signals, while a loop filter  348  converts the correction current CC 2  into a frequency-control signal Sf 2 . A VCO  350  produces four phases P 1 -P 4  of a clock signal, the frequencies of which vary in response to changes in control signal Sf 2 . Relative to VCO  324  in PLL  305 , VCO  350  is a low-Q oscillator with a wide tuning range. In the depicted example, VCO  350  operaties at either 2.5 GHz or 3.125 GHz, exhibiting a tuning range of about 25%. Narrower or wider tuning ranges may also be used in other embodiments.  
      An optional transmit phase interpolator, or phase mixer,  354  selects from and interpolates between a pair of the phase vectors P 1 -P 4  to produce a transmit clock TXClk. Phase vectors P 1 -P 4  are also conveyed from PLL  310  to a pair of additional phase interpolators, illustrated as a single block  356 . These phase interpolators combine phase vectors from VCO  350  to produce a receive edge clock RXEClk and a receive data clock RXDClk. A frequency divider  357  then conveys clock signals TX, RXE, and RXD unaltered if a first rate-select signal RateSel 2  is asserted (a logic one) and divides each of clock signals TX, RXE, and RXD by two if select signal RateSel 2  is deasserted (a logic zero). In either case, divider  357  produces transmit clock signal TXClk and the receive edge and data clock signals RXEClk and RXDClk, respectively. Clock buffers  358  and  360  then buffer the respective transmit and receive clock signals and pass them to respective transmit and receive circuitry (see  FIG. 4 ). Clock architectures in other embodiments generate different types of clock signals to support different types of synchronous circuits. Different embodiments may be used whenever high quality clock signals are required, particularly where the required clock signals may operate over a range of frequencies and are derived from a relatively low-frequency reference clock signal.  
      The case in which the clock signals are divided by two is discussed below: assume for the moment that divider  357  merely passes the input clock signals. Transmit clock TX is conveyed to a divider  362  that selectively divides the transmit clock frequency by four or five, depending upon the level of select signal RateSel 1 . Transmit clock TX and receive clocks RXE and RXD are conveyed to divider  357 , which selectively divides those signals by one or two, depending upon the level of select signal RateSel 2 . The following Table 4 shows the combinations of signals RateSel 1  and RateSel 2  that produce the three available frequencies for transmit clock signal TXClk and receive clock signals RXEClk and RXDClk.  
                               TABLE 4                       RateSel1   RateSel2   Div2   Div3   TX/RX Clks                                                        0   1   1/2   1/4   1.25   GHz       0   0   1/1   1/4   2.5   GHz       1   0   1/1   1/5   3.125   GHz                  
 
      Setting select signals RateSel 1  and RateSel 2  to zero and one, respectively, causes dividers  357  and  362  to divide their respective input signals by two and four, respectively. PLL  310  thus locks when VCO  350  oscillates at 4×625 MHz, or 2.5 GHz. Divider  357  divides this frequency by two, so the transmit and receive clock signals oscillate at 1.25 GHz. When both select signals RateSel 1  and RateSel 2  are zero, divider  357  passes the signals from interpolators  354  and  356  unaltered (divided by one) and divider  362  divides transmit clock TX by four. PLL  310  thus locks when VCO  350  oscillates at 2.5 GHz. With divider  357  set to divide by one, the transmit and receive clocks TXClk, RXEClk and RXDClk oscillate at 2.5 GHz. Finally, setting select signals RateSel 1  and RateSel 2  to one and zero, respectively, causes divider  362  to divide transmit clock signal TX by five and divider  357  to pass clock signals TX, RXE, and RXD unaltered. PLL  310  thus locks when VCO  350  oscillates at 3.125 GHz. Divider  357  is set to divide by one, so transmit and receive clocks TXClk, RXEClk, and RXDClk oscillate at 3.125 GHz. The clock multiplication circuitry of IC  300  can thus support various clock rates, such as to allow compatibility with different standards or to allow for various operational modes with different power-to-performance tradeoffs.  
      Phase detector  340 , charge pump  346 , and loop filter  348  adjust the frequency of VCO  350  to maintain a fixed phase relationship between the two input signals to phase detector  340 , and consequently the phase relationship between intermediate reference clock IRClk and transmit clock TXClk.  
      VCO  350  is a low-Q oscillator based upon a ring oscillator (not shown). This type of VCO advantageously offers the desired frequency range and multiple output phases. Unfortunately, low-Q oscillators produce relatively high phase noise. PLL  310  is therefore tuned to exhibit a relatively high loop bandwidth to lower the noise contribution of VCO  350 . In this embodiment, PLL  310  exhibits a loop bandwidth of between about 40 and 62.5 MHz. Increasing the loop bandwidth renders PLL  310  more susceptible to input noise (i.e., phase noise on intermediate reference clock signal IRClk). Recall, however, that PLL  305  is adapted to remove the jitter on reference clock RClk, leaving intermediate reference clock IRClk relatively clean. The loop bandwidth of PLL  310  can thus be increased for improved VCO noise immunity. Also important, the loop bandwidth of a PLL is limited to about 10% of the input frequency, so the pre-multiplication of the reference clock signal by PLL  305  to 625 MHz facilitates the increased bandwidth for PLL  310  of up to 62.5 MHz. For a more detailed discussion of the impact of loop bandwidth on VCO noise, see the above-incorporated Razavi reference.  
      In addition to the aforementioned, PLL  305  includes a buffer  331 . Buffer  331  blocks intermediate reference clock IRClk in response to a standby signal Stby that is asserted to save power by disabling the distribution of intermediate clock signal IRClk without disabling processor clock PClk.  
       FIG. 4  depicts an IC  400  in accordance with an embodiment that employs a two-stage clocking architecture of the type described above in connection with  FIGS. 2 and 3 . IC  400  includes a PLL  401 , some core logic  403 , and N+1 transceivers with associated serial link channels. For brevity, the following discussion is limited to just one of a number of transceivers  405  and an associated pair of serial communication channels, an outgoing channel  402  and an incoming channel  404 .  
      PLL  401  is similar or identical to PLL  305  of  FIG. 3 , and generates a clean intermediate reference clock signal IRClk and a processor clock PClk in the manner discussed above. Core logic  403  is e.g. a graphics processor. Transceiver  405  includes a transmit section  406 , a receive section  407 , and a phase-lock loop (PLL)  409  shared by both transmit and receive sections  405  and  407 . PLL  409  approximates PLL  310  of  FIG. 3 , though the boundaries are drawn in  FIG. 4  to exclude the transmit phase interpolator. Each transceiver  405  includes PLL  409  in the depicted embodiment, but one PLL  409  can also be shared among a plurality of transceivers. In the latter case, transmit clock TxClk and phase vectors P 1 -P 4  can be shared, each transceiver deriving edge and data receive clocks from the respective received signal using the phase vectors. In still other embodiments, the phase vectors alone are shared, in which case one or more of the phase vectors can be used to generate the transmit clocks.  
      Receive section  407  is of a well-known type, and is thus not described in detail. In brief, receive section  407  includes a phase detector  425  and a sampler  411 , each of which samples received data from channel  404 . Phase detector  425  provides an output signal to a receiver phase controller  413 , which controls the sample timing of the received signal via a pair of phase interpolators  415  and  416  that derive edge and data clocks EdClk and DaClk, respectively, by combining selected ones of a plurality of differently phased reference clocks P 1 -P 4  from PLL  409 . Sampler  411 , thus properly timed, samples the incoming data and provides the resulting sampled data to a deserializer  422  for conversion to parallel input data RxD 0 , and to phase controller  413 .  
      Transmit section  406  is also of a well-known type, and conventionally includes a resynchronizer  420  that re-times parallel transmit data TxDO timed to a local clock LClk to transmit clock TxClk. The resulting re-timed parallel data TxDr is then fed to a serializer  423 . Serial transmit data TxDs from serializer  423  is then conveyed to a transmitter  426  for transmission over channel  402 . A transmit phase interpolator  430  coupled to the output of PLL  409  is optionally included to match the delay through the PLL feedback loop to the delay through the receive interpolators. In one embodiment, resynchronizer  420  is of a type described in U.S. patent application Ser. No. 10/282,531 entitled “Method and Apparatus for Fail-Safe Resynchronization with Minimum Latency,” which is incorporated herein by reference. An article entitled “Equalization and Clock Recovery for a 2.5-10-Gb/s 2-PAM/4-PAM Backplane Transceiver Cell,” by Jared L. Zerbe, et al. (IEEE JSSC, December 2003) details an example of a transceiver similar to transceiver  405  but employing a different clock architecture.  
      The foregoing embodiments can be adapted for use with other communication schemes.  FIG. 5  depicts an IC  500  in accordance with an embodiment extended to derive 750 MHz, 1.5 GHz, and 3 GHz clock signals for SATA interfaces. At present, SATA data rates are e.g. 1.5 Gbps (SATAI), 3.0 Gbps (SATAII) and 6.0 Gbps (SATAIII). IC  500  is similar to IC  300  of  FIG. 3 , like-labeled elements being the same or similar. Detailed discussions of like-labeled elements are omitted for brevity. IC  500  derives transmit and receive clock signals TXClk, RXEClk, and RXDClk using cascaded PLL stages  505  and  510 . These stages are similar to stages  305  and  310  of IC  300 , but the number of supported clock rates is increased.  
      Referring first to PLL  505 , the fixed-ratio dividers of PLL  305  are replaced with a pair of selectable dividers  515  and  520 . Dividers  515  and  520  can be set to divide by twenty-five and four, respectively, to produce a 625 MHz intermediate clock IRClk in the manner described above; alternatively, dividers  515  and  520  can be set to divide by twenty and five, respectively, to produce a 500 MHz intermediate clock IRClk in support of SATA operational modes. The intermediate-reference frequency can be selected using an internal or external mode-select signal IRM.  
      Turning to PLL  510 , the selectable dividers of PLL  310  are replaced with dividers  525  and  530 , each of which offers the selection of an additional factor. Divider  525  is extended to divide signal TX by six, thus fixing the oscillation frequency of VCO  350  at 3.0 GHz when intermediate frequency IRClk is at 500 HMz (i.e., 6×500 MHz=3 GHz). The divide-by-six setting of divider  525  is used in each SATA mode, and can be selected using the same mode signal used to control the output of PLL  505  to 500 MHz.  
      The transmit and receive clock rates supportive of SATA interfaces are 750 MHz, 1.5 GHz, and 3.0 GHz. Divider  530  selectively divides the 3.0 GHz output from VCO  350  in the SATA modes by four, two, or one to achieve these respective rates for clock signals TXClk, RXEClk, and RXDClk. The following Table 5 shows the combinations of signals IRM, RateSel 1 , and RateSel 2  that produce the six available frequencies for transmit clock signal TXClk and receive clock signals RXEClk and RXDClk in the embodiment of  FIG. 5 .  
                                   TABLE 5                       IRM   RateSel1   RateSel2   Div2   Div3   TX/RX Clks                                                            0   0   1   1/2   1/4   1.25   GHz       0   0   0   1/1   1/4   2.5   GHz       0   1   0   1/1   1/5   3.125   GHz       1   1   1   1/4   1/6   750   MHz       1   0   1   1/2   1/6   1.5   GHz       1   0   0   1/1   1/6   3.0   GHz                  
 
 Thus, depending on the selected mode, IC  500  can develop six different transmit and receive clock frequencies in support of six different communication schemes. SATA communication schemes can employ rising and falling clock edges (DDR), so the 750 MHz, 1.5 GHz, and 3.0 GHz clock signals can support 1.5 Gbs, 3.0 Gbs, and 6.0 Gbs SATA data rates, respectively. 
 
      An output of the design process for an integrated circuit, or a portion of an integrated circuit, may be a computer-readable medium (e.g., a magnetic tape or an optical or magnetic disk) encoded with data structures or other information defining circuitry that may be physically instantiated as an integrated circuit or portion of an integrated circuit. These data structures are commonly written in Caltech Intermediate Format (CIF) or GDSII, a proprietary binary format. Those of skill in the art of mask preparation can develop such data structures from schematic diagrams of the type detailed above.  
      In the foregoing description and in the accompanying drawings, specific terminology and drawing symbols are set forth to provide a thorough understanding of the present invention. In some instances, the terminology and symbols may imply specific details that are not required to practice the invention. For example, the interconnection between circuit elements or circuit blocks may be shown or described as multi-conductor or single conductor signal lines. Each of the multi-conductor signal lines may alternatively be single-conductor signal lines, and each of the single-conductor signal lines may alternatively be multi-conductor signal lines. Signals and signaling paths shown or described as being single-ended may also be differential, and vice-versa. Similarly, signals described or depicted as having active-high or active-low logic levels may have opposite logic levels in alternative embodiments. As another example, circuits described or depicted as including metal oxide semiconductor (MOS) transistors may alternatively be implemented using bipolar technology or any other technology in which a signal-controlled current flow may be achieved. With respect to terminology, a signal is said to be “asserted” when the signal is driven to a low or high logic state (or charged to a high logic state or discharged to a low logic state) to indicate a particular condition. Conversely, a signal is said to be “deasserted” to indicate that the signal is driven (or charged or discharged) to a state other than the asserted state (including a high or low logic state, or the floating state that may occur when the signal driving circuit is transitioned to a high impedance condition, such as an open drain or open collector condition). A signal driving circuit is said to “output” a signal to a signal receiving circuit when the signal driving circuit asserts (or deasserts, if explicitly stated or indicated by context) the signal on a signal line coupled between the signal driving and signal receiving circuits. A signal line is said to be “activated” when a signal is asserted on the signal line, and “deactivated” when the signal is deasserted. In any case, whether a given signal is an active low or an active high will be evident to those of skill in the art.  
      While the present invention has been described in connection with specific embodiments, variations of these embodiments will be obvious to those of ordinary skill in the art. For example: 
          1. one or more of the PLLs can be replaced with a delay-locked loop (DLL) or a multiplying delay-locked loop (M-DLL);     2. one or more VCOs in systems in accordance with the foregoing embodiments can be replaced with controllable delay lines;     3. current-controlled elements can be used in place of one or more of the voltage-controlled elements;     4. the VCOs can be provided with voltage regulators to provide an extra measure of isolation between the supply noise and VCO output jitter; and     5. some systems that support both DDR and SDR may simply ignore one type of edge (rising or falling) in SDR mode to achieve a half data-rate mode compared to the DDR mode, and additional edges can be ignored to support even lower fractional data rates. 
 
 Moreover, some components are shown directly connected to one another while others are shown connected via intermediate components. In each instance the method of interconnection, or “coupling,” establishes some desired electrical communication between two or more circuit nodes, or terminals. Such coupling may often be accomplished using a number of circuit configurations, as will be understood by those of skill in the art. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description. Only those claims specifically reciting “means for” or “step for” should be construed in the manner required under the sixth paragraph of 35 U.S.C. Section 112.