Abstract:
A signaling system is described. The signaling system comprises a transmit device, a receive device including a partial response receive circuit, and a signaling path coupling the transmit device and the receive device. The receive device observes an equalized signal from the signaling path, and includes circuitry to use feedback from the most recent previously resolved symbol to sample a currently incoming symbol. The transmit device equalizes transmit data to transmit the equalized signal, by applying weighting based on one or more data values not associated with the most recent previously resolved symbol value.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 14/683,081, filed Apr. 9, 2015, which is a continuation of U.S. patent application Ser. No. 13/460,732, filed Apr. 30, 2012, now U.S. Pat. No. 9,025,678, which is a continuation of U.S. patent application Ser. No. 12/710,615, filed Feb. 23, 2010, issuing on May 1, 2012 as U.S. Pat. No. 8,170,163, which is a continuation of U.S. patent application Ser. No. 10/966,070, filed Oct. 18, 2004, now U.S. Pat. No. 7,715,501, which is a continuation of U.S. patent application Ser. No. 10/662,872 filed Sep. 16, 2003, now U.S. Pat. No. 7,397,848, which claims priority from U.S. Provisional Application No. 60/461,729 filed Apr. 9, 2003, all of which are hereby incorporated by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    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 
       [0003]    Electrical pulses transmitted on a band-limited signaling path disperse in time as they travel from source to destination. In systems in which data is transmitted as a sequence of level-encoded electrical pulses, such time-domain dispersion results in a blending of neighboring pulses; an effect known as dispersion-type inter-symbol interference (ISI). Dispersion-type ISI becomes more pronounced at faster signaling rates, ultimately degrading the signal quality to the point at which distinctions between originally transmitted signal levels may be lost. 
         [0004]      FIG. 1  illustrates a prior-art signaling system having an equalizing output driver  109  and an equalizing receiver  103  to mitigate dispersion-type ISI resulting from signal transmission on a signaling path  102 . The receiver  103  includes a sampling circuit  105  to generate digitized samples  106  of the incoming signal, a shift register  107  to store some number (N) of the most recently received samples, and an equalizer  112  to generate an equalization signal  114  based on samples stored in the shift register  107 . Ideally, the equalization signal  114  represents the residual signal level on path  102  of the N prior received samples in the incoming signal so that, by subtracting the equalization signal  114  from the incoming signal in difference circuit  115 , the dispersion-type ISI resulting from the prior transmissions is canceled. Because the prior decisions of the sampling circuit  105  are fed back to the sampling circuit input in the form of the equalization signal  114 , the receiver  103  is commonly referred to as a decision feedback equalizer (DFE). 
         [0005]    One major limitation of the DFE  103  is that the time delay in the overall feedback path from sampling circuit  105  to difference circuit  115  makes it difficult to generate the equalization feedback signal  114  in time to equalize the signal level of the immediately following data value if the least latent sample (i.e., the most recently captured sample  106 ) is included in the equalization feedback signal  114 . Including the least latent sample in the equalization signal is particularly challenging in modern high-speed signaling systems in which incoming symbols are present on the signal path  102  for extremely brief intervals (e.g., less than a nanosecond for signal rates above one Gigabit per second). One solution to the least-latent sample problem is to omit one or more of the least-latent samples from contributing to generation of the decision-feedback equalization signal. Unfortunately, the least latent sample, being nearest in time to the incoming symbol, tends to be the largest contributor to dispersion-type ISI and therefore a primary objective of cancellation by the DFE. Consequently, in signaling systems in which the least-latent sample is omitted from contribution to decision-feedback equalization, transmit-side pre-emphasis is often used to decrease the dispersion-type ISI caused by the least-latent symbol. That is, when a given symbol is transmitted by the equalizing output driver  109 , one or more previously transmitted symbols stored in shift register  113  (i.e., the least latent symbols relative to the outgoing symbol) are used to pre-shape the outgoing waveform to reduce the dispersion-type ISI observed at the receiver. Unfortunately, as can be seen in the raw and equalized pulse responses depicted in  FIG. 2 , forcing the least-latent sample, D N−1 , to zero (or near zero) results in significant attenuation of the overall signal level, thereby reducing signaling margins and ultimately limiting the data rate of the signaling system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    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: 
           [0007]      FIG. 1  illustrates a prior-art signaling system; 
           [0008]      FIG. 2  illustrates raw and equalized pulse responses observed in the prior art signaling system of  FIG. 1 ; 
           [0009]      FIG. 3  illustrates a signaling system that includes a partial response receiver according to an embodiment of the invention; 
           [0010]      FIG. 4  illustrates a relationship between clock and data signals in one embodiment of the signaling system of  FIG. 3 ; 
           [0011]      FIG. 5  illustrates the bimodal distribution of a binary signal observed at a signal receiver when the primary source of dispersion-type ISI is the signal transmitted in the immediately preceding symbol time; 
           [0012]      FIG. 6  illustrates the four partial response signal levels depicted in  FIG. 5  relative to a common mode level, L CM ; 
           [0013]      FIG. 7  illustrates a partial response receive circuit according to an embodiment of the invention; 
           [0014]      FIG. 8  illustrates a double data rate signaling protocol in which two symbols are transmitted in succession during each cycle of a sampling clock signal; 
           [0015]      FIG. 9  illustrates a partial response receive circuit for use in a double data rate signaling system; 
           [0016]      FIG. 10  illustrates even and odd pipelines of sample values generated within the partial response receive circuit of  FIG. 9 ; 
           [0017]      FIG. 11  illustrates the use of embedded scoping to generate a time-based trace of an incoming data signal; 
           [0018]      FIG. 12  illustrates a signaling system according to an embodiment of the invention; 
           [0019]      FIG. 13  illustrates a waveform trace of a pulse response captured by an embedded scope within the signaling system of  FIG. 12 ; 
           [0020]      FIG. 14  illustrates a partial response receiver according to an embodiment of the invention that adaptively generates partial response threshold levels; 
           [0021]      FIG. 15  illustrates a embodiment of the adaptive module of  FIG. 14 ; 
           [0022]      FIG. 16  illustrates an alternative circuit arrangement that may be used in place of the averaging circuits of  FIG. 15 ; 
           [0023]      FIG. 17  illustrates a comparator that may be used within the receiver circuits and level sampling circuits of  FIGS. 7, 9 and 14 ; 
           [0024]      FIG. 18  illustrates an embodiment of the current DAC of  FIG. 17 ; 
           [0025]      FIG. 19  illustrates an alternative comparator embodiment that may be used within the receiver circuits and level sampling circuits of  FIGS. 7, 9 and 14 ; 
           [0026]      FIG. 20  illustrates a partial response receiver according to another alternative embodiment of the invention; 
           [0027]      FIG. 21  illustrates the operation of the adaptive module of  FIG. 20  according an embodiment of the invention; 
           [0028]      FIG. 22  illustrates an embodiment of a dual mode receiver that may be operated in either a multi-level signaling mode or a partial response mode; 
           [0029]      FIG. 23  illustrates a multi-level signaling protocol used within dual mode receiver of  FIG. 22 ; 
           [0030]      FIG. 24  illustrates a partial response receiver having a clock data recovery function according to an embodiment of the invention; 
           [0031]      FIG. 25  illustrates a number of possible data signal transitions when the incoming data signal has the bimodal distribution shown in  FIG. 4 ; 
           [0032]      FIG. 26  illustrates an embodiment of a clock recovery circuit that adjusts the phase of edge clock signal and sampling clock signal based on selected transitions detected in the incoming data signal; 
           [0033]      FIG. 27  illustrates a potential source of non-convergence in a system that adaptively generates partial response threshold levels; 
           [0034]      FIG. 28  illustrates an embodiment of a dual-mode, partial response receiver with clock data recovery; 
           [0035]      FIG. 29  illustrates signal transitions between successive 4-PAM (Pulse Amplitude Modulation) symbols; 
           [0036]      FIG. 30  illustrates data signal transitions that may be used for clock recovery when the dual mode receiver of  FIG. 28  is operated in a partial response mode; 
           [0037]      FIG. 31  illustrates a transition detect circuit and sample select circuit that may be used within the clock recovery circuit of  FIG. 28 ; 
           [0038]      FIG. 32  illustrates additional data signal transitions that may be used for clock recovery when the dual mode receiver of  FIG. 28  is operated in a partial response mode; 
           [0039]      FIG. 33  illustrates a threshold select circuit according to an embodiment of the invention; 
           [0040]      FIG. 34  illustrates a distribution  770  of signal levels in a system in which the two most recently received symbols are the primary source of residual channel response; 
           [0041]      FIG. 35  illustrates a partial response receiver according to an embodiment of the invention that operates in accordance with the partial response states shown in  FIG. 34 ; 
           [0042]      FIG. 36  illustrates a partial response receiver according to another embodiment of the invention; 
           [0043]      FIG. 37  illustrates a constellation of possible signal levels in a 4-PAM signaling system; 
           [0044]      FIG. 38  illustrates a 4-PAM partial response receiver according to an embodiment of the invention; 
           [0045]      FIG. 39  illustrates an input circuit that includes multiple partial response receivers according to an embodiment of the invention; 
           [0046]      FIG. 40  illustrates an input circuit according to an alternative embodiment of the invention; 
           [0047]      FIG. 41  illustrates a partial-response signaling system according to an embodiment of the invention; 
           [0048]      FIG. 42  illustrates a raw pulse response and a pulse response equalized by the signaling system of  FIG. 41  to enable partial-response data reception; and 
           [0049]      FIG. 43  illustrates residual inter-symbol interference that may be equalized by transmit preemphasis within the signaling system of  FIG. 41 . 
       
    
    
     DETAILED DESCRIPTION 
       [0050]    In the following 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. Additionally, the prefix symbol “l” attached to signal names indicates that the signal is an active low signal (i.e., the asserted state is a logic low state). A line over a signal name (e.g., ‘ &lt;signal name&gt; ’) is also used to indicate an active low signal. 
         [0051]    A novel signal receiving circuit for use in electrical signaling systems is disclosed in various embodiments. In one embodiment, the signal receiving circuit includes a pair of comparators for comparing an incoming electrical signal against respective threshold levels, with each threshold level being set according to the level of a dispersed component of a previously received signal. More specifically, one of the threshold levels corresponds to the dispersion-type ISI introduced by a previously transmitted signal representative of a first digital state, and the other of the threshold levels corresponds to the dispersion-type ISI introduced when the previously transmitted signal is representative of a second digital state. In the case of a binary signaling system, the previously transmitted signal is resolved into one of two states by the receiving circuit, then used to select which of the comparators is to source the sample to be used for selection of the subsequent pair of samples. By this arrangement, each of the comparators compares the incoming signal against a threshold that is offset from a nominal value according to a respective, anticipated level of ISI, with the comparator having its threshold level offset in the direction that corresponds to the state of the previously received signal ultimately being selected as the sample source. Because samples are captured on the assumption of dispersion-type ISI resulting from both possible states of the previously captured sample (i.e., the least-latent sample), both samples are available for later selection when the state of the previously captured sample is resolved. Thus, by establishing separate threshold levels according to the different possible levels of dispersion-type ISI and comparing the incoming signal against each threshold level, the timing problems associated with generating an equalization signal that includes the least latent sample are avoided. Because the threshold level supplied to each of the comparators includes an offset according to the residual, partial response of the signaling path to the previously transmitted signal, a receive circuit having such offset-threshold comparators is referred to herein as a partial response receiver. 
         [0052]    In another embodiment of the invention, a multi-level signal receiver (i.e., receiver capable of receiving a symbol representative of more than a single binary bit), may be switched to a binary signaling, partial response mode. In one implementation, a multi-PAM receiver (Pulse Amplitude Modulation receiver—a type of multi-level signal receiver) includes multiple comparators for distinguishing different possible levels (e.g., 4 levels in a 4-PAM receiver, 8 levels in an 8-PAM receiver, etc.) such that no additional comparators are necessary to support partial response operation. Instead, when the partial response mode is enabled, the threshold levels supplied to the comparators are adjusted to match the partial response levels for the least-latent symbol (or set of N least-latent symbols) and the post processing of the comparator outputs is switched from a symbol decoding operation to a sample selection operation. 
         [0053]    In other embodiments of the invention, methods and circuits for generating partial response threshold levels are disclosed. Also, methods and circuits for transmitting a partial-response data signal that includes a partial response from selected prior-transmitted bits are disclosed. Further, methods and circuits for recovering a clock signal from a partial response data signal are also disclosed. 
         [0054]    Signaling System Overview 
         [0055]      FIG. 3  illustrates a signaling system  117  in which embodiments of the present invention may be used. The system  117  includes an equalizing transmitter  118  and equalizing receiver  116  coupled to one another via a high-speed signal path  122 , and a controller  141  coupled to the transmitter  118  and the receiver  116  via relatively low-speed signal paths  142 A and  142 B, respectively. In one embodiment, the signal path  122  is formed by component signal paths  122 A,  122 B and  122 C (e.g., transmission lines that introduce respective, nonzero propagation delays and exhibit respective impedance characteristics), each disposed on respective circuit boards that are coupled to one another via circuit board interfaces  125  and  127  (e.g., connectors). In a specific implementation, signal path  122 B is formed on a backplane and signal paths  122 A and  122 C are formed on respective daughterboards (e.g., line cards) that are removably coupled to the backplane via circuit board interfaces  125  and  127 . The transmitter  118  and receiver  116  are implemented in respective integrated circuit (IC) devices that are mounted on the daughterboards. The controller, which may be a general or special purpose processor, state machine or other logic circuit, is implemented within a third integrated circuit device mounted to a yet another circuit board. In the embodiment of  FIG. 3 , signal paths  142 A and  142 B are used to convey configuration information from the controller  141  to the transmitter  118  and receiver  116 , respectively, and may be disposed on the same circuit board (or circuit boards) as signal path  122  or implemented by an alternative structure such as a cable. The controller may alternatively be coupled to the transmitter  118  and receiver  116  by a shared signal path such as a multi-drop bus. The operation of the controller  141  is discussed in greater detail below. In alternative embodiments, the IC devices containing the transmitter  118 , receiver  116  and controller  141  may be mounted to a common structure with the signaling paths  122 ,  142 A and  142 B coupled directly to the IC devices (e.g., all three ICs mounted to a circuit board and coupled to one another via circuit board traces, or all three ICs packaged within a single multi-chip module with signal paths  122  and  142  formed between the ICs by bond wires or other conducting structures). Also, the transmitter  118 , receiver  116  and controller  141 , or any subset thereof, may be included within the same IC device (e.g., system on chip) and the signaling paths  122  and/or  142  implemented by a metal layer or other conducting structure within the IC device. 
         [0056]    The equalizing transmitter  118  transmits data on the signaling path  122  during successive time intervals, referred to herein as symbol times. In one embodiment, illustrated by the timing diagram of  FIG. 4 , each symbol time, T S , corresponds to a half cycle of a transmit clock signal, TCLK, such that two data values (e.g., values A and B) are transmitted on signaling path  122  per transmit clock cycle. The transmitted data signal arrives at the input of the equalizing receiver  116  after propagation time, T P , and is sampled by the receiver  116  in response to edges of a receive clock signal, RCLK. The receive clock signal may be received within the receive circuit via an external clock line, or may be a recovered version of a reference clock signal (e.g., recovered by a delay-locked loop or phase locked loop circuit). In other embodiments, discussed below, the receive clock signal may be recovered from the transmitted data signal. Still referring to  FIG. 4 , the receive clock signal has a quadrature phase relation to data valid windows (i.e., data eyes) in the incoming data signal such that each sample is captured at the midpoint of a data eye. In alternative embodiments, the sampling instant may be skewed relative to data eye midpoints as necessary to satisfy signal setup and hold time requirements in the receiver  116 . Also, more or fewer symbols may be transmitted per cycle of the transmit clock signal. 
         [0057]    The equalizing transmitter  118  includes a transmit shift register  124 , output driver  121  and transmit equalizer  129 ; the transmit equalizer  129  itself including a shift register  120  and a bank of output drivers  131 . At the start of each symbol time, the data value at the head (i.e., output) of the transmit shift register  124 , referred to herein as the primary data value, is driven onto the signal path  122  by the output driver  121 , and the transmit equalizer  129  simultaneously drives an equalizing signal onto the signal path  122 . This type of equalization is referred to herein as transmit preemphasis. In one embodiment, the signal driven onto the signal path  122  by the output driver  121  (referred to herein as the primary signal) is a multi-level signal having one of four possible states (e.g., defined by four distinct signal ranges) and therefore constitutes a symbol representative of two binary bits of information. In alternative embodiments, the primary signal may have more or fewer possible states and therefore represent more or fewer than two binary bits. Also, the primary signal may be single-ended or differential (an additional signal line is provided to carry the complement signal in the differential case), and may be a voltage or current mode signal. 
         [0058]    Each of the output drivers  131  within the transmit equalizer  129  form either a pre-tap driver or post-tap driver according to whether the source data value has already been transmitted (post-tap data) or is yet to be transmitted (pre-tap data). In the specific embodiment of  FIG. 3 , the equalizer includes N post-tap drivers sourced by data values within the shift register  120  and one pre-tap driver sourced by a data value within the transmit shift register  124 . Accordingly, the resultant equalizing signal driven onto the data path  122  will have a signal level according to data values having symbol latencies of −1, 1, 2, . . . , N, where the symbol latency of a given data value refers to the number of symbol times by which transmission of the data value precedes the transmission of the primary value. Different numbers of post-tap and pre-tap drivers may be provided in alternative embodiments, thereby allowing for equalization based on values having different symbol latencies. 
         [0059]    Still referring to  FIG. 3 , the equalizing receiver  116  includes a partial-response receive circuit  123 , buffer  132  (e.g., shift register), tap select circuit  128  and tap select logic  139 . Data signals are sampled by the partial response receive circuit  123  to generate digitized samples that are stored in the buffer  132  for eventual use by application logic (not shown). Because the buffered data is stored for at least a predetermined time and represents historical data up to a predetermined number of symbol latencies, the buffered data forms a source of post-tap data values that may be selected by the tap select circuit  128  to source equalizer taps in a receive-side equalizer circuit. Because a subset of buffered data values may be selected according to the precise symbol latencies of reflections and other high-latency distortions, a relatively small number of buffered data values may be selected to form receive-side equalization taps having latencies that match the latencies of the distortions. By this arrangement, high latency distortions may be reduced by receive-side equalization without dramatically increasing the parasitic capacitance of the receiver (i.e., as would result from a large number of receive-side equalization taps). In one embodiment, the tap select logic  139  is a configuration circuit that outputs a tap select signal  134  according to a configuration value. The configuration value may be automatically generated by system  117  (e.g., at system startup) or may be empirically determined and stored within the configuration circuit or elsewhere within system  117 . 
         [0060]    In the embodiment of  FIG. 3 , buffer  132  is formed by a shift register having a dead range component  133  having M storage elements and a selectable-range component  135  having R storage elements, the tap select circuit  128  being coupled to the selectable-range component  135  to select the subset of tap data sources therefrom. In alternative embodiments, the dead range component of the buffer  132  may include fewer than M storage elements or even zero storage elements, depending on the time required to receive data and transfer data into the buffer  132 . Also, the tap select circuit  128  may be coupled to one or more storage elements within the dead range component  133  to enable the size of the dead range to be programmed according to the configuration of the transmit circuit  118 . Finally, as discussed below, the buffer  132  may include one or more parallel registers in addition to (or instead of) the shift register formed by components  133  and  135 . 
         [0061]    Still referring to  FIG. 3 , numerous alternative types of equalization circuits may be used within the receiver  116 . For example, in one embodiment, the receiver  116  includes an output driver  140  (illustrated in dashed outline in  FIG. 3  to indicate its optional nature) to drive an equalizing signal onto the signal path  122  (and therefore to the input of the partial response receive circuit  123 ) coincidentally with the symbol time of an incoming signal. In another embodiment, the partial response receive circuit  123  includes a preamplifier having an equalizing subcircuit. 
         [0062]    Multi-Modal Signal Distribution—Partial Response 
         [0063]      FIG. 5  illustrates the bimodal distribution of a binary signal observed at a signal receiver when the primary source of dispersion-type ISI is the signal transmitted in the immediately preceding symbol time. That is, the symbol sampled at time T N−1  is the primary source of dispersion-type ISI in the symbol sampled at time T N ; the symbol sampled at time T N  is the primary source of dispersion-type ISI in the symbol sampled at time T N+1 ; and so forth. Referring to the signal levels at time T N , it can be seen that if the if the preceding symbol was a logic ‘1’, the partial response to the preceding symbol will raise the signal level at time T N  to one of the circled levels ‘11’ or ‘01’ (the second bit of the ‘11’ and ‘01’ bit sequences corresponding to the state of the signal at time T N−1 ). If the preceding symbol was a logic ‘0’, the partial response to the preceding symbol will lower the signal level at time T N  to one of the square-designated levels ‘10’ or ‘00’. In a single-ended signaling system, the incoming signal is sampled and compared with a reference voltage level at the midpoint between the steady state extremes (i.e., midway between the ‘11 . . . 1’ and ‘00 . . . 0’ signal levels). Accordingly, the worst case voltage margins (i.e., smallest voltage differences between the signal and reference voltage) occur when the signal transitions from a ‘1’ to a ‘0’ or vice-versa; the ‘10’ and ‘01’ signaling levels becoming indistinguishable when the partial response level (i.e., residual signal level from the preceding symbol transmission) is large. In a differential signaling system, the incoming signal and its complement are sampled, for example, by a differential amplifier which amplifies the difference between the complementary signal levels. As in the single-ended case, the worst case voltage margins occur when the differential signal transitions from a ‘1’ to a ‘0’ or vice-versa; the ‘10’ and ‘01’ signaling levels becoming indistinguishable when the partial response pushes the ‘10’ and ‘01’ levels to the common mode (i.e., the average of a differential signal pair). 
         [0064]      FIG. 6  illustrates the four partial response signal levels depicted in  FIG. 5  relative to a common mode level, L CM . In the case of a differential signaling system, the partial response to the preceding symbol may be viewed as increasing or decreasing the differential amplitude (i.e., the amplitude difference between the signals that form the differential signal pair) relative to a nominal differential amplitude. Normalizing the positive and negative signal levels for the nominal differential amplitude to 1 and −1, respectively, the common mode level, L CM , becomes zero, and the four possible signal levels become 1+α, 1−α, −1+α and −1−α, where α represents the magnitude of the partial response to the preceding symbol. Thus, when the preceding symbol, D N−1 , is a ‘0’, the incoming symbol, D N , is represented by a signal level at either 1−α or −1−α, depending on whether D N  is a ‘1’ or ‘0’ Similarly, when D N−1 =1, the incoming symbol is represented by a signal level at either 1+α or −1+α according to the state of the current symbol. In the former case (i.e., when D N−1 =0), the two complementary signal levels, −1−α and 1−α, have a common mode of −α, as shown in box  151 . In the latter case (i.e., when D N−1 =1), the two complementary signal levels, 1+α and −1+α, have a common mode of +α, as shown in box  153 . 
         [0065]    Partial Response Receiver 
         [0066]      FIG. 7  illustrates a partial response receive circuit  200  according to an embodiment of the invention that exploits the bi-modal characteristic of the signal levels depicted in  FIGS. 5 and 6 . The partial response circuitry includes a compare circuit  202  to sample the input data signal, D N , and a decision circuit  204  to generate an output data value (i.e., received data value) based on data samples generated by the compare circuit  202 . Rather than a single comparator that distinguishes between binary signaling levels based on whether the sampled signal is above or below a common mode threshold, the compare circuit  202  includes two comparators  201  and  203  having threshold levels that are offset from the common mode threshold by the two possible partial responses to the preceding symbol. That is, the threshold level of the comparator  201  is set the +α level, and the threshold level of the comparator  203  is set to the −α level. By this arrangement, if the preceding symbol was a ‘1’ (i.e., D N−1 =1), the comparator  201  will resolve the incoming signal as being a ‘1’ or ‘0’ by determining whether the signal level is above or below the partial response level, +α. Conversely, if D N−1 =0, the comparator  203  will resolve the incoming signal as being a ‘1’ or ‘0’ by determining whether the signal is above or below the partial response level, −α. Because both comparisons are performed for each incoming symbol, the selection of which comparator output represents the state of the symbol may be delayed until the state of the preceding symbol is resolved. In the partial response receive circuit  200 , for example, the sample values output by the comparators  201  and  203  are output to the decision circuit  204  where they are optionally stored in storage elements  208  and  209  (e.g., D flip-flops or other types of storage circuits) and provided to respective input ports of a select circuit  205  (e.g., a multiplexer). The sample value selected by the select circuit  205  is stored in a storage circuit  207  in response to a sampling clock signal  210  (or other timing control signal) at which point the sample value becomes the D N−1  sample value. The D N−1  sample value stored within storage circuit  207  is fed back to the select input of the select circuit  205  to select one of the two sample values generated by the comparators  201  and  203 . That is, the D N−1  sample value is used to select, via select circuit  205 , which of the comparators  201  and  203  will source the D N  sample. In an embodiment that includes the storage elements  208  and  209 , the output of storage circuit  207  becomes the D N−2  sample value and is used to select one of the D N−1  sample values output from the storage elements  208  and  209 . 
         [0067]    Reflecting on the operation of the partial response receive circuit  200  of  FIG. 7  in reference to the signal distribution shown in  FIG. 6 , it can be seen that the signaling margin (i.e., difference between signal levels representative of ‘1’ and ‘0’ states) within either of the comparators  201 ,  203  exceeds the signaling margin between the worst-case partial response states. That is, the signaling margin between the worst-case partial response states (10-to-01) is (1−α)−(−1+α)=2−2α, while the signaling margin in either pair of the separated signal constellation is (1−α)−(−1−α)=2. Thus, as alpha grows, the signaling margin in a single-comparator receiver decreases, going to zero as α approaches 1. By contrast, the signaling margin in the partial response receive circuit  200  of  FIG. 7  remains substantially constant at 2, even as α grows. In effect, by offsetting the threshold of the comparators  201  and  203  by the partial response levels +α and −α, respectively, the partial response to the preceding symbol is canceled, making the full response of the incoming symbol available to resolve the symbol state. 
         [0068]    Still referring to  FIG. 7 , a threshold generator  211  is used to generate the ±α thresholds supplied to the comparators  201  and  203 . In the case of a single-ended signaling system, the threshold values ±α may be voltage levels applied to reference inputs of the comparators  201  and  203 , respectively, to enable pseudo-differential signal detection (i.e., comparison of the incoming signal level with the threshold voltage level to resolve the digital state of the sample value). In the case of a differential signaling system, the threshold values ±α may be voltages or currents applied to offset the common modes of the comparators to the ±α voltage levels depicted in  FIG. 6 . The threshold values ±α may also be digital values for controlling digital-to-analog converters (DACs) within the comparators  201  and  203 . In one embodiment, the threshold generator  211  includes circuitry for a one-time or periodic determination of the ±α threshold levels, for example in one or more calibration operations. In other embodiments, the ±α threshold levels are adaptively generated based on incoming signal levels produced by selected data patterns. Embodiments of the threshold generator  211  are discussed in further detail below. 
         [0069]    Multi-Data-Rate Signaling 
         [0070]    In the partial response receive circuit  200  of  FIG. 7 , a single symbol is captured during each cycle of the sampling clock signal  210 . That is, a rising (or falling) edge of the sample clock is used to capture samples of the incoming data signal. In a multi-data rate system, multiple symbols are captured per cycle of the sampling clock signal  210 . In such systems, clock generation circuitry is provided to generate multiple instances of the sampling clock signal  210  that are phase-distributed through a period (1/frequency) of the sampling clock signal.  FIG. 8 , for example, depicts a double data rate signaling protocol in which two symbols are transmitted in succession during each period (i.e., cycle time) of a sampling clock signal. Accordingly, two instances of the sampling clock signal are provided: an even-phase sampling clock signal, SCLK E , to sample even-numbered symbols D N , D N+2 , D N+4  . . . ; and an odd-phase sampling clock signal, SCLK O , to sample odd-numbered symbols D N−1 , D N+1 , D N+3  . . . . This technique may be extended to achieve virtually any data rate, including quad data rate (4 symbols per sampling clock cycle), octal data rate (8 symbols per sampling clock cycle), decade data rate (10 symbols per sampling clock cycle), and so forth. 
         [0071]      FIG. 9  illustrates a partial response receive circuit  215  for use in a double data rate signaling system. The partial response receive circuit  215  includes an odd-phase receive circuit  216  clocked by an odd-phase sampling clock signal  210   O  (SCLK O ), and an even-phase receive circuit  217  clocked by an even-phase sampling clock signal  210   E  (SCLK E ). The odd- and even-phase receive circuits  216 ,  217  are similar to the partial response receive circuit  200  of  FIG. 7 , except that, due to the interleaved nature of the samples they generate, the select circuit  225  of the odd-phase receive circuit  216  is controlled by a latched instance of a sample selected by the select circuit  239  in the even-phase receive circuit  217  and, conversely, the select circuit  239  of the even-phase receive circuit  217  is controlled by a latched instance of a sample selected by the select circuit  225  in the odd-phase receive circuit  216 . Samples D N   + /D N   −  are generated by comparators  232  and  233 , stored in storage circuits  235  and  237 , and then selected by select circuit  239  to form the even-phase pipeline (EVEN PIPE) illustrated in  FIG. 10  (the output of the select circuit  239  optionally being buffered in storage circuit  243  in response to the even-phase sampling clock signal  210   E ). Similarly, samples D N+1   + /D N+1   −  are generated by comparators  218  and  219 , stored in storage circuits  221  and  223 , then selected by the select circuit  225  form the odd-phase pipeline (ODD PIPE) illustrated in  FIG. 10  (the output of the select circuit  225  optionally being buffered in storage circuit  227  in response to the odd-phase sampling clock signal  210   O ). Thus, when an odd phase sample D N−1  is selected by select circuit  225 , the D N−1  sample is latched within latch element  241  of the even-phase receive circuit (thereby making D N−1  available for a full cycle of the even-phase clock signal  210   E ) and thereafter used to select the subsequent even-phase sample D N . The selected even phase sample D N  is then latched within latch element  228  of the odd-phase receiver and thereafter used to select the subsequent odd-phase sample, D N+1 . For higher data rates, the number of partial-cycle receive circuits (circuits  216  and  217  each being a half-cycle receive circuit) may be increased according to the data rate. For example, in a quad data rate system, a partial-response receiver includes four quarter-cycle receive circuits interconnected such that sample N within a first quarter-phase receive circuit is used to select sample N+1 within a second quarter-phase receive circuit; sample N+1 within the second quarter-phase receive circuit is used to select sample N+2 in a third quarter-phase receive circuit; sample N+2 is used to select sample N+3 in a fourth quarter-phase receive circuit; sample N+3 is used to select sample N+4 in the first quarter phase receiver; and so forth. In the remainder of this description, various partial response receive circuit embodiments are described in the context of a single data rate (SDR) signaling system. Each of the embodiments disclosed may be modified as described in reference to  FIGS. 8-10  to support multi-data rate signaling. 
         [0072]    Threshold Level Generation 
         [0073]    In one embodiment of the invention, a technique referred to herein as embedded scoping is used to determine the ±α threshold levels applied within the comparators of the partial response receive circuit. Embedded scoping involves iteratively receiving a sequence of symbols in a receiver and comparing the received symbol sequence with a local generation of the sequence to confirm error-free reception. With each receive-and-confirm iteration, a threshold voltage used to distinguish between symbol values in the incoming signal is offset from a calibrated level by a progressively larger amount until a symbol in the sequence no longer matches the expected value. The threshold voltage offset at which the failure occurs is referred to herein as a pass/fail offset and represents a measure of the signal level at the sampling instant at which the failure occurred. Thus, by sweeping the threshold voltage through a range of threshold voltages until the pass/fail offsets for each symbol in the symbol sequence have been detected, a sample plot for the incoming signal may be developed. Further, by sweeping the receive clock signal through an incremental sequence of phase offsets, and determining the pass/fail offset at each phase offset, a complete trace of the incoming signal may be generated. Also, the granularity and start stop points of the phase offsets and/or threshold voltage steps may be controlled (e.g., by configuring a programmable circuit or register) to enable the waveform trace to be constrained to selected points of interest in the incoming signal (e.g., ±N° from an intended sampling instant, N representing a sweep angle). 
         [0074]      FIG. 11  illustrates the use of embedded scoping to generate a time-based trace  290  of an incoming data signal  286 . The range of threshold voltage offsets over which the incoming signal  286  is sampled is indicated by V T , and the range of phase offsets at which the signal is sampled is indicated by φ. Each sample point within the sweep is indicated by a respective dot within a grid of sample points  280 . Note that the sweep may be obtained by stepping the voltage threshold through the range of V T  values for each value of φ, or, alternatively, by stepping the clock phase through the range of φ values for each value of V T . 
         [0075]    Still referring to  FIG. 11 , reference numeral  288  indicates a pair of samples for which a pass/fail condition is detected. A corresponding pass/fail offset (PFO) is determined according to the difference between the calibrated V T  level (V T (CAL)) and the average of the V T  offsets between the pass and fail samples, and recorded as a measure of the incoming signal. That is, the pass/fail offset may be used to establish a data point within the trace  290  as shown. After sweeping through all the sample points within the grid  280  (which sweep may be repeated numerous times to obtain an average and to discard statistical outliers), a measure of the incoming signal is obtained as illustrated graphically by the trace  290 . 
         [0076]    Embedded scoping has a number of benefits over traditional signal measurement techniques. First, because the technique is non-invasive (i.e., no probe contact), the electrical characteristics of the system under test are unaltered, thereby yielding potentially more accurate results. Also, the trace is generated from the perspective of the receive circuit itself, meaning that any non-ideal characteristics of the receive circuit are accounted for in the resulting signal trace information. Finally, because all components needed for embedded scoping may be included within a finished signaling system, embedded scoping may be used to perform numerous run-time analyses in addition to partial response measurement including, without limitation, determining the latency and amplitude of reflections and other distortions within the signaling system. 
         [0077]      FIG. 12  illustrates a signaling system  300  according to an embodiment of the invention. The signaling system  300  includes a transmit device  301  and receive device  309  that employ embedded scoping to determine partial response amplitudes. The transmit device  301  includes a pattern generator  303 , data selector  305 , equalizing transmitter  307  and application logic  302 . The application logic  302  performs the core function of the transmitting device (e.g., signal processing, instruction processing, routing control, or any other function) and provides transmit data (TX DATA) to a first input of the data selector  305 . During normal operation, the application logic  302  outputs a logic low scope signal  306  (SCOPE) to the data selector  305  to select the transmit data to be passed to the equalizing transmitter  307  for transmission to the receive device  309  via signal path  122  (which may include or be connected to numerous sources of discontinuity such as connectors, vias, stubs, etc.). During a scoping mode of operation, the application logic  302  drives the scope signal  306  high to enable a scoping mode of operation within the transmit device  301 . In the scoping mode, the data selector  305  selects a repeating single-symbol pulse sequence (e.g., a test signal such as: 00100 . . . 00100 . . . 00100 . . . ) generated by the pattern generator  303  to be transmitted to the receive device  309 . The receive device  309  includes a partial response receiver  310  to receive the incoming data signal, a pattern register  311  to store a local version of the single-symbol pulse sequence, a multiplexer  312  to enable the pattern register  311  to be switched between load and barrel-shifting modes, a XOR gate  313  to compare the received data sequence with the locally generated sequence, and application logic  315  (or other logic) to generate a threshold voltage adjust signal (THRESH ADJ) to step the threshold voltage used within the partial response receive circuit through their scoping ranges. In on embodiment, the thresholds applied to the multiple comparators of the partial response receive circuit are set to the same nominal starting value and stepped together for purposes of embedded scoping. In an alternative embodiment, only one comparator of the partial response receive circuit is used when scoping mode is enabled. The application logic may additionally generate a clock adjust signal (not shown) to step the sampling clock through a sequence of phase offsets within a cycle of the sampling clock signal. The application logic  315  additionally builds a trace record (i.e., data indicative of the incoming data sequence) based on the output of XOR gate  313 . 
         [0078]    When the receive device  309  is in a scoping mode of operation, the multiplexer  312  is initially set to load the pattern register  311  with the output of the partial response receiver  310 . After a desired sequence of data (e.g., the single-symbol pulse sequence) is shifted into the pattern register  311 , the multiplexer  312  is set to enable the barrel-shifting mode of the pattern register  311 . That is, the multiplexer  312  selects the output of the pattern register  311  to be fed back to the input of the pattern register  311  so that the contents of the pattern register  311  are continuously rotated through the pattern register  311  (i.e., a barrel shifting operation). By this arrangement, the data sequence loaded into the pattern register  311  is repeatedly output, bit by bit, to a first input of the XOR gate  313 . The data sequence received by the partial response receiver  310  is input to a second input of the XOR gate  313  so that the received data sequence is compared, bit by bit, with the data sequence stored within the pattern register  311 . By selecting the length of the repeatedly transmitted data sequence to match the storage size of the pattern register  311 , the pattern register contents are repeatedly compared with a newly received version of the same data sequence (i.e., putatively the same data sequence). Any reception error will result in a mismatch between the received value and the corresponding value within the pattern register and therefore, when compared by XOR gate  313 , will result in an error signal being output from the XOR gate  313  to the application logic  315 . The application logic  315  may then record the adjusted threshold voltage (and optionally the clock phase offset) at which the error occurred as a signal level within the waveform trace. 
         [0079]      FIG. 13  illustrates a sample waveform trace  320  of a pulse response captured by an embedded scope within the signaling system of  FIG. 12 . As shown, the waveform starts and ends at a steady-state low level which corresponds to the −1−α level discussed in reference to  FIGS. 5 and 6 . A pulse (i.e., D=1) is received at time T 3 , and the partial response of the pulse is received at time T 4 . Due to the preceding zero-valued symbol, the signal level sampled at time T 3  corresponds to the 1−α level. Similarly, due to the preceding one-valued symbol, the signal level sampled at time T 4  corresponds to the −1+α level. The difference between the −1+α level and the −1−α level may be determined by the application logic  315  of  FIG. 12  (or other circuitry) and used to determine ±α. That is, α=((−1+α)−(−1−α))/2. The normalized signal level, 1, may be used in certain clock recovery operations (discussed below) and may be determined from the pulse level and the steady-state low level (i.e., 1=((1−α)−(−1−α))/2). Once determined, the α level may be applied to the comparators of the partial response receive circuit to enable partial response operation. 
         [0080]      FIG. 14  illustrates a partial response receiver  325  that includes circuitry for adaptively generating the partial response threshold levels ±α. The partial response receiver  325  includes a partial response receive circuit  200 , a level sampler  327 , and an adaptive module  329 . The partial response receive circuit  200  operates generally as described above in reference to  FIG. 7  to generate a pair of sample values based on comparisons (in comparators  201  and  203 ) of the incoming signal D N  with offset thresholds ±α. The samples are stored in storage elements  208  and  209  as samples D N−1   +  and D N−1   −  (the ‘+’ and ‘−’ designating the samples as corresponding to relatively positive and negative partial responses). A select circuit  205  selects one of the D N−1   +  and D N−1   −  samples to be the selected D N−1  sample based on the state of the D N−2  sample stored in storage circuit  207 . As discussed above, in a multi-data rate system, the D N−2  sample may be supplied by a counterpart partial response receive circuit clocked by a phase shifted version of the sampling clock signal  210 . In either case, single- or multi-data rate, two or more time-adjacent sample values are supplied to (or buffered within) the adaptive module  329 . In the embodiment of  FIG. 14 , sample values D N−2  and D N−1  are supplied to the adaptive module  329 , though higher or lower latency samples may be provided in alternative embodiments. 
         [0081]    The level sampler  327  includes four comparators  331   1 - 331   4  for comparing the signal level of the incoming symbol, D N , with each of the four signal levels that correspond to the bimodal signal distribution of  FIG. 5 . That is, comparator  331   4  compares the incoming signal with a threshold level at 1+α and generates a corresponding error sample E 11  according to whether the incoming signal is above or below the 1+α level. Similarly, comparators  331   3 ,  331   2  and  331   1  compare the D N  signal level with threshold levels at 1−α, −1+α and −1−α, respectively, and generate corresponding error samples E 10 , E 01  and E 00 . Each of the error samples is buffered in a respective storage circuit  335   1 - 335   4  before being provided to the adaptive module  329 . By this arrangement, the error samples arrive at the adaptive module  329  with the same latency as the D N−1  sample generated by the partial response receive circuit  200 . 
         [0082]    The adaptive module  329  responds to the data samples from the partial response receive circuit  200  and the error samples from the level sampler  327  by selectively updating the threshold values supplied to the comparators  201  and  203  within the partial response receive circuit  200  and comparators  331   1 - 331   4  within the level sampler  327 . For example, when the data samples indicate a 11 state (i.e., D[N−1:N−2]=11), the adaptive module  329  increases or decreases the 1+α threshold level according to whether the incoming signal level is indicated by error sample E 11  to be above or below the 1+α threshold level. The adaptive module similarly updates the 1−α, −1+α, and −1−α thresholds based on error samples E 10 , E 01  and E 00  when the data samples indicate the 10, 01 and 00 states, respectively. As discussed below, the ±α levels may be derived from the 1+α, 1−α, −1+α and −1−α threshold levels, or a subset thereof. 
         [0083]      FIG. 15  illustrates an embodiment of the adaptive module  329  of  FIG. 14 . The adaptive module includes a 2:4 decoder  353 , up/down counters  357   1 - 357   4 , averaging circuits  361  and  365 , digital-to-analog converters (DACs)  359   1 - 359   6 , and, optionally, a filter circuit  351 . The 2:4 decoder  353  activates one of four enable signal lines  354   1 - 354   4  (i.e., asserts an enable signal on the signal line) according to the state of the input data samples D N−1  and D N−2  (also expressed herein as D[N−1:N−2]). Each of the counters  357  has an up/down input (U/D) coupled to receive a respective one of the four error samples (i.e., E 00 , E 01 , E 10  and E 11 ), a count enable input (CEN) coupled to a respective one of the enable signal lines  354 , and a strobe input coupled to receive the sample clock signal  210 . By this arrangement, during each cycle of the sample clock signal  210 , the counter  357  coupled to the activated enable signal line  354  is incremented or decremented according to the state of the corresponding error sample. Thus, when D[N−2:N−1]=11, counter  357   4  is incremented if error sample E 11  indicates that the incoming signal level is above the 1+α threshold level, and decremented if E 11  indicates that the incoming signal level is below the 1+α threshold level. The count values maintained within counters  357   3 ,  357   2  and  357   1  are similarly incremented and decremented according to error samples E10, E01 and E00, respectively, when enabled by their respective enable signal lines  354 . In one embodiment, the filter circuit  351  is used to filter the incoming error samples, for example, by requiring a predetermined number of same-state error samples to be received within a given time before the corresponding count value is adjusted. Other types of error sample filtering may be applied in alternative embodiments. Also, the filter  351  may be omitted altogether. 
         [0084]    The count values maintained within the counters  357   1 - 357   4  are output as control values to respective DACs  359   1 - 359   4  to generate the 1+α, 1−α, −1+α and −1−α levels. Thus, the adaptive module  329  operates to selectively update the 1+α, 1−α, −1+α and −1−α threshold levels according to the state of the received data values D[N−1:N−2]. 
         [0085]    The averaging circuits  361  and  365  generate the ±α threshold levels by averaging the control values output from counters  357   1 - 357   4 . Averaging circuit  365 , for example, includes a summing circuit  366  to sum the C 1+α  and C −1+α  control values from counters  357   4  and  357   2 , respectively, and a divide-by-2 element  367  (which may be achieved without active circuitry by dropping the least significant bit of the sum) to generate a control value C α  that corresponds to the desired α threshold level (i.e., (C 1+α +C −1+α )/2=C α ). Averaging circuit  361  similarly includes a summing circuit  362  to sum the count values C 1−α  and C −1−α  from counters  357   3  and  357   1 , and a divide-by-2 element  363  to generate control value C −α . In one embodiment, each of the control values generated by the counters  357  and the averaging circuits  361 ,  365  are input to respective DACs  359   1 - 359   6  to generate the threshold levels used within the comparators of the partial response receive circuit  200  and level sampler  327  of  FIG. 14 . In an alternative embodiment, each of the comparators within the partial response receive circuit  200  and level sampler  327  include internal DACs and receive respective threshold values in the form of the control values themselves (i.e., C α , C −α , C 1+α , C 1−α , C −1+α , C −1−α ). Also, in a differential embodiment (and in certain single-ended embodiments), the C +α  and C −α  values may be complements of one another so that one of the averaging circuits  361  or  365  may be replaced by a multi-bit inverter. Further, the C 1+α  and C −1−α  values may be complements, and the C 1−α  and C −1+α  values may be complements such that two of the comparators within the level sampler  327  of  FIG. 14  and two of the counters  357  within the adaptive module  329  (and two of the DACs  359 , if used) may be omitted. 
         [0086]      FIG. 16  illustrates an alternative circuit arrangement  375  that may be used in place of the averaging circuits  361  and  365  of  FIG. 15 . Instead of averaging the C 1+α  and C −1+α  count values to generate the C +α  value, C +α  is generated by halving the difference between the C 1+α  and the C 1−α  control values (i.e., C α =((C 1+α )−(C 1−α ))/2). Similarly, the C −α  value is generated by halving the difference between the C −1−α  and the C −1+α  control values. Thus, the circuit of  FIG. 16  includes a difference circuit  376  to generate 2α by subtracting the C 1−α  control value from the C 1+α  control value, and a divide-by-2 element  377  (which may be a implemented by dropping the least significant bit of the difference) to generate C α  by halving the 2α value. Difference circuit  379  and divide-by-2 element  380  are used in a similar manner to generate C −α  from count values C −1+α  and C −1−α  in a corresponding manner. 
         [0087]    Differential Comparator 
         [0088]      FIG. 17  illustrates a comparator  400  that may be used within the receiver circuits and level sampling circuits of  FIGS. 7, 9 and 14 . The comparator  400  includes a preamplifier  401  and a sampling circuit  425 . The preamplifier  401  includes a pair of differential amplifiers  402  and  403  each biased by a respective current DAC (IDAC)  411  and  413 , and each having first and second output nodes  418  and  419  coupled to a supply voltage via a respective resistive element, R. The resistive elements may be implemented, for example, using diode-configured transistors, biased transistors, resistors, or any other active or passive circuitry for establishing a resistance. Transistors  405  and  404  within differential amplifier  402  have widths W 1  and W 2 , respectively, with W 1  being greater than W 2 . Transistors  408  and  407  within differential amplifier  403  also have respective widths W 1  and W 2 . A differential input signal composed of signal components D and /D is coupled to each of the differential amplifiers  402 ,  403  such that D is coupled to transistors  404  and  408  and /D is coupled to transistors  405  and  407 . By this arrangement, when C α  and /C α  are substantially equal (e.g., in an 8-bit control word, C α =128 and /C α =127), the differential amplifiers  402  and  403  are substantially balanced, operating in effect as a single differential amplifier having component transistors of width W 1 +W 2 . Thus, if D is greater than /D, transistors  404  and  408  will collectively sink more current than transistors  405  and  407 , thereby causing the voltage on output node  418  to be pulled down (i.e., via the resistive element, R, coupled to the output node  418 ) more than the voltage on output node  419 . 
         [0089]    When the preamplifier  401  is balanced (i.e., C α  substantially equal to /C α ), the voltages on the preamplifier output nodes  418  and  419  are substantially equal when D and /D are at the common mode potential (i.e., as when D and /D cross one another in transition). Thus, the effective threshold of the preamplifier  401 , and therefore the comparator  400  as a whole, occurs at the common mode of D and /D. By contrast, when the preamplifier is imbalanced by increasing C α  relative to /C α , equal values of D and /D result in output node  419  being pulled lower than output node  418 , due to the fact that transistor  405  is wider than transistor  404  (and therefore has a greater gain), and that the compensating (balancing) effect of differential amplifier  403  is diminished by the reduced control value /C α . Thus, increasing C α  relative to /C α  increases the effective threshold of the preamplifier above the common mode. By increasing C α  to the point at which the threshold between ‘0’ and ‘1’ signal levels is set to a, a differential comparator having a threshold level at a is achieved. By reversing the connections of the C α  and /C α  values to the current DACs of a counterpart comparator (not shown), a differential comparator having a threshold level at −α is achieved. 
         [0090]    The sampling circuit  425  includes a differential amplifier  426  formed by transistors  423  and  424 , a sense amplifier  427  formed by back-to-back coupled inverters  428  and  429 , and a storage circuit  436  formed by a set-reset flip-flop. The differential amplifier  426  includes control inputs coupled to the output nodes  418  and  419 , respectively, of the preamplifier  401 , and output nodes  431  and  433  coupled to source terminals of the inverters  428  and  429 , respectively. A biasing transistor  430 , switchably controlled by the sampling clock signal  210  (or other sample control signal), is coupled between the differential amplifier  426  and a ground reference (or other low voltage reference). The sampling clock signal  210  is additionally coupled to control inputs of positively-doped MOS (PMOS) transistors  434  and  435  which are coupled between a supply voltage (e.g., V DD ) and output nodes of the inverters  428  and  429 . By this arrangement, when the sample clock signal  210  is low, transistor  430  is switched off, and transistors  434  and  435  are switched on to pre-charge the output nodes of the inverters  428  and  429  to the supply voltage. The output nodes of the inverters  428  and  429  are coupled to active-low set and reset inputs, respectively, of the storage circuit  436 , so that the content of the storage circuit  436  is maintained through the low half-cycle of the sample clock signal  210 . When the sample clock signal  210  goes high, biasing transistor  430  is switched on and draws current through the two transistors  424  and  423  of the differential amplifier  426  in proportion to the voltages developed on the output nodes  418  and  419  of the preamplifier  401 . Thus, if the voltage developed on node  419  is higher than the voltage on node  418 , the current drawn by biasing transistor  430  will flow primarily through transistor  423 . Conversely, if the voltage developed on node  418  is higher than the voltage on  419 , the current drawn by biasing transistor will flow primarily through transistor  423 . Transistors  434  and  435  are switched off in response to the high-going sample clock signal  210 , so that the pre-charged outputs of the inverters  428  and  429  are discharged by currents flowing through transistors  423  and  424 . By this operation, if the incoming signal (D) exceeds the common mode voltage, ((D+/D) divided by 2), by more than the +α threshold level (i.e., the incoming signal exceeds the +α threshold level), the current drawn by biasing transistor  430  will flow primarily through transistor  423 . Consequently, the output node of inverter  429  will be discharged more rapidly than the output node of inverter  428 , driving the output of inverter  429  low and driving the output of inverter  428  high (i.e., the PMOS transistor within inverter  428  is switched on and the NMOS transistor within inverter  428  is switched off). The low output of inverter  429  is applied to the active-low set input of the storage circuit  436 , causing the storage circuit  436  to store a logic ‘1’ sampled data value. By contrast, if the incoming signal level does not exceed the +α threshold level, the current drawn by biasing transistor  430  will flow primarily through transistor  424 , thereby driving inverter  428  low (and driving inverter  429  high) to store a logic ‘0’ sampled data value within storage circuit  436 . 
         [0091]      FIG. 18  illustrates an embodiment of the current DAC  411  of  FIG. 17 . The current DAC  411  includes control transistors  437   0 - 437   N−1  and biasing transistors  439   0 - 439   N−1 . Each of the control transistors  437   0 - 437   N−1  is coupled in series (e.g., source to drain) with a corresponding one of the biasing transistors  439   0 - 439   N−1  to form a transistor pair that is coupled between a reference voltage (ground in this example) and an output node  438  (i.e., the node to be connected to the source terminals of the transistors which form the differential amplifier  402 ). Gate terminals of the control transistors  437   0 - 437   N−1  are coupled to receive respective component signals, C α [0]−C α [N−1], of a multi-bit control value, C α  (or, control value /C α ). Each of the control transistors  437   0 - 437   N−1  has a binary weighted gain such that a current of I REF ×2 i  (where i represents the i th  transistor in the positions 0, 1, 2, . . . , N−1) flows through control transistor  437   i  when the corresponding control signal component is high. Thus, if all the constituent bits of the control value C α  [N−1:0] are high, then I REF  flows through control transistor  437   0 , I REF ×2 flows through transistor  437   1 , I REF ×4 flows through control transistor  437   2 , and so forth to control transistor  437   N−1  which conducts I REF ×2 N−1 . Accordingly, control transistors  437   0 - 437   N−1  are designated x1, x2 . . . , x2 N−1  transistors, respectively. By this arrangement, the control value C α  [N−1:0] may be set to any of 2 N  values to select bias currents that range from 0 to I REF ×2 N−1  in increments of I REF . The biasing transistors  439   0 - 439   N−1  have gate terminals coupled to receive a bias voltage, VBIAS, that is adjusted as necessary (e.g., by a biasing circuit) to establish or maintain a desired I REF . 
         [0092]    In one embodiment, the relative gains (i.e., transconductance values) of the various transistors used to implement the current DAC  411  (and therefore drive strengths of the sub-drivers) are established by adjusting the width-length ratio (i.e., W/L) of individual control transistors  437  and/or biasing transistors  439 . For example, the width-length ratio of the x2 control transistor  437   1  is twice the width-length ratio of the x1 control transistor  437   0 , the width-length ratio of the x4 control transistor  437   2  is twice the width-length ratio of the x2 control transistor  437   1 , and so forth. The biasing transistors  439  may have similar gain ratios relative to one another (e.g., x1, x2, x4, x2 N−1  as shown in  FIG. 18 ). Other techniques for adjusting the relative gains of the control transistors  437  and biasing transistors  439  may be used in alternative embodiments. Also, weightings other than binary weightings may be used. For example, in one embodiment, each of the control transistors  437  has an equal gain to each of the other control transistors  437  such that the current drawn by the current DAC  411  is proportional to the number of logic ‘1’ bits in the control value, C α . 
         [0093]      FIG. 19  illustrates an alternative embodiment of a differential comparator  450  that may be used to implement the comparators depicted in  FIGS. 7, 9 and 14 . The comparator  450  includes a sampling circuit  452  and an offset control circuit  440 . The sampling circuit  452  is implemented in generally the same manner as the sampling circuit  425  of  FIG. 17  (and includes differential amplifier  426 , sense amplifier  427 , biasing transistor  430 , and storage circuit  436 ), except that the input signal lines carrying D and /D are coupled directly to the control terminals of transistors  423  and  424 , respectively. The offset control circuit  440  includes a differential amplifier  446  having output nodes coupled to nodes  431  and  433  of the sampling circuit  452 . Control terminals of the transistors  447  and  449  of the differential amplifier  446  are biased by respective voltage DACs (e.g., implemented by current DACs  443  and  445  coupled to respective resistive pull-up elements  451  and  453 ) controlled by C α  and /C α , respectively. By this arrangement, when the sample clock signal goes high the current through output node  433  of the sampling circuit  452  is a sum of the currents drawn by transistor  423  of the sampling circuit  452  and transistor  447  of the offset control circuit  440 . Similarly, the current through node  431  of the sampling circuit  452  is a sum of the currents drawn by transistor  424  of the sampling circuit  452  and transistor  449  of the offset control circuit  440 . As discussed above in reference to  FIG. 17 , when the current through node  431  exceeds the current through node  433 , a logic ‘1’ is stored within storage circuit  436  and, conversely, when the current through node  433  exceeds the current through node  431 , a logic ‘0’ is stored within storage circuit  436 . 
         [0094]    When the DAC control values C α  and /C α  are substantially the same, the comparator  450  is balanced and the effective threshold occurs at the common mode of the D and /D signal levels. That is, if D exceeds the common mode voltage, V CM =((D+/D) divided by 2), the current through node  433  exceeds the current through node  431 , causing a logic ‘1’ to be captured as the sampled data value. As C α  is increased and /C α  decreased, the effective threshold of the differential amplifier is increased such that D must be higher than /D by an amount necessary to overcome the additional current drawn by transistor  449  of the offset control circuit. Thus, by increasing C α  and decreasing /C α , the effective threshold of the sampling circuit may be set to the partial response level, α. That is, a logic ‘1’ is output as the sampled data value if the difference between the D exceeds the common mode voltage, V CM  by more than a, and a logic ‘0’ is output otherwise. A counterpart comparator having a −α threshold may be provided by switching the C α  and /C α  inputs to the DACs  443  and  445 . 
         [0095]    Threshold Level Generator with Single Level-Adapting Comparator 
         [0096]      FIG. 20  illustrates a partial response receiver  500  according to another alternative embodiment. The partial response receiver  500  includes a partial response receive circuit  200  that operates generally as described in reference to  FIGS. 7 and 14 , a level sampler  501 , and an adaptive module  503 . In contrast to the level sampler  327  of  FIG. 14 , the level sampler  501  includes a single comparator  507  that generates an error sample, E DLEV , according to whether the incoming signal exceeds or is below a data level threshold generated by the adaptive module  503 . In the embodiment of  FIG. 20 , the data level threshold is established by a multi-bit control value, C DLEV , generated by the adaptive module. In alternative embodiments, the data level threshold may be established by an analog voltage or current generated by the adaptive module  503 . The error sample may be buffered in one or more storage circuits  509  before being forwarded to the adaptive module  503 , thereby aligning the latency of the error sample with data samples generated by the partial response receive circuit  200 . 
         [0097]    The adaptive module  503  receives the error sample from the level sampler  501  and data samples D N−1  and D N−2  from the partial response receive circuit  200  and, in response, generates control values, C α  and /C α , to establish the ±α levels within the partial response receive circuit  200 , and the control value, C DLEV , to establish the data level threshold within the level sampler  501 . In one embodiment, the adaptive module  503  initially updates the C DLEV  value upon detecting reception of the 1+α data pattern (i.e., D[N−1:N−2]=11) until the comparator  507  indicates that the data level threshold matches the 1+α data level. Thereafter, the adaptive module  503  updates the C DLEV  value in response to the 1−α data pattern (10) until the D LEV  threshold matches the 1−α data level. The control value, Cα, may be generated, for example, by halving the difference between the two C DLEV  values (i.e., C 1+α  and C 1−α ), and the /Cα control value may be generated by complementing the C α  value. 
         [0098]      FIG. 21  illustrates the operation of the adaptive module  503  of  FIG. 20  according one embodiment. Initially, at block  521 , the control values, C α  and C DLEV , and an internally maintained control value, C 2α , are each set to a nominal zero value. In one embodiment, the nominal zero value corresponds to a DAC midpoint value. For example, in an 8-bit DAC, a midpoint value of 1000 0000b (the ‘b’ indicating binary notation) or 128 decimal may be used as the nominal zero value. The C −α  value may be generated by subtracting the C α  value from the full-scale DAC value. Thus, in the eight-bit example, when C +α =128, C −α =(2 8 −1)−128=127. It will be appreciated that this result may be obtained by complementing C α  (i.e., /C α =C −α ). 
         [0099]    At decision block  523 , a history of increment and decrement operations applied to C DLEV  value is evaluated to determine whether the threshold level that corresponds to the 1+α signal level has been reached (i.e., UPPER DLEV FOUND). In one embodiment, if, over the last N updates to the C DLEV  value, the difference between the number of increment operations and the number of decrement operations is less than a predetermined value, the data level threshold is deemed to be dithering about the 1+α signal level and the upper data level threshold is therefore considered found. If the upper data level threshold has not been found, then the sample values generated by the partial response receive circuit (D[N−1:N−2]) are evaluated in decision block  525  to determine whether a signal level at the 1+α level has been received. If so, the error sample generated by the level sampler, E DLEV , is evaluated in decision block  527  to determine whether the incoming signal level is greater than or less than the threshold level established by the present value of C DLEV . If the error sample is a ‘1’, the incoming signal level is greater than the threshold level, and the C DLEV  value is incremented at  529  to increase the data level threshold. If the error sample is a ‘0’, the signal level is less than the data level threshold and the C DLEV  value is decremented at  531  to decrease the data level threshold. As discussed above in reference to  FIG. 15 , some level of filtering may be applied before incrementing or decrementing the C DLEV  value. After the C DLEV  value has been incremented or decremented (i.e., in block  529  or  531 ), the C DLEV  increment/decrement history is evaluated again at decision block  523  to determine whether the upper data level threshold has been found. 
         [0100]    Returning to decision block  525 , if D[N−1:N−2] is not equal to ‘11’, the error sample, E DLEV , is not evaluated and the operation at  525  is repeated for a subsequent set of data samples. Thus, in the level-adapting loop formed by blocks  523 - 531 , the adaptive module selectively updates the C DLEV  value, and therefore the data level threshold according to the state of the data samples generated by the partial response receive circuit (i.e., updates the C DLEV  value in response to detection of the 1+α sample pattern). 
         [0101]    If, at decision block  523 , the upper data level is deemed to be found, the history of increment and decrement operations is cleared and a second level adapting loop is begun at decision block  535 . At decision block  535 , the history of increment and decrement operations is evaluated to determine whether a data level threshold corresponding to the 1−α data level has been found (i.e., LOWER DLEV FOUND). In one embodiment, if, over the last N updates to the C DLEV  value, the difference between the number of increment operations and the number of decrement operations is less than a predetermined value, the data level threshold is deemed to be dithering about the 1−α signal level and the lower data level threshold is therefore considered found. If the lower data level threshold has not been found, then the sample values generated by the partial response receive circuit, D[N−1:N−2], are evaluated in decision block  537  to determine whether a signal level at the 1−α level has been received. If not, decision block  537  is repeatedly entered for subsequent sample value pairs until a signal level at the 1−α level has been received. When data samples that correspond to the 1−α level are detected, the error sample generated by the level sampling circuit, E DLEV , is evaluated at decision block  539  to determine whether the incoming signal level is above or below the data threshold level established by the present value of C DLEV . If the error sample is a ‘0’, as it will be initially, the signal level is below the data threshold level, and the C DLEV  value is decremented at  543  to decrease the DLEV threshold level, and the C 2α  control value is incremented to track the difference between the upper and lower data level thresholds. If the error sample is determined to be a ‘1’ at decision block  537 , then C DLEV  is incremented and C 2α  decremented at block  541 . By this arrangement, when the data level threshold reaches the 1−α data level, the control value, C 2α , will be representative of twice the +α threshold level. Accordingly, at block  545 , the C α  value is generated by dividing C 2α  by 2 (e.g., by right shifting or otherwise dropping the least significant bit of C 2α ). The C −α  value may be generated by complementing the C α  value. Thereafter, the process is repeated starting at decision block  523 . Alternatively, an updated C 2α  value may be generated by counting the return increments from the lower data level threshold to the upper data level threshold, and another updated value of C 2α  generated on a subsequent return to the lower data level threshold, and so forth. The C α  value may be updated after each regeneration of the C 2α  value. Also, rather than finding the 1+α and 1−α data levels, the 1+α and −1+α data levels may be determined, and the corresponding data level control values averaged to generate the C α  value. 
         [0102]    Dual Mode Receiver 
         [0103]      FIG. 22  illustrates an embodiment of a dual mode receiver  550  that may be operated in either a multi-level signaling mode or a partial response mode. When a mode select signal  572  (MSEL) is in a first state (e.g., a logic low state), a two-bit per symbol, multi-level signaling mode is selected. In one embodiment, illustrated in  FIG. 23 , the signal level of each incoming symbol falls into one of four voltage ranges distinguished by three threshold levels. The four voltage ranges are designated 10, 11, 01 and 00 in  FIG. 23  according to the pair of data bits represented at each level. Because each symbol constitutes a pulse having one of four possible amplitudes, the incoming multi-level signal is referred to herein as a 4-PAM (Pulse Amplitude Modulation) signal. Different PAM levels (e.g., 8-PAM, 10-PAM, etc.) may be used in alternative embodiments. 
         [0104]    In the multi-level signaling mode, comparator  557  compares the incoming signal with a midlevel threshold (referred to herein as the zero threshold) to determine the state of the most significant bit (MSB) of the symbol. In the embodiment of  FIGS. 22 and 23 , the MSB is resolved to be a ‘1’ if the signal level exceeds the zero threshold, and a ‘0’ if the signal level does not exceed the zero threshold. Comparators  553  and  555  compare the incoming signal with threshold levels +α and −α, respectively. For example, in a 4-PAM signal mode, the +α threshold level is set to the steady-state low level plus ⅚ of the difference between the steady-state high and low levels, and the −α threshold is set to the steady-state low level plus ⅙ of the difference between the steady-state high and low levels. Referring to  FIG. 23 , if the incoming signal level exceeds the +α threshold level, both comparators  553  and  555  generate a logic high sample value, and if the signal level is below the −α threshold level, both comparators  553  and  555  generate a logic low sample value. By contrast, if the incoming signal level is between the +α and −α threshold levels, the comparators  553  and  555  generate outputs having different states. Thus, by assigning the least significant bit (LSB) of the incoming symbol to be a logic ‘0’ in the case of a signal level above the +α threshold level or below the −α threshold level, and a logic ‘1’ in the case of a signal level between the +α and −α thresholds, the LSB may be generated by exclusive ORing the sample values generated by the comparators  553  and  555 . Accordingly, exclusive OR gate  561  is coupled to receive the sample values generated by the comparators  553  and  555  (i.e., after the sample values are buffered in storage circuits  554  and  556 ), and outputs the LSB to a first port of select circuit  565 . When the mode select signal selects the 4-PAM mode of the dual-mode receiver, the LSB is selected to be passed to next stage storage circuits  567  and  578 , and ultimately is selected by select circuit  579  to be provided to the adaptive module  581 . Thus, in 4-PAM mode, the adaptive module  581  receives an LSB and MSB (the MSB bit being stored in succession in storage circuits  558 ,  571  and  573  for latency alignment with the LSB) for each set of sample values generated by the comparators  553 ,  555 , and  557 . 
         [0105]    It should be noted that the threshold levels and data signal levels described in reference to  FIG. 23  may be used in both differential and single-ended signaling systems. In the case of single-ended signaling systems, the voltage levels of the thresholds and data signals are expressed as an offset from a common, substantially fixed reference voltage, such as a ground reference. In differential signaling systems, the data signals and thresholds are each represented by differential signal pair, with the voltage level of the data signal or threshold being the voltage difference between the two differential signals (e.g., subtracting the voltage level of one signal from the other). 
         [0106]    Still referring to the 4-PAM mode of the dual mode receiver  550 , the comparator  551  generates an error sample E DLEV  that is buffered within storage circuits  552  and  570 , then provided to the adaptive module  581 . In one embodiment, the adaptive module generates the 4-PAM mode threshold level control values C 0 , C +α  and C −α  (i.e., the control values supplied to comparators  557 ,  553 ,  555 , respectively) by determining the data signal level at MSB/LSB=11, and then the signal level at MSB/LSB=00. For example, the C +60  value is offset from the 00 signal level by two-thirds of the difference between the 11 and 00 signal levels, C 0  is set to the midpoint between the 11 and 00 signal levels, and C −α is offset from the 00 signal level by one-third of the difference between the 11 and 00 signal levels. As with the adaptive module described in reference to  FIG. 14 , the error sample generated by the comparator  551  may be filtered to obtain the steady state 11 signal level and the steady state 00 signal level. 
         [0107]    Still referring to  FIG. 22 , when the mode select signal  572  is high, the dual mode receiver  550  is switched to the partial response mode. In the partial response mode, the comparators  553  and  55  are used to compare the incoming data signal against the partial response threshold levels +α and −α, respectively. The samples values generated by the comparators  553  and  555  are buffered in storage circuits  554  and  556 , respectively, then provided to select circuit  563  which selects one of the samples according to the state of the previously received sample. That is, one of the D N−1  samples stored in storage circuits  554  and  556  is selected to be stored in storage circuit  567  according to the D N−2  sample previously stored in the storage circuit  567 . The D N−2  sample is stored in the storage circuit  578  to generate the D N−3  sample value. During a given symbol time, both the D N−2  and D N−3  samples are provided to the adaptive module  581  via select circuits  579  and  575 , respectively. Thus, in 4-PAM mode, the adaptive module  581  receives the MSB/LSB pair for each incoming 4-PAM symbol, and in partial response mode, the adaptive module  581  receives the D[N−2:N−3] sample values that represent one of the four states of the bimodal signal illustrated in  FIG. 4 . 
         [0108]    Still referring to  FIG. 22 , in partial response mode, the comparator  551  and adaptive module  581  operate to generate the +α and −α levels in the manner described in reference to  FIGS. 20 and 21 . Alternatively, because the MSB comparator  557  is not otherwise used in the partial response mode, the comparator  557  may be used to determine the level of the 1−α partial response state, thereby enabling both the 1+α and 1−α signal levels to be determined simultaneously. The ±α levels may then be generated based on the 1+α and the 1−α signal levels (i.e., C α =((C 1+α )−(C 1−α ))/2, and C −α =/Cα). In another alternative embodiment, the MSB comparator may be used to determine the −1+α signal level, thereby enabling ±α to be determined by averaging the 1+α and −1+α signal levels. The error signal generated by the MSB comparator, E 10  (or E 01 ) is illustrated by dashed line  574  to indicate its optional nature. 
         [0109]    Reflecting on the dual mode receiver  550  of  FIG. 22 , it can be seen that much of the circuitry provided for 4-PAM operation (e.g., the four comparators ( 551 ,  553 ,  555  and  557 ), storage circuits ( 552 ,  554 ,  556 ,  558 ,  567 ,  570 ,  571 ,  578  and  573 ) and adaptive module  581 ) is re-used in the 2-PAM partial response mode, thereby providing partial response operation with relatively little additional hardware overhead. The mode select signal  572  may be provided from an external source or from a configuration control circuit within the integrated device that includes the dual mode receiver  550 . In one embodiment the configuration control circuit is a configuration register that may be programmed by one or more other integrated circuits within a system (e.g., a host processor or similar control device) to establish the mode of the dual mode receiver  550 . The configuration control circuit may also be a non-volatile control circuit such as a non-volatile memory, fusible circuit or similar circuit that may be programmed with a mode select value. In yet other embodiments, the configuration control circuit may dynamically change the state of the mode select signal in response to detecting selected system conditions (e.g., detection of threshold error rate when in one operating mode or the other). 
         [0110]    Clock Recovery 
         [0111]      FIG. 24  illustrates an embodiment of a partial response receiver  600  that performs a clock data recovery (CDR) function. That is, the partial response receiver  600  recovers both data and clocking information from the incoming data signal, D N . The partial response receiver  600  includes a data receive and level sampling circuit  601 , adaptive module  603 , clock recovery circuit  605 , edge-sampling comparator  607 , and one or more storage circuits  609 . The data receive and level sampling circuit  601  operates as described above in reference to  FIGS. 14 and 20  to generate one or more error samples, E DLEV , and data samples D[N−1:N−2]. The data receive and level sampling circuit  601  may also be operable in a multi-PAM mode as described in reference to  FIG. 22 . The adaptive module  603  generates one or more threshold control values that are supplied to the data receive and level sampling circuit  601 , including one or more data level control values, C DLEV , and partial response control values, C +α  and C −α . The control values are used to establish threshold values in the partial response compare circuits (i.e., the ±α comparators) and one or more level adapting comparators as discussed above. 
         [0112]    The clock recovery circuit  605  generates a sampling clock signal  210  (SCLK) and edge clock signal  610  (ECLK) in response to a transition sample, T N−1 , generated by the edge-sampling comparator  607  and the data and error samples generated by the data receive and level sampling circuit  601  (i.e., E DLEV  and D[N−1:N−2]). The sampling clock signal  210  is used to time the operation of comparators and storage circuits within the data receive and level sampling circuit  601  and, at least in one embodiment, is phase aligned with midpoints in the incoming data eyes (i.e., midpoint of data valid intervals in the incoming data signal, D N ). In an alternative embodiment, the sampling clock signal  210  may be offset from the midpoints in the incoming data eyes, for example, to accommodate asymmetric setup and hold time requirements in the comparators or other circuitry in the data receive and level sampling circuit  601 . Note that while only a single sampling clock signal  210  is shown, multiple sampling clocks may be generated by the clock recovery circuit to enable receipt of multi-data rate signals. For example, in a double data rate system, the clock recovery circuit may generate SCLK and /SCLK to enable capture of data and signal level samples in both odd and even phases of the sampling clock signal  210 . 
         [0113]    The clock recovery circuit  605  adjusts the phase of the edge clock signal  610  to maintain phase alignment between the edge clock signal  610  and transition points between incoming data eyes (i.e., the edge clock signal  610  is edge aligned with data valid intervals in the incoming data signal). The edge clock signal  610  is supplied to the edge-sampling comparator  607  where it is used to time the sampling of transitions in the incoming data signal, and to one or more storage circuits  609  provided to latency-align the transition sample, T N−1 , with the data sample, D N−1 . In the case of a low-to-high data signal transition, a logic ‘1’ transition sample (i.e., T N−1 =1) indicates that the edge clock transition occurred after the transition in the data signal and therefore that the edge clock signal  610  lags the data signal transition. Conversely, a logic ‘0’ transition sample indicates that the edge clock transition occurred prior to the low-to-high data signal transition and therefore that the edge clock signal  610  leads the data signal transition. The transition samples from edge-sampling comparator  607  and data samples from the data receive and level sampling circuit  601  are used within the clock recovery circuit  605  to adjust the phase of the edge clock signal  610  as necessary to maintain alignment between the edge clock signal  610  and transitions in the incoming data signal. In one embodiment, the sampling clock signal  210  is maintained at a substantially constant phase offset from the edge clock signal  610  such that phase alignment between the edge clock signal  610  and data signal transitions yields a desired phase alignment between the sampling clock signal  210  and midpoints in the incoming data eyes. 
         [0114]      FIG. 25  illustrates a number of possible data signal transitions when the incoming data signal has the bimodal distribution shown in  FIG. 5 . That is, if the incoming data signal has one of the four bimodal signal levels corresponding to the partial response states 11, 10, 01, 00, then the signal may transition from either the 11 or 10 level to the 01 level (i.e., D[N−1:N−2]=11 or 10 and D[N:N−1]=01), and the signal may transition from either the 00 or 01 level to the 10 level. Note that signal transitions from the 10 level to the 11 level (illustrated by dashed line  631 ) and from the 01 level to the 00 level (illustrated by dashed line  632 ) are also possible. 
         [0115]    Considering the transitions from 10-to-01 and from 01-to-10, it can be seen that each of these transitions crosses the midpoint threshold (i.e., zero threshold level) at a time, T 1 , midway between the centers of the data eyes  628  and  630 . That is, if the edge clock signal  610  is phase aligned to the transitions in the incoming data signal, the 10-to-01 and 01-to-10 data signal transitions cross the midpoint threshold coincidentally (i.e., in phase alignment) with the edge clock transition. Accordingly, the 10-to-01 and 01-to-10 transitions may be compared with the zero threshold level (designated ‘0’ in  FIG. 25 ) to generate transition samples for adjusting the phase of the edge clock signal. Note that, because the signal swings in the transitions from 11-to-01 and 00-to-10 are not symmetric with respect to the zero threshold level, the 11-to-01 and 00-to-10 transitions do not cross the zero threshold level at the same time as the 10-to-01 and 01-to-10 transitions, but rather cross the zero threshold level at some time later (indicated by circle  636 ). Consequently, use of the 11-to-01 and 00-to-10 transitions to determine zero-crossing times (i.e., times at which the zero threshold level is crossed) may introduce phase error and/or bi-modal jitter in the recovered edge and sampling clock signals  610  and  210 . Accordingly, in one embodiment of the invention, transitions in the incoming data signal are selected according to their zero-crossing characteristics, with the selected transitions being used to adjust the phase of the edge and sampling clock signals  610  and  210 . 
         [0116]      FIG. 26  illustrates an embodiment of a clock recovery circuit  650  that adjusts the phase of edge clock signal  610  and sampling clock signal  210  based on selected transitions detected in the incoming data signal. The clock recovery circuit  650  includes a transition detect circuit  651 , early/late detector  661 , early/late counter  663 , majority detector  665 , interpolator  667  and reference loop  669 . In the embodiment of  FIG. 26 , the transition detect circuit  651  asserts a transition detect signal  652  (TDET) upon detecting a 01-to-10 or a 10-to-01 transition in the incoming data signal. More specifically, incoming data samples D N−1  and D N−2  are compared in exclusive-OR gate  653  to determine whether a transition has occurred, and signals D N  and D N−2  are compared in exclusive-NOR gate  657  to determine whether the D N  state matches the D N−2  state. The outputs of the exclusive OR and exclusive-NOR gates  653 ,  657  are supplied to AND gate  659  which generates the transition detect signal  652 . By this arrangement, the transition detect signal  652  goes high if D[N:N−2]=010 or 101 (i.e., in response to a 01-to-10 transition or a 10-to-01 transition). The transition detect signal  652  is applied to a count enable input (CEN) of the early/late counter  663  to enable an early/late count value to be incremented or decremented according to the output of the early/late detector  661 . In one embodiment, the early late detector  661  is implemented by an exclusive OR gate, and therefore asserts an early signal  654  (e.g., a logic high signal) if the transition sample, T N , does not match data sample D N , and a late signal (e.g., logic low signal  654 ) if the T N  matches D N . That is, if the transition sample is captured after the transition from D N−1  to D N , the transition sample will match the D N  sample and thereby indicate that the edge clock signal transition is late relative to the data signal transition. Conversely, if the transition sample is captured before the transition from D N−1  to D N , the transition sample will not match the D N  sample and thereby indicate that the edge clock signal transition is early relative to the data signal transition. 
         [0117]    In one embodiment, the early/late counter  663  is initialized to a zero level, and is then incremented in response to an early signal (i.e., from the early/late detector  661 ) and decremented in response to a late signal. By this arrangement, the sign bit (e.g., the MSB) of the early/late count maintained within the early/late counter  663  indicates whether more early signals than late signals, or more late than early signals have been received from the early/late detector (i.e., the count value will underflow to a negative value if more late signals than early signals are detected). Accordingly, after a predetermined number of transition detect assertions (or after a predetermined time), the majority detector  665  evaluates the sign of the early/late count and asserts an up/down signal  668  to the interpolator accordingly. The early/late count value may then be reset to zero in order to count a subsequent set of early/late signal assertions. 
         [0118]    In one embodiment, the interpolator  667  maintains a digital control word that is incremented in response to a logic high up/down signal  668  and decremented in response to a logic low up/down signal  668 . The most significant bits of the digital control word are used to select a pair of phase vectors from the set of N phase vectors  672  generated by the reference loop  669 , and the least significant bits of the digital control word are used to interpolate between the selected pair of phase vectors. As the control word is incremented, the interpolation is incrementally shifted from a leading one of the phase vectors to a lagging one of the phase vectors, thereby incrementally retarding the phase of the edge and sampling clock signals  610 ,  210 . Conversely, as the control word is decremented, the interpolation is incrementally shifted toward the leading one of the selected phase vectors, thereby incrementally advancing the phase of the edge and sampling clock signals  610 ,  210 . 
         [0119]    In one embodiment, the reference loop  669  is formed by a delay locked loop (DLL) that receives a reference clock signal  670  and, in response, generates a plurality of phase vectors  672  that are phase distributed within a cycle time of the reference clock signal  670 . Alternatively, the reference loop  669  may be a phase locked loop (PLL) that multiplies the reference clock frequency to generate a plurality of phase vectors  672  having a higher frequency than the reference clock frequency. In another alternative embodiment, the reference loop  669  may include an internal timing reference generator (e.g., a ring oscillator or other clock generating circuit) so that no reference clock signal  670  is required. Also, as discussed above, the interpolator  667  may generate any number of sampling clock and edge clock signals. For example, in a double data rate system, the interpolator generates an edge clock signal and complement edge clock signal, and a sampling clock signal and complement sampling clock signal, the sampling clock signal being offset from the edge clock signal by a quarter cycle (90 degrees) of the edge clock signal. The quarter cycle offset may be achieved, for example, by a second interpolator that maintains a control word having a 90 degree digital offset from the control word used to generate the edge clock signal. Other techniques may be used to generate the edge clock-to-sample clock offset in alternative embodiments. In a quad data rate system, the interpolator generates four edge clock signals and four sampling clock signals, the combined set of eight clock signals being distributed in phase through a cycle time of the edge clock signal (i.e., 45 degree increments between successive clock edges). This approach may be extended to support virtually any data rate. 
         [0120]    It should be noted that numerous changes may be made to the clock recovery circuit of  FIG. 26  without departing from the scope of the present invention. For example, in one alternative embodiment, the up/down signal is a two-bit signal in which the 00 state indicates a hold condition in which control word maintained within the interpolator is not adjusted. In such an embodiment, the majority detector may receive the entire early/late count from the early/late counter, and output the up/down signal in the 00 state if the count value indicates a balanced reception of early and late detections (e.g., the early/late count is zero). Alternatively, the majority detector may be omitted altogether and the sign of the early/late count value output directly to the interpolator to control the phase adjustment of the edge and sampling clock signals. 
         [0121]    Returning to  FIG. 25 , it can be seen that the 11-to-01 transition occurs between the 1+α and −1+α signal levels, and therefore crosses the midpoint between the centers of data eyes  628  and  630  (i.e., time T 1 ) at the +α threshold level. That is, the 11-to-01 transition crosses the +α threshold coincidentally with the desired transition time of the edge clock signal  610 . Similarly, the 00-to-10 transition crosses the −α threshold coincidentally with the desired transition time of the edge clock signal  610 . Moreover, the 11-to-01 and 00-to-10 transitions have a faster slew rate than the 10-to-01 and 01-to-10 transitions and therefore tend to yield more accurate timing edges. That is, the signal passes through the metastable region more quickly, thereby generating less timing jitter. Accordingly, in one embodiment, additional edge comparators are provided to generate transition samples at the +α and/or −α thresholds. Additional circuitry is also provided within the clock recovery circuit  650  of  FIG. 26  to detect the 11-to-01 and/or 00-to-10 transitions and, in response, to update the early/late counter according to the corresponding transition samples. By this arrangement, the overall number of data signal transitions that are used for clock recovery is increased, thereby relaxing the transition density required in the incoming signal for clock recovery purposes. 
         [0122]    In partial response receiver embodiments in which the ±α levels are adapted (i.e., as opposed to being initially calibrated or determined empirically and one-time programmed), the ±α levels may initially be incorrect (e.g., started at 0) so that edge comparison based on the ±α levels may be undesirable. In such a system, a zero-threshold edge comparator (e.g., edge-sampling comparator  607  of  FIG. 24 ) may be used to recover the edge and sampling clock signals initially, with switchover to ±α edge comparators occurring after the ±α levels are determined. Transition samples generated by zero-threshold edge comparator may then be ignored, or the samples may continue to be used. 
         [0123]    In a system that recovers edge and sampling clock signals based on ±α threshold crossings, and that adapts ±α levels based on a difference between the 1+α and 1−α signal levels, a non-convergence of the ±α threshold levels may occur in certain situations. Referring to bimodal distribution shown in  FIG. 27 , it can be seen that as the sampling clock phase moves left (i.e., in the increasingly lagging direction), the difference between signal levels  682  and  684  increases. Unfortunately, the increased difference between signal levels  682  and  684  yields larger magnitudes for the ±α threshold levels which, as can be seen in the diagram of  FIG. 25 , shifts the phase of the edge and sampling clock signals further in the lagging direction (i.e., further to the left), thereby further increasing the clock phase error. If the clock phase error becomes too large, accurate data and edge samples are no longer received by the partial response receiver so that the ±α threshold levels will not converge to the desired setpoints. 
         [0124]    Still referring to  FIG. 27 , one solution to the non-convergence problem is to limit the data samples used to adapt the ±α threshold levels to those indicated by bolded lines  681  and  683 . That is, in the case of a sample for which D[N:N−1]=11, the +α level is updated only if the preceding state was 10 (i.e., D[N−1:N−2]=10). Similarly, in the case of a sample for which D[N:N−1]=10, the −α threshold level is updated only if the preceding state was 01 (i.e., D[N−1:N−2]=01). By this arrangement, as the edge and sampling clock signals begin to lag the desired phase offsets (i.e., T SAMPLE  moving to the left in the diagram of  FIG. 27 ), the difference between the +α and −α threshold levels will decrease, thereby producing a counter-effect to shift the edge and sampling clock signals in the leading direction. The level-adapting procedure described in reference to  FIG. 21  may be modified to accommodate the above change by changing the evaluation in decision block  525  to D[N−1:N−3]=110 and changing the evaluation in decision block  537  to D[N−1:N−3]=101. Note that these sample patterns are illustrated in  FIG. 27  as being patterns for bits D[N:N−2]. In general, the data samples relied upon for level adaptation may have any latency. 
         [0125]      FIG. 28  illustrates an embodiment of a dual-mode, partial response receiver  700  with clock data recovery. The partial response receiver  700  includes a dual-mode data receive/level sampling circuit  701 , adaptive module  703 , edge comparators  707 ,  708 ,  709 , and clock recovery circuit  705 . The dual-mode data receive/level sampling circuit  701  and the adaptive module  703  operate in generally the same manner as the circuitry described in reference to  FIG. 22 . That is, when the mode select signal selects a 4-PAM mode of operation, the data receive and level sampling circuit samples incoming 4-PAM signals to generate an MSB and LSB per captured sample, and generates a data level error sample (E DLEV ) that allows the adaptive module to determine the full scale difference between steady-state high and low signal levels. The +α, −α and zero threshold levels are then established by the adaptive module  703  at the C α =⅔ full-scale, C −α =⅓ full-scale and C 0 =½ full-scale levels, respectively (other threshold levels may be used in alternative embodiments). When the mode select signal selects a 2-PAM mode of operation, the data receive and level sampling circuit  701  generates data samples by selecting between partial-response comparators (i.e., the comparators receiving the ±α threshold levels) and supplies the selected samples to the adaptive module  703  in pairs to enable determination of which of partial response state is reflected by the error sample, E DLEV , generated by data level comparator (or data level comparators). The adaptive module  703  then adjusts the ±α threshold levels and the zero threshold level according to the incoming data signal levels determined by the data level comparator. 
         [0126]    The edge comparators  707 ,  708  and  709  capture transition samples T N−2 (+α), T N−2 (−α) and T N−2 (0), respectively, of the incoming data signal in response to transitions of an edge clock signal, ECLK, and provide the transition samples to the clock data recovery circuit  705 . The 4-PAM/partial response data samples captured by the data receive and level sampling circuit  701  (i.e., MSB/LSB in 4-PAM mode, D[N−2:N−3] in partial response mode) are also provided to the clock data recovery circuit  705 . The clock data recovery circuit selectively adjusts the phases of the edge clock signal  610  and sampling clock signal  210  based on the transition samples and data samples. 
         [0127]      FIG. 29  illustrates the possible signal transitions between successive 4-PAM symbols. As shown, from each of four possible signal levels, the incoming data signal may transition to any of three other signal levels. For example, a signal level above +α (corresponding to data state 10) may transition to a signal level between +α and 0 (data state 11); a signal level between 0 and −α (data state 01); and a signal level below −α (data state 00). Examining the different possible transitions, it can be seen that any transitions that cross all three threshold levels will cross the zero threshold level at the timing center, T 1 , between the desired data sampling instants. Similarly, transitions that cross a single threshold level will cross either the zero threshold level, the +α threshold level or the −α threshold level at T 1 . By contrast, any transitions that cross two threshold levels, but not three, do not cross the zero, +α or −α threshold levels at T 1 . Enumerating the different transitions that cross the zero, +α and −α threshold levels at T 1  as transition types (1), (2) and (3), respectively, it can be seen that type (1) transitions are those in which the LSB remains unchanged at either ‘1’ or ‘0’, while the MSB changes state (i.e., (MSB N  xor MSB N−1 ) &amp; (LSB N  xnor LSB N−1 )); type (2) transitions are those in which the MSB remains high while the LSB changes state (i.e., MSB N  &amp; MSB N−1  &amp; (LSB N  xor LSB N−1 )); and type (3) transitions are those in which the MSB remains low, while the LSB changes state (i.e., /MSB N  &amp; /MSB N−1  &amp; (LSB N  xor LSB N−1 )). Thus, in one embodiment, when the mode select signal  712  selects a 4-PAM mode of operation within the partial response receiver  700  of  FIG. 28 , the clock recovery circuit adjusts the phase of the edge clock signal and sampling clock signal in response to the data samples generated by the data receive and level sampling circuit  701  and the transition samples generated by comparators  707 ,  708 ,  709  in accordance with the following table: 
         [0000]    
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Center Time 
                 T N   
                 T N   
                 T N   
                 Early/Late 
               
               
                 D N−1 (4P) 
                 D N (4P) 
                 Crossing At: 
                 (+α) 
                 (−α) 
                 (0) 
                 Count Adj. 
               
               
                   
               
             
             
               
                 00 
                 01 
                 −α 
                 X 
                  0/1* 
                 X 
                 +1/−1 
               
               
                 00 
                 11 
                 — 
                 X 
                 X 
                 X 
                 — 
               
               
                 00 
                 10 
                 0 
                 X 
                 X 
                 0/1 
                 +1/−1 
               
               
                 01 
                 00 
                 −α 
                 X 
                 0/1 
                 X 
                 −1/+1 
               
               
                 01 
                 11 
                 0 
                 X 
                 X 
                 0/1 
                 +1/−1 
               
               
                 01 
                 10 
                 — 
                 X 
                 X 
                 X 
                 — 
               
               
                 11 
                 00 
                 — 
                 X 
                 X 
                 X 
                 — 
               
               
                 11 
                 01 
                 0 
                 X 
                 X 
                 0/1 
                 −1/+1 
               
               
                 11 
                 10 
                 +α 
                 0/1 
                 X 
                 X 
                 +1/−1 
               
               
                 10 
                 00 
                 0 
                 X 
                 X 
                 0/1 
                 −1/+1 
               
               
                 10 
                 01 
                 — 
                 X 
                 X 
                 X 
                 — 
               
               
                 10 
                 11 
                 +α 
                 0/1 
                 X 
                 X 
                 −1/+1 
               
               
                   
               
               
                 (*‘0/1’ means ‘0’ or ‘1’ and corresponds to the +1 or −1 adjustment to the early/late count) 
               
             
          
         
       
     
         [0128]    Still referring to  FIG. 28 , when the partial response receiver  700  is operated in the partial response mode, the transitions of interest are as shown in  FIG. 30 . That is, a partial-response state transition from 10-to-01 or 10-to-01 crosses the zero threshold level at the desired edge clock transition time, T 1 ; a state transition 11-to-01 crosses the +α threshold level at T 1 ; and a state transition from 00-to-10 crosses the −α threshold level at T 1 . Enumerating the partial response mode transitions that cross the zero, +α and −α threshold levels at T 1  as transition types (1), (2) and (3), respectively, it can be seen that type (1) transitions are those in which the current data sample, D N , does not match the immediately preceding data sample, D N−1 , which, in turn, does not match the twice-removed data sample, D N−2  (i.e., (D N  xor D N−1 ) &amp; (D N−1  xor D N−2 )); type (2) transitions are those in which the current data sample, D N , is low, and the two immediately preceding data samples, D N−1  and D N−2 , are high (i.e., /D N  &amp; D N−1 &amp; D N−2 ); and type three (3) transitions are those in which the current data sample, D N , is high, and the two immediately preceding data samples, D N−1  and D N−2 , are low (i.e., D N  &amp; /D N−1  &amp; /D N−2 ). Thus, in one embodiment, when the mode select signal  712  selects a partial response mode of operation within the partial response receiver  700 , the clock recovery circuit  705  adjusts the phase of the edge clock and sampling clock signals  610 ,  210  in response to the data and transition samples generated by circuit  701  and comparators  707 ,  708 ,  709  in accordance with the following table: 
         [0000]    
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Center Time 
                 T N   
                 T N   
                 T N   
                 Early/Late 
               
               
                 D[N−1:N−2] 
                 D[N:N−1] 
                 Crossing At: 
                 (+α) 
                 (−α) 
                 (0) 
                 Count Adj. 
               
               
                   
               
             
             
               
                 00 
                 10 
                 −α 
                 X 
                 0/1 
                 X 
                 +1/−1 
               
               
                 01 
                 00 
                 — 
                 X 
                 X 
                 X 
                 — 
               
               
                 01 
                 10 
                 0 
                 X 
                 X 
                 0/1 
                 +1/−1 
               
               
                 10 
                 01 
                 0 
                 X 
                 X 
                 0/1 
                 −1/+1 
               
               
                 10 
                 11 
                 — 
                 X 
                 X 
                 X 
                 — 
               
               
                 11 
                 01 
                 +α 
                 0/1 
                 X 
                 X 
                 −1/+1 
               
               
                   
               
             
          
         
       
     
         [0129]    Still referring to  FIG. 30 , if the +α and −α threshold levels are initially set to zero as indicated by reference numeral  741 , it can be seen that the edge clock alignment will initially converge to a point that is phase delayed relative to the desired edge sampling point, T 1 . As the levels of ±α progress toward their ultimate setpoints at  742  and  743 , however, the edge clock phase alignment will converge to the desired sampling point, T 1 . 
         [0130]      FIG. 31  illustrates a transition detect circuit  725  and sample select circuit  740  that may be used within the clock recovery circuit  705  of  FIG. 28 . The transition detect circuit receives the data sample pair generated by the data receive and level sampling circuit  701  and generates a transition detect signal  732  (TDET) in accordance with the states of successive data sample pairs, the mode select signal  712  (MSEL) and a transition select signal  728  (TS[3:1]). The transition detect circuit additionally generates component transition detect signals  733   1 ,  733   2  and  733   3  (i.e., TDET(0), TDET(+α) and TDET(−α), respectively) which are output to the transition select circuit  740 . The transition select circuit includes AND gates  741   1 ,  741   2 , and  741   3  to gate the transition samples T N (0), T N (+α) and T N (−α), according to the state of the corresponding transition select signals  733   1 - 733   3 . The outputs of the AND gates  741  are input to OR gate  743  to generate an early signal  734 . The transition detect signal  732  is itself generated by a logic OR combination of the component transition detect signals  733  in OR gate  733 . By this arrangement, if any of the component transition detect signals  733  is asserted (e.g., to a logic high state), the transition detect signal  732  is asserted, and the state of the corresponding transition sample T N  is output as the early signal  734 . 
         [0131]    The transition detect circuit  725  includes a set of combinatorial logic circuits  727   1 ,  727   2  and  727   3  that generate type (1), type (2) and type (3) transition detect signals  730  for both 4-PAM and partial response data states in accordance with the type (1), type (2) and type (3) transition types described in reference to  FIGS. 29 and 30 . In the embodiment of  FIG. 31 , combinatorial logic circuit  727   1  generates a 4-PAM type (1) transition detect signal  730   1A  and a 2-PAM, partial response type (1) transition detect signal  730   1B  as follows:
       Signal  730   1A =(MSB N  xor MSB N−1 ) &amp; (LSB N  xnor LSB N−1 ); and   Signal  730   1B =(D N  xor D N−1 ) &amp; (D N−1  xor D N−2 ).
 
Similarly, combinatorial logic circuits  727   2  and  727   3  generate 4-PAM type (2) and type (3) transition detect signals  730   2A  and  730   3A , and 2-PAM, partial response type (2) and type (3) transition detect signals,  730   2B  and  730   3B , as follows:
   Signal  730   2A =(MSB N  &amp; MSB N−1 ) &amp; (LSB N  xor LSB N−1 );   Signal  730   3A =(/MSB N  &amp; /MSB N−1 ) &amp; (LSB N  xor LSB N−1 );   Signal  730   2B =/D N  &amp; D N−1  &amp; D N−2 ; and   Signal  730   3B =D N  &amp; /D N−1  &amp; /D N−2 .
 
It should be noted that, in both partial-response mode and 4-PAM mode, two successive pairs of data samples are used within the transition detect circuit  725  (e.g., MSB/LSB N  and MSB/LSB N−1  in 4-PAM mode; D N /D N−1  and D N−1 /D N−2  in partial response mode) to generate the transition detect signals  730 . One or more of the data sample pairs may be buffered within a storage circuit within transition detect circuit  725  or elsewhere within the dual mode receiver and made available to the various combinatorial logic circuits  727 . Also, the latency of the data samples referred to in  FIG. 31 , though depicted as N, N−1 and N−2, may be any latency necessary to match the latency of the data samples output from the data receive and level sampling circuit  701  of  FIG. 28 .
       
 
         [0138]    Select circuits  729   1 ,  729   2  and  729   3  each have a first input port coupled to receive a respective one of the 4-PAM type (1), type (2) and type (3) transitions detect signals  730   1A ,  730   2A  and  730   3A , respectively, and a second input port coupled to receive a respective one of the 2-PAM, partial response type (1), type (2) and type (3) transition detect signals  730   1B ,  730   2B  and  730   3B , respectively. The mode select signal  712  is coupled to a select input of each of the select circuits  729  so that, when the mode select signal  712  is low to select a 4-PAM mode of operation, the 4-PAM transition detect signals  730   1A ,  730   2A  and  730   3A  are supplied to inputs of respective AND gates  731   1 ,  731   2  and  731   3 , respectively. By contrast, when the mode select signal  712  is high, the 2-PAM, partial response transition detect signals  730   1B ,  730   2B  and  730   3B  are supplied to respective inputs of the AND gates  731 . In the embodiment of  FIG. 31 , the transition select signal  728  includes three component signals, TS[1], TS[2] and TS[3], that are input to AND gates  731   1 ,  731   2  and  731   3 , respectively, to gate the generation of the component transition detect signals  733   1 ,  733   2 , and  733   3 . Thus, the transition select signal  728  may be used selectively enable the different types of data signal transitions to be used for clock recovery purposes. For example, if TS[3:1]=001, then detection of type (1) transitions (i.e., crossing the zero-threshold at the desired time) is enabled, but detection of type (2) and type (3) transitions (i.e., crossing the ±α thresholds at the desired time) is disabled. When TS[3:1]=111, then detection of all three types of transitions depicted in  FIGS. 29 and 30  is enabled. Other settings of the transition select signal may be used to enable detection of the different transition types in other combinations. In one embodiment, the transition select signal is generated according to a transition select value stored in a configuration circuit that may be run-time programmed (e.g., by a host processor or other system control component). The transition select value may alternatively be programmed into a non-volatile storage circuit (e.g., at production time) or similar hardware configurable circuit (e.g., fuse-blowing to establish desired configuration). 
         [0139]    The transition detect signal  732  and early signal  734  generated by the transition detect circuit  725  and transition select circuit  740  may be used to control the phase of one or more sampling clock signals and one or more edge clock signals in the manner above in reference to  FIG. 26 . For example, in one embodiment the transition detect signal  732  is applied to the count enable input of an early/late counter, and the early signal  734  is applied to the up/down input. The transition detect signal  732  and early signal  734  may alternatively be supplied to other logic circuitry that determines whether detected transitions occur early or late relative to an edge clock signal. 
         [0140]    Numerous changes may be made to the transition detect circuit  725  and transition select circuit  740  without departing from the spirit and scope of the present invention. For example, if the data signal transitions to be used for clock recovery is a subset of the three transition types shown in  FIGS. 29 and 30 , one or more component signals of the transition select signal (and corresponding components of the combinatorial logic  727 , select circuitry  729  and gating circuitry  731 ,  741 ) may be omitted. Moreover, if the transition types are fixed for a given application (e.g., all three types of the transitions depicted in  FIGS. 29 and 30 , or any subset or single one thereof), the transition select signal may be omitted altogether along with the combinatorial logic and gating circuitry for any unused types of transitions. 
         [0141]      FIG. 32  illustrates the full set of transitions that may be detected when the partial response receiver  700  of  FIG. 28  is operated in the partial response mode. In particular, transitions  750  and  751  (i.e., transitions from 10-to-11 and from 01-to-00), designated type (4) and type (5) transitions, respectively, can be seen to cross the nominal 1 and −1 threshold levels (referred to herein as unity threshold levels), respectively, at the desired edge sampling point, T 1 . Note that, in these transitions, the input signal slews substantially faster than in the 01-to-10 and 10-to-01 transitions and therefore is a potentially more accurate (and/or more jitter-free) source of clock phase information. Accordingly, in one embodiment, additional edge comparators are provided in the circuit of  FIG. 28  (not shown), and corresponding additional logic is added to the transition detect and select circuits ( 725 ,  740 ) of  FIG. 31  to enable clock recovery based on type (4) and/or type (5) transitions. Alternatively, in one embodiment, the zero-threshold comparator  709  of  FIG. 28  is re-allocated to a unity threshold comparator function after the ±α threshold levels are established (e.g., converge to the points  742  and  743  depicted in  FIG. 30 ).  FIG. 33  illustrates an embodiment of a threshold select circuit  760  that is used to select between control values C 0  and C 1  for the zero and unity threshold levels, respectively. Initially a threshold select signal  762  (LSEL) is supplied in a first state to multiplexer  761  (or other select circuit) to select the C 0  control value to be provided to edge comparator  709 , thereby enabling the 10-to-01 and 01-to-10 transitions to be used for clock recovery purposes as the ±α levels are adapted. After the ±α levels have reached convergence points (e.g., begin to dither), the threshold select signal  762  is switched to an alternate state to select the C 1  control value to be used for generation of edge samples. The C 1  control value may be generated by the adaptive module  703  of  FIG. 28  using the techniques discussed above in reference to  FIG. 22 . The C 0  and/or C 1  control values may be buffered in registers  763  and  765  as shown in  FIG. 33  (or other storage circuits), or provided directly to the multiplexer  761  from the adaptive module. 
         [0142]    Referring again to the partial response receiver  700  of  FIG. 28 , it should be noted that the receiver may alternatively be a single-mode 4-PAM circuit having the clock recovery function described above in reference to  FIGS. 28, 29 and 31  (i.e., omitting the combinatorial circuitry and mode select circuitry used to enable the partial response mode). That is, the 4-PAM circuit may include data receive circuitry for capturing a sequence of 2-bit data samples (or more bits per data sample in the case of M-PAM, M&gt;4), and clock recovery circuitry that captures transition samples at one or more of the 0, +α and −α threshold levels, and that selectively uses the transition samples to recover a sampling clock signal and edge clock signal according to the state of successive data samples. Conversely, the circuitry necessary to enable the 4-PAM mode of operation may be omitted to provide a partial-response receiver with clock recovery circuitry that captures transition samples at one or more of the 0, +α and −α threshold levels, and that selectively uses the transition samples to recover a sampling clock signal and edge clock signal according to the state of successive data samples. 
         [0143]    In another alternative embodiment, the partial response receiver  700  of  FIG. 28  may include a third, non-partial-response operating mode in which binary signal reception and clock recovery are performed without regard to partial response. That is, the +α and −α comparators within the data sampling circuit and edge sampling circuit may be disabled, and the zero-threshold comparators within the data and edge sampling circuits being used to sample data and edges in the incoming signal. Alternatively, in the non-partial-response mode, the ±α threshold levels may be set to zero such that all the comparators within the data and edge sampling circuits perform a zero-threshold comparison. In yet other embodiments, the partial response receiver  700  of  FIG. 28  may be operable in either a 4-PAM or binary mode, with partial response signal detection being separately enabled in either mode. That is, the partial response receiver  700  may be operated in either a binary non-partial response mode; a binary partial response mode; a 4-PAM non-partial response mode; or a 4-PAM partial response mode. Moreover, the number of least-latent data samples included in the partial response may be extended to any number. More generally, the receive circuit  700  may be operated in PAM modes ranging from M-PAM to (M-X)-PAM (where M is any integer and X is any integer less than M−1), and may be operated with or without regard to partial response in each PAM mode. Circuitry to support extended-bit partial response operation, and 4-PAM partial response operation is discussed below in greater detail. 
         [0144]    Extending the Partial Response Compensation to Include Additional Data History 
         [0145]    Thus far, partial response receivers have been described in terms of a bimodal distribution based on the current symbol, D N , and the preceding symbol, D N−1 . In alternative embodiments, partial responses to any number of preceding symbols may be accounted for in the generation of data samples.  FIG. 34 , for example, illustrates a distribution  770  of data-plus-partial-response signal levels in a system in which the two most recently received symbols are the primary source of residual channel response (i.e., the channel memory includes a partial response, a, to the immediately preceding symbol, D N−1 , and a partial response, β, to the twice-removed preceding symbol, D N−2 ). For purposes of simplicity, the signal level constellation shown in  FIG. 34  assumes a substantially equal distribution of the eight partial response signal levels: 1+α+β, 1+α−β, 1−α+β, 1−α−β, −1+α+β, −1+α−β, −1−α+β, and −1−α−β. Other signal distributions may result depending upon the channel characteristics. As shown, the constellation may be viewed as defining four signal ranges  771   1 - 771   4 , each range being centered about one of the four possible partial response levels, α+β, α−β, −α−β, and −α−β. 
         [0146]      FIG. 35  illustrates a partial response receiver  800  that operates in accordance with the partial response states shown in  FIG. 34 . Four comparators  801   1 ,  801   2 ,  801   3  and  801   4  are provided to compare the signal level of an incoming symbol, D N , against the four partial response threshold levels α+β, α−β, −α−β and −α−β, respectively. The outputs of each of the comparators  801  are stored in a first stage of storage circuits  802   1 - 802   4  (e.g., in response to a sampling clock signal, not shown), with the sample values that correspond to the +α threshold levels (i.e., D N−1 (α+β) and D N−1 (α−β)) being supplied to a first select circuit  810  and the sample values that correspond to the −α threshold levels (i.e., D N−1 (−α+β) and D N−1 (−α−β)) being supplied to a second select circuit  812 . Each of the first and second select circuits  810  and  812  selects between its respective pair of input samples according to the state of a previously resolved sample value, D N−3 , stored in storage circuit  820 . More specifically, if the resolved sample value, D N−3 , is a ‘1’, then β, the partial response to D N−3 , is a positive value and the select circuits  810  and  812  select the sample values D N−1 (α+β) and D N−1 (−α−β), respectively, that correspond to the positive β state. If the D N−3  sample is a logic ‘0’ value, then β is a negative value and the select circuits  810  and  812  select the sample values D N−1 (α−β) and D N−1 (−α−β) that correspond to the negative β state. The D N−1  sample values selected by the select circuits  810  and  812  correspond to the +α and −α partial response states, respectively, and are stored in storage circuits  814  and  816  to provide D N−2  sample values D N−2 (α) and D N−2 (−α). The D N−2 (α) and D N−2 (−α) samples are output from the storage circuits  814  and  816  to respective inputs of select circuit  818 . The state of the D N−3  sample value indicates the sign of the partial response contribution in the D N−2  signal. That is, if D N−3 =1, α is positive, and if D N−3 =0, α is negative. Accordingly, the D N−3  sample value is supplied to a select input of select circuit  818  to select either D N−2 (+α) or D N−2 (−α). Thus, the partial response receiver  800  of  FIG. 35  simultaneously compares the incoming data signal against four different partial response thresholds, then selects one of the four sample values to be the output sample value (D N−3 ) based on the previously resolved state of the α and β partial response components. Partial response components from more than two previously received symbols may be accommodated in a similar manner by increasing the number of comparators to match the number of partial response levels to be resolved, and then selecting the desired partial response sample on the basis of the partial response components indicated by a previously resolved sample value. 
         [0147]      FIG. 36  illustrates a partial response receiver  840  according to another embodiment of the invention. The partial response receiver  840  includes a partial response receive circuit  800 , level sampler  841  and adaptive module  850 . The partial response receive circuit  800  operates generally as described in reference to  FIG. 35  to generate sample value D N−3 , which is supplied to the adaptive module  850 . The level sampler  841  operates in generally the same manner as the level sampling circuit of  FIG. 14 , except that eight comparators  842   1 - 842   8  are provided to error samples for each of the eight possible partial response levels depicted in  FIG. 34  (i.e., E 000 , E 001 , . . . , E 110 , E 111 ). The level sampler additionally includes storage circuits  844  to latency-align the error samples generated by comparators  842  with the data sample D[N−3] generated by the partial response receive circuit  800 . The adaptive module  850  operates similarly to the adaptive module described in reference to  FIGS. 14-16 , except that control values for each of the eight partial response threshold levels applied within comparators  842  are updated in response to reception of a corresponding one of the eight possible data patterns for sample values D[N−3:N−5] (note that data samples D N−4  and D N−5  may be generated by buffering data samples received from the partial response receive circuit  800  within the adaptive module  850 ). That is, when D[N−3:N−5]=111, control value C 1+α+β  is incremented or decremented according to the state of the error sample, E 111 , generated by comparator  842   8 ; when D[N−3:N−5]=110, C 1+α+β  is incremented or decremented according to the state of error sample E 110 , and so forth to error sample E 000  which is used to update C −1−α−β  when D[N−3:N−5]=000. 
         [0148]    In one embodiment, each of the four threshold control values supplied to the partial response receive circuit  800  (i.e., C α+β , C α−β , C −α+β  and C −α−β ) are generated by averaging respective pairs of the control values output to the level sampler  841 . That is: 
         [0000]        C   α+β =( C   1+α+β   +C   −1+α+β )/2; 
         [0000]        C   α−β =( C   1+α−β   +C   −1+α−β )/2; 
         [0000]        C   −α+β =( C   1−α+β   +C   −1−α+β )/2; and 
         [0000]        C   −α−β =( C   1−α−β   +C   −1−α−β )/2 
         [0149]    Alternatively, the threshold control values supplied to the partial response receive circuit  800  may be generated by halving the difference between respective pairs of the control values output to the level sampler  841  (e.g., C α+β =(C 1+α+β −C 1−α−β )/2). 
         [0150]    Partial Response, Multi-PAM Receiver 
         [0151]    The partial response operation described above in the context of binary signaling may be extended to multi-PAM receivers.  FIG. 37 , for example, illustrates a constellation of possible signal levels in a 4-PAM signaling system. More specifically, assuming that a given data symbol has one of the four levels +3, +1, −1 or −3 centered about a nominal zero value, and that the primary partial response is due to the immediately preceding symbol, then sixteen partial response states become possible. That is, a symbol nominally transmitted at level +3 may have any of the states 3+3α, 3+α, 3−α, or 3−3α when observed at the receiver, depending on whether the signal level for the preceding symbol was +3, +1, −1 or −3. Similarly, a symbol nominally transmitted at levels +1, −1 or −3 may have any of the four states that correspond to the additive or subtractive effect of the preceding symbol level. As shown in  FIG. 37 , by grouping the possible partial response states according to the signal level of the preceding symbol, it can be seen that if the preceding symbol was a +3, the subsequently transmitted symbol will have a signal level that is offset by 3α and therefore that ranges from −3+3α to +3+3α. Similarly, if the preceding symbol was a +1, the subsequently transmitted symbol will have a signal level that is offset by 1α, and so forth for preceding symbols at −1 (offset=−1α) and −3 (offset=−3α). Accordingly, by providing four 4-PAM receive circuits each having thresholds offset by respective one of the 3α, +α, −α and −3α partial response levels, a partial response mode of operation may be enabled within a 4-PAM receiver. 
         [0152]      FIG. 38  illustrates a 4-PAM partial response receiver  875  according to an embodiment of the invention. The receiver  875  includes four 4-PAM receive circuits  877   1 - 877   4  (i.e., each capable of resolving the signal level of an incoming symbol into one of four possible two-bit combinations) each having a threshold level that is offset according to one of the four partial response levels (3α, α, −α, or −3α). In the case of receive circuit  877   4 , for example, the 3α offset (corresponding to a prior symbol level of 10) is applied to each of the internal levels generated to resolve the 4-PAM signal. That is, instead of comparing the incoming signal with levels of −2, 0, and 2, the incoming signal is compared with −2+3α, 3α and 2+3α (i.e., the threshold values between the 3+3α, 1+3α, −1+3α, −3+3α constellation depicted in  FIG. 37 ). Similarly, 4-PAM receive circuit  877   3  compares the incoming signal with thresholds offset by α (i.e., −2+α, α and 2+α), comparator  877   2  compares the incoming signal with thresholds offset by −α (i.e., −2−α, −α, and 2−α) and comparator  877   1  compares the incoming signal with thresholds offset by 3α (i.e., −2−3α, −3α and 2−3α). By this arrangement, the incoming signal is resolved into four 2-bit sample values according to each of the four possible partial responses to the preceding symbol. The preceding symbol, MSB/LSB[N−1], having been selected according to the state of MSB/LSB[N−2], is stored in storage element  881  and applied to the select input of select circuit  879  to select the one of the four sample values generated in accordance with the incoming signal level. While the partial response operation has been described in terms of a 4-PAM receiver that enables partial response selection based on a single preceding symbol, the circuits and operation described may be extended to other PAM levels (e.g., 8-PAM, 10-PAM, etc.) and/or to include the partial response to additional preceding bits. 
         [0153]      FIG. 39  illustrates an input circuit  885  that includes multiple partial response receivers  887   1 - 887   M  according to an embodiment of the invention. Each of the partial response receivers  887   1 - 887   M  is coupled to a respective one of signal lines  883   1 - 883   M  to receive a corresponding one of data signals D 1 -D M . In one embodiment, each of the data signals D 1 -D M  is a serial stream of data values (e.g., data eyes) that is sampled within the corresponding partial response receiver  887  to generate a corresponding one of sample values, S 1 -S M . The sampled values S 1 -S M  are delayed relative to the corresponding data values by one or more symbol times, i, according to the latency introduced by the partial response receiver  887 . Thus, during the symbol time in which data value D M [N] is valid at the input to partial response receiver  887   M , the partial response receiver outputs sample value S M [N-i]. Partial response receivers  887   1 - 887   M−1  similarly output respective sample values S 1 [N-i]-S M−1 [N-i] during the symbol times in which data values D 1 [N]-D M−1 [N] are valid. 
         [0154]    In one embodiment, each of the signal lines  893   1 - 893   M  forms a unidirectional or bidirectional point-to-point signaling link between a first IC device that includes input circuit  885  and a second IC device. In the case of a bi-directional link, the first IC device may additionally include transmit circuitry coupled to the signal lines  893 , and the second IC device may include a counterpart input/output circuitry having an input circuit similar to input circuit  885 . The first and second IC devices may be in separate IC packages coupled to a common substrate (e.g., circuit board) or coupled to different substrates (e.g., coupled to respective daughterboards with signal paths  893   1 - 893   M  formed in part by signal traces on the daughterboards and on a backplane, or the first IC device coupled to a daughterboard and the second IC device coupled to a motherboard). Alternatively, the first and second IC devices may be included within the same IC package (e.g., a multi-chip module, chip-stack package, paper-thin package or combination of integrated circuit dice within a package). In another embodiment, each of the signal lines  893   1 - 893   M  forms a unidirectional or bidirectional multi-drop signaling link coupled to three or more IC devices, with the IC devices or any subset thereof being disposed on the same or different substrates, and within the same or different IC packages. 
         [0155]    In either the point-to-point or multi-drop embodiments, the signal lines  893   1 - 893   M  may constitute a parallel signaling path  894  for transmission of multi-bit data values. For example, each of the sample values S 1 -S M  for a given symbol time (i.e., corresponding to data values D 1 -D M  recovered from path  894 ) may represent a unified digital value (in which S M  is a most significant bit (MSB) and sample S 1  is a least significant bit (LSB), M being any integer value greater than one. Also, subsets of bits within the sample values S 1 -S M  may constitute respective unified digital values (e.g., S 1 -S X  being a first value ordered from LSB to MSB, S X+1 -S Y  being a second data value ordered from LSB to MSB, and so forth to S Z+1  to S M  which constitute a last data value ordered from LSB to MSB). Further, any number of successively received samples (i.e., corresponding to a packetized transmission of values) may form a unified digital value. For example, S 1 [N-i]-S M [N-i], S 1 [N-i-1]-S M [N-i-1], . . . , S 1 [N-i-Q]-S M [N-i-Q] may form a unified digital value having a total of Q×M bits (Q being any integer value greater than one) in which one bit is an LSB and one bit is an MSB. Alternatively, a set of successively received samples may form multiple unified digitized values, as when bits of a first digitized value are generated in successive symbol times by partial response receivers  887   1 - 887   X , bits of a second digitized value are generated in successive symbol times by partial response receivers  887   X+1 - 887   Y , and so forth. 
         [0156]    In the embodiment of  FIG. 39 , each partial response receiver  887  includes a partial response receive circuit  888 , level sampler  889 , adaptive module  890 , edge sampler  891  and clock data recovery circuit  892 . The partial response receive circuit  888  may be a multi-mode receive circuit as described above (i.e., capable of being switched between a multi-level signaling mode and a partial-response mode), or may be dedicated to partial-response operation. Also, the partial response receive circuit may operate on incoming binary or multi-level signals (e.g., 2-PAM, 4-PAM, 8-PAM, 10-PAM, etc.) having any data rate (e.g., single data rate, double data rate, quad data rate, octal data rate, decade data rate, etc.). Thus, the partial response receive circuit  888  may be implemented by any of the partial response receive circuit embodiments described herein. Similarly, the level sampler  889  and adaptive module  890  may be implemented by any of the level sampler and adaptive module embodiments described herein. For example, though depicted as outputting a single error sample, E DLEV , the level sampler  889  may output multiple error samples to the adaptive module  890 , and the adaptive module  890  may output any number of control values to the level sampler  889 , partial response receive circuit  888  and edge sampler  891  (e.g., as shown in  FIGS. 14, 28 and 36 ) in addition to or instead of C DLEV , C α ,/C α  and C 0 . Also, though depicted in  FIG. 39  as recovering a sample clock signal (SCLK) and edge clock signal (ECLK), the edge sampler  891  and clock data recovery circuit  892  may recover any number of clock signals as described above in reference to  FIGS. 24 and 26  (e.g., for use in multi-data rate embodiments). Further, the edge sampler  891  and clock data recovery circuit  892  may recover phase information (i.e., for adjusting the phase of the recovered clock signals) from any or all transitions of the incoming data signal  893  as described above. The edge sampler  891  and clock data recovery circuit  892  may be omitted from the partial response receiver  887  in an embodiment in which clock signals, strobe signals or other timing signals are provided by another source, such as an external or internal clock generator, or separate clock recovery circuit. 
         [0157]      FIG. 40  illustrates an input circuit  895  according to an alternative embodiment of the invention. The input circuit  895  is similar to the input circuit  885  of  FIG. 39  (and may be implemented in all the alternative embodiments described in reference to  FIG. 39 ), except that the circuitry used to generate partial response thresholds (or representative control values) and sampling clock signals is shared among multiple partial response receive circuits  888   1 - 888   M . That is, a single level sampler  889  and corresponding adaptive module  890  are used to generate control values C α ,/C α  and C 0  (and additional or different threshold control values in multi-level signaling embodiments and embodiments that do not include clock recovery circuitry) that are supplied to each of the partial response receive circuits  888   1 - 888   M , and a single edge sampler  891  and corresponding clock data recovery circuit  892  are used to generate the sampling clock signal (SCLK) that is supplied to each of the partial response receive circuits  888   1 - 888   M . Multiple sampling clock signals may be generated and shared among the partial response receive circuits  888   1 - 888   M  in multi-data rate embodiments. By sharing the circuitry for generating control thresholds and/or the circuitry for generating sampling clock signals, the amount of circuitry within the input circuit  895  is substantially reduced relative to input circuit  885 , reducing production and operational cost of the host integrated circuit device (i.e., due to reduced die area consumption, layout complexity, test and verification time, power consumption, etc.). Note that, in an embodiment in which the sampling clock signal is provided by circuitry other than clock data recovery circuitry (e.g., external clock source, internal clock generated), the edge sampler  891  and clock data recovery circuit  892  may be omitted. Alternatively, programmable selection of the clock source may be used to select either the recovered sampling clock (i.e., recovered by operation of the edge sampler  891  and clock data recovery circuit  892 ) or another clock source to provide a sampling clock signal to the partial response receive circuits  888   1 - 888   M . As in the embodiment of  FIG. 39 , the signal lines  893   1 - 893   M  may be used to deliver distinct serial transmissions, or transmission of related groups of bits (e.g., the M bits received during a given symbol time representing one or more multi-bit digital values) as, for example, in the case of a parallel bus or any other signaling system in which the threshold control values and/or timing control signals are expected to be substantially similar (e.g., substantially same thresholds and/or clock phases) across multiple signaling paths. Also, while a single level sampler  889  and corresponding adaptive module  890  are depicted in  FIG. 40  as generating threshold control values C α ,/C α  and C 0  for each of the partial response receive circuits  888   1 - 888   M , in an alternative embodiment, multiple level sampler/adaptive module pairs may be used to generate threshold control values for respective subsets of the partial response receive circuits  888   1 - 888   M . Respective edge sampler/clock data recovery circuit pairs may similarly be used to generate sampling clock signals for corresponding subsets of partial response receive circuits  888   1 - 888   M . 
         [0158]    Still referring to  FIG. 40 , the partial response receive circuits  888   1 - 888   M  may be implemented by any of the embodiments described herein including, without limitation, the embodiments described as described in reference to  FIGS. 17-19 . Also, in embodiments in which a preamplifier (e.g., preamplifier  401  of  FIG. 17 ) or offset control circuit (e.g., offset control circuit  440  of  FIG. 19 ) is used to adjust the effective threshold of a differential comparator above or below a common mode, such preamplifier or offset control circuit may be shared among multiple partial response receive circuits  888 , thereby reducing the circuitry within the partial response receive circuits  888   1 - 888   M  to that of a sampling circuit (e.g., sampler  425  of  FIG. 17  or sampler  452  of  FIG. 19 ). Respective preamplifiers (or offset control circuits) may also be shared among subsets of the partial response receive circuits  888   1 - 888   M . 
         [0159]    Transmit-Side Equalization in System with Partial Response Receiver 
         [0160]      FIG. 41  illustrates a signaling system  950  having a transmit device  951  and receive device  953  coupled to one another via a signaling path  122 . The transmit device  951  includes an equalizing transmit circuit  957  and tap control logic  955 , and the receive device  953  includes a partial response receive circuit (which may, for example, be a multi-mode circuit or any other of the partial response receive circuits disclosed herein), level sampler  973  and adaptive module  975 . Both the transmit device  951  and receive device  953  may include numerous other circuits not shown in  FIG. 41  (e.g., application logic, additional transmit circuits and/or receive circuits, etc.). The equalizing transmit circuit  957  includes a number of output drivers  961   0 - 961   N−1  to drive a symbol onto signaling path  122  in response to a transmit timing signal (not shown) and in accordance with a set of weighting values W 0 -W N−1  and the state of data values D 0 -D N−1 . Each of the data values D 0 -D −(N−1)  is stored within a shift register  959  (or similar storage circuit) and includes a number of constituent bits in accordance to the number of bits encoded into each symbol transmission on the signaling path  102 . For example, in a 4-PAM system, each of the data values, D 0 -D −(N−1) , includes two constituent bits. More generally, in a M-PAM system, each of the data values includes log 2 (M) constituent bits. After each symbol transmission, the contents of the shift register  959  are shifted forward so that a new data value is shifted into position D 0 , and the data value at position D −(N−1)  is overwritten by the data value previously at position D −(N−2) . 
         [0161]    The weighting values W 0 -W N−1  are generated by the tap control logic  955  and are used to establish the signal drive strength of each of the output drivers  961 . In one embodiment, the output driver  961   0  drives the symbol to be transmitted during a given symbol time in accordance with weighting value W 0 , and the output drivers  961   1 - 961   N−1  constitute a set of post-tap drivers for equalizing the output signal according to the post-tap data values stored within positions D −1 -D −(N−1)  of the shift register  959  and the weighting values W 1 -W N−1 . Though not shown in  FIG. 41 , pre-tap output drivers may also be provided to equalize the output signal according to pre-tap data values (and pre-tap weighting values), and yet other output drivers may be provided to cancel cross-talk from neighboring signal lines. Also, one or more of the output drivers  961  may be dynamically or statically allocated between pre-tap equalization, post-tap equalization and cross-talk cancellation functions, for example, through configuration register settings or externally received configuration control signals. 
         [0162]    The partial response receive circuit  971  includes R compare circuits to generate respective partial response values according to the number of expected partial response signal levels. For example, in a system in which the least-latent symbol (i.e., the most recently received symbol, N-1-i, relative to incoming symbol N-i) is the only expected contributor to the partial response, two compare circuits are provided to generate partial response values according to the two possible states of least-latent symbol (i.e., R=2). In a system in which the K most recently received symbols are expected to contribute to the partial response, 2 K  compare circuits are provided to generate respective data samples based on comparisons with the K partial response threshold levels (C PR ) generated by the adaptive module  975 . 
         [0163]    In one embodiment, the tap control logic  955  of transmit device  951  generates the weighting values W 0 -W N−1  in accordance with the number of symbols desired to contribute to the partial response observed at the receive device  953 . Referring to  FIG. 42 , for example, in a system in which the partial response is to be limited to the least latent symbol, D −1  (i.e., K=1), the tap control logic  955  generates weighting values W 0 -W N−1  to equalize the dispersion-type ISI produced by symbol transmissions after D −1  and prior to D 0 . That is, referring to the raw pulse response shown in  FIG. 42 , the partial response of the signaling path  122  to symbols D +1 , and D −2  to D −(N−1)  is equalized by appropriate settings of the weighting values applied to a pre-tap output driver (not shown in  FIG. 41 ) and post-tap output drivers  961   2 - 961   N−1 . By this arrangement, the equalized signal observed by the receive device  953  (i.e., equalized for partial response as depicted in  FIG. 42 ) includes a contribution from the symbol to be received, D 0 , and a partial response to the immediately preceding symbol, D −1 , with the partial response to all other transmissions being attenuated (e.g., to a negligible or substantially reduced level). In alternative embodiments, the tap control logic  955  may enable partial response to one or more additional symbols to remain in the signal observed by the receive device  953  depending on the desired number of partial response contributors. Also, in a multi-mode circuit in which a partial response receive circuit may optionally be operated in a non-partial response mode, the tap control logic  955  may generate weighting values W 0 -W N−1  as necessary to cancel (or reduce) the partial response to post-tap and pre-tap data values. 
         [0164]    Still referring to  FIG. 41 , the level sampler  973  within the receive device  953  operates generally as described above (e.g., in reference to the various embodiments described in reference to  FIGS. 14-15 and 20-21 ) to compare the incoming signal with a data level threshold and generate error samples (E DLEV ) that are used to adjust the data level threshold(s), C DLEV , and the partial response thresholds, C PR . As discussed above, the data level threshold and/or the partial response thresholds, C PR , may be digital values that are used to establish the desired threshold levels within compare circuits of the partial response receive circuit  971  and the level sampler  973 , or any other types of control values (e.g., analog voltage and/or current levels that are themselves the threshold levels). 
         [0165]    As discussed above in reference to  FIG. 15 , the adaptive module may include filtering circuitry to filter the error samples generated by the level sampler  973  such that the threshold levels generated by the adaptive module are updated based on multiple error samples, rather than on a sample-by-sample basis. For example, in one embodiment, the adaptive module includes a finite impulse response (FIR) filter to update the threshold levels based on a succession of error samples applied to the filter. Alternatively, the adaptive module may include an infinite impulse response filter (IIR) or any other type of filter for filtering the error samples. 
         [0166]    In one embodiment, the data level error samples, E DLEV , generated by the level sampler  973  are returned to the transmit device  951  via a back channel  968  (e.g., out-of-band signaling, passage via an intermediary device such as the controller  141  of  FIG. 3 , or other communication path that does not consume otherwise usable bandwidth on the signaling path  122 ). The error samples are received within the tap control logic  955  and used to adjust the tap weights, W 0 -W N−1 , as necessary to reduce ISI (or other sources of distortion) resulting from transmission of symbols other than symbols desired to contribute to the partial response. Referring to  FIG. 43 , for example, after the data level threshold generated by the adaptive module  975  converges to a nominal data level  985  (DLEV NOM ), further jitter detected in the signal data level  986  may be treated as residual ISI due resulting from transmission of symbols other than symbols desired to contribute to the partial response. The tap control logic may increase and decrease the weights of the post-tap output drivers and/or pre-tap output drivers as necessary to reduce the residual ISI. In alternative embodiments, updates to the tap weights themselves (e.g., values to be added and/or subtracted from weights W 0 -W N−1  or a subset thereof) may be determined within the receive device (or another device such as controller  141  of FIG.  3 ) and provided to the transmit device for application within the tap control logic  955 . In yet other embodiments, one or more replacement tap weights may be determined within the receive device (or other device such as controller  141  of  FIG. 3 ) and provided to the transmit device for application in place of corresponding tap weights W 0 -W N−1 . 
         [0167]    Section headings have been provided in this detailed description for convenience of reference only, and in no way define, limit, construe or describe the scope or extent of such sections. Also, while the invention has been described with reference to specific embodiments thereof, it will be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.