Abstract:
A transceiver comprises a first interface to receive a first signal, through a first channel, from a memory device. A transmitter transmits a second signal that represents the first signal, through a second channel, to a master device. A plurality of registers stores a plurality of values provided by the master device. The plurality of values includes a first value that specifies a transmit timing adjustment to the second signal to transmit to the master device by the transmitter.

Description:
This is a continuation of U.S. patent application Ser. No. 10/699,116 filed on Oct. 31, 2003; which is a continuation of U.S. patent application Ser. No. 09/458,582 filed on Dec. 9, 1999 (now U.S. Pat. No. 6,643,752). 

   FIELD OF THE INVENTION 
   The present invention relates to communication systems, and more particularly to a communication path that includes one or more latency-aligning transceivers. 
   BACKGROUND 
     FIG. 1  illustrates a prior art memory system that includes multiple integrated circuit memory devices  120  coupled to a memory controller  110  via a bidirectional communication channel  140 . Because each memory device  120  consumes physical space along the channel, the number of memory devices that can be coupled to the channel  140 , and to some extent the storage capacity of the memory system, is limited by the length of the channel  140 . The length of the channel  140  is itself limited by a number of practical considerations. For example, signals attenuate as they propagate down the channel  140 , constraining the channel length to one that provides a tolerable signal level at the memory IC farthest from the controller  110 . Similarly, channel capacitance increases with channel length, limiting the frequency response of the channel. Accordingly, the channel length usually must be limited to support the desired operating frequency of the memory system. 
   One technique for increasing the number of memory devices that can be used in a memory system without unacceptable loss in signaling margin or frequency response is to use buffering circuits to segment the communication path into multiple smaller channels. Unfortunately, buffers add latency that can be problematic, particularly in synchronous memory systems which rely on deterministic timing relationships. For example, in some memory systems, memory operations are pipelined by transmitting commands in the intervening time between transmission of an earlier command (e.g., a read command) and responsive transmission of the corresponding data (e.g., the read data). When buffers are positioned along the channel&#39;s length, however, the time intervals between command and response transmissions vary arbitrarily depending on the positions of the addressed memory devices (i.e., memory devices positioned downstream from one or more buffers or repeaters exhibit greater effective response delay than memory devices coupled directly to the memory controller). This significantly complicates command pipelining. 
   Thus, it is desirable to provide a memory subsystem that can support a large number of memory devices without degrading the reliability and performance of the memory system. 
   SUMMARY 
   A memory system including one or more transceivers with latency alignment circuitry is disclosed in various embodiments. The memory system includes a communication path that is segmented into a primary channel and one or more stick channels by appropriate placement of the latency aligning transceivers. In one embodiment, the transceivers buffer clock, control and data signals while also aligning the latency in the round-trip path between the memory controller and the stick channel driven by the transceiver to a clock cycle boundary. When memory devices that have adjustable response delays are coupled to the different stick channels in the memory system, the memory system can be configured so that the total response latency is substantially the same for each memory IC in the memory system. This simplifies command pipelining significantly, permitting commands to be packed densely within the available channel bandwidth. As discussed below, stick channels themselves can feed one or more additional transceivers, making any number of interconnection topologies possible. 
   These and other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
       FIG. 1  illustrates a prior art memory system. 
       FIG. 2  is a block diagram of a memory system according to one embodiment of the present invention. 
       FIG. 3A  is a timing diagram of a data transfer operation in the memory system of  FIG. 2 . 
       FIG. 3B  is a timing diagram of the data transfer from a master device to a memory device. 
       FIG. 3C  is another timing diagram of a data transfer from the master device to a memory device. 
       FIG. 4  illustrates the response latency of a memory transaction according to one embodiment. 
       FIG. 5  illustrates the scaleability of a memory system according to one embodiment. 
       FIG. 6  is a block diagram of a transceiver according to one embodiment. 
       FIG. 7  illustrates the synchronization and transceiver logic of a transceiver  220  according to one embodiment. 
       FIG. 8  is a diagram of a transceiver that includes circuitry for preventing a latch-up condition. 
   

   DETAILED DESCRIPTION 
     FIG. 2  is a block diagram of a memory system  200  according to one embodiment of the present invention. The memory system  200  includes a master device  210  (e.g., a memory controller) coupled to a plurality of memory devices  260 A– 260 I via a communication path formed by a primary channel  215  and stick channels  275 A– 275 D. In one embodiment, the master device, transceivers and memory devices transmit signals on the communication path through current-mode signaling. That is, each conductor in a given channel  275 A– 275 D is pulled up to a predetermined voltage level through a termination impedance and may be driven to at least one lower voltage level by sinking an appropriate amount of current. Although the termination impedances are depicted in  FIG. 2  as being coupled to the ends of the channels  275 A– 275 D, the termination impedances may alternatively be placed at any point along their respective channels, including within the master device  210 , or within a transceiver or memory device coupled to the channel. 
   In an alternative embodiment, voltage mode signaling may be used in which the master device, transceivers and memory devices output digital voltage levels to the bus to effect digital signaling. In voltage mode embodiments, the bus may be allowed to float or the bus may be pulled up or down through termination impedances. 
   In the embodiment of  FIG. 2 , a clock generator  230  generates a clock signal  240  called clock-to-master (CTM) that propagates toward master device  210 . A second clock signal  250 , preferably having the same frequency as CTM  240 , propagates away from the master device  210  and is called clock-from-master (CFM). CTM  240  is used to clock the transmission of information to master device  210  on the primary channel  215 , while CFM  250  is used to clock transmission of information from the master device  210  to memory device  260 A and transceivers  220 A and  220 B. Together CTM and CFM provide for source synchronous transmission of data (i.e., data travels with clock) in both directions on the primary channel  215 . In one embodiment, CTM  240  and CFM  250  are the same signal, with the conductors that carry CFM  250  and CTM  240  being coupled to one another at or near the master device  210  (e.g., within the master device  210 , at a pin of the master device  210  or at another point just outside the master device  210 ). In alternative embodiments, clock signals CTM  240  and CFM  250  may be separately generated. For example, master device  210  may include a clock generator circuit that generates CFM  250  in a predetermined phase relationship to CTM  240 . 
   Regardless of whether CTM  240  and CFM  250  are the same signal or separately generated, CTM  240  and CFM  250  will have a different phase relationship at different points along the primary channel due to the fact that they are traveling in different directions. For example, if CFM and CTM are in phase at master device  210 , then at transceiver  220 B, they will be out of phase by the amount of time it takes for CTM  240  to travel from the transceiver  220 B to the master  210  plus the time it takes for CFM  250  to travel from the master  210  to the transceiver  220 B. This phase difference between CTM and CFM, referred to herein as t TR , is different at each point along the primary channel. 
   Each of transceivers  220 A– 220 C serves as a bi-directional repeater between a host channel (i.e., a channel used to deliver signals from the master device  210 ) and at least one stick channel. More specifically, transceiver  220 B serves as a bi-directional repeater between host channel  215  (the primary channel) and stick channel  275 C; transceiver  220 C serves as a bi-directional repeater between host channel  275 C and stick channel  275 D; and transceiver  220 A serves as a bi-directional repeater between host channel  215  and each of stick channels  275 A and  275 B. In one embodiment, each of the transceivers  220 A– 220 D provides regenerative gain and drive capability and resynchronizes signal transmissions between the clock domain of the host channel and the stick channel. It should be noted that the channel topology depicted in  FIG. 2  is merely an example—numerous alternative channel topologies may be constructed without departing from the spirit and scope of the present invention. 
   By using transceivers  220 A– 220 D to segment the overall communication path into multiple segments, the resistive and capacitive loading of any given length of the communication path may be kept below a tolerable threshold. This permits the communication path to be extended to support more memory devices without unacceptable loss of signal margin due to resistive or capacitive loading. 
   Although each of transceivers  220 A– 220 C is shown in  FIG. 2  as supporting one or two stick channels, a given transceiver may support any number of stick channels up to a practical limit. Also, though the primary channel  215  and stick channels  275 A– 275 D are each shown as supporting one or two memory devices, more memory devices may be supported by the channel segments in alternate embodiments. Similarly, any number of transceivers up to a practical limit may be hosted by a given channel segment. 
   In one embodiment, each of the transceivers uses the clock signals that correspond to its host channel to generate one or more clock signals for the stick channel (or channels) that it serves. For example, transceiver  220 B generates a clock signal “clock-to-end” (CTE)  270 C based on clock signals CTM  240  and CFM  250 . CTE  270 C is folded back at the end of stick channel  275 C to provide clock signal “clock-to-transceiver” (CTT)  280 C, which in turn is used to generate clock signal “clock-from-transceiver (CFT)  290 C. Similarly, transceiver  220 C generates clock signals CTE  270 D, CTT  280 D and CFT  290 D based on clock signals CTT  280 C and CFT  290 C, and transceiver  220 A generates clock signals CTE  270 A, CTT  280 A, CFT  290 A, CTE  270 B, CTT  280 B and CFT  290 B from clock signals CTM  240  and CFM  250 . 
   The relationship between CTM  240  and CFM  250  described above applies to the clock signals CTT and CFT generated for each stick channel. For example, in the embodiment of  FIG. 2 , CTT and CFT for a given stick channel are the same signal, with their respective conductors being coupled together at or near the transceiver for the stick channel (e.g., within the transceiver, at a pin of the transceiver or at another point just outside the transceiver). In alternative embodiments, CTT and CFT may be separately generated. For example, a given transceiver may include a clock generator circuit that generates CFT in a predetermined phase relationship to CTT. 
   Regardless of whether CTT and CFT are the same signal or separately generated, CTT and CFT will have a different phase relationship at different points along the stick channel they serve. This phase difference between CTT and CFT for a given stick channel is analogous to the phase difference, t TR , between CTM  240  and CFM  250  discussed above, and is referred to herein as t-stick TR . As discussed below, transceivers  220 A– 220 D perform a latency alignment function by adjusting the transfer latency from host channel to stick channel according to the phase difference between the host channel&#39;s clocks (i.e., t TR  when the host channel is the primary channel  215  and t-stick TR  when the host channel is a stick channel). 
   In one embodiment, the CFT and CTT clocks on stick channels (stick clocks) are synchronized to CTM  240  on the primary channel  215 . Requests/commands from the master device  210  are received with CFM and resynchronized to CFT for retransmission on the stick channel. This timing relationship is discussed below in further detail. 
     FIG. 3A  is a timing diagram of a data transfer operation in the memory system  200  of  FIG. 2 . More specifically,  FIG. 3A  illustrates the timing of a data transfer from memory device  260 G to master device  210 . Data C is available on stick channel  275 C at the falling edge of StickClk  330 . In the embodiment shown, TxClk  320  is the equivalent of CTM  240  and StickClk  330  is 180 degrees out of phase with TxClk  320 . Data C is transferred onto the primary channel  215  at the second falling edge of TxClk  320  at time T 2 . The overall propagation delay from the primary channel  215  to the stick channel  275  (i.e., the latency incurred crossing transceiver  220 B) is t LAT(SP) . In the embodiment shown, t LAT(SP)  is 1.5 clock cycles in duration. 
     FIG. 3B  illustrates the timing of a data transfer in the opposite direction—from master device  210  to memory device  260 G. The primary channel  215  has data A on it at a first time, at a falling edge of RxClk  310 . For one embodiment, RxClk  310  is equivalent to CFM  250 . CFM  250  lags CTM  240  by time t TR  so that RxClk  310  lags TxClk  320  by time t TR . As discussed above, time t TR  is twice the time of flight down the bus, which is the difference in phase between CTM and CFM at the pin of the slave device (transceiver). Generally period t TR  should be less than one cycle (e.g. 0.8 t CYCLE ), otherwise the timing relationship may be confusing (i.e. 2.2 cycles looks just like 0.2 cycles). In alternative embodiments, circuitry for tracking multiple cycles may be used so that t TR  need not be limited to less than a clock cycle. 
   At the falling edge of RxClk  310 , data A is available to the transceiver. For one embodiment, transceiver latches data A at this time. The data A is available on the stick channel  275 C on the falling edge F of stick clock  330 , after the rising edge  2 R. The overall propagation delay from the primary channel  215  to the stick channel  275 C is t LAT(PS) . 
     FIG. 3C  is a timing diagram of a data transfer from the master device  210  to the memory device  260 G when t TR  is relatively large (e.g., 0.8 tcycle). As shown, data B is available on primary channel  215  at a falling edge of RxClk  310  and then on the stick channel  275 C at time T 2 , the first falling edge after the second rising edge  2 R of StickClk  330 . The overall propagation delay from the primary channel  215  to the stick channel  275  is t LAT(PS) . 
   Referring to  FIGS. 3B and 3C , it can be seen that the transfer latency from primary channel to stick channel (t LAT(PS) ) is dependent upon the time t TR . More specifically, t LAT(PS)  is given by a predetermined number of clock cycles less the round trip time on the channel between the transceiver and the master device, t TR . In an embodiment having the timing characteristic shown in  FIGS. 3B and 3C , the latency incurred crossing the transceiver in the direction of the stick channel may be expressed mathematically as t LAT(PS) =2.5 cycles−t TR . Accordingly, when t TR  is larger, t LAT(PS)  is smaller (compare  FIGS. 3B and 3C ). Thus, the transceiver  220 B effectively adjusts the time delay to repeat signals from the primary channel  215  on the stick channel  275 C to compensate for the flight time down the primary channel in each direction. The result of this compensation is that the roundtrip latency between the master device and a stick channel (not counting t-stick TR  or the latency required for the target memory device to respond) is aligned to a clock cycle boundary. Said another way, the round-trip latency between the master device and a stick channel is independent of the distance on the primary channel between the transceiver and the master device  210 . 
     FIG. 4  illustrates the response latency of a memory transaction in greater detail. As shown, the overall response latency perceived by the master device is made up of the following latencies: 
   
     
       
             
             
             
           
         
             
                 
             
           
           
             
               1. 
               Flight time on primary channel 215 from 
               0.5t TR   
             
             
                 
               master device 210 to transceiver 220 
             
             
               2. 
               Time to cross transceiver 220 from primary 
               t LAT(PS)  = 
             
             
                 
               channel 215 to stick channel 275 
               (X cycles) − t TR   
             
             
               3. 
               Flight time on stick channel from 
               0.5tstick TR   
             
             
                 
               transceiver 220B to memory device 260G 
             
             
               4. 
               Response latency of memory device 
               t DEVLAT   
             
             
               5. 
               Flight time on stick channel from memory 
               0.5tstick TR   
             
             
                 
               device 260G to transceiver 220B 
             
             
               6. 
               Time to cross transceiver 220 from stick 
               t LAT(SP)  = Y cycles 
             
             
                 
               channel 275 to primary channel 215 
             
             
               7. 
               Flight time on primary channel 215 from 
               0.5t TR   
             
             
                 
               transceiver 220B to master device 210 
             
             
                 
               Total 
               (X + Y) cycles + 
             
             
                 
                 
               t-stick TR  + t DEVLAT   
             
             
                 
             
           
        
       
     
   
   Note that, because the time to cross the transceiver  220  from primary channel  215  to stick channel  275  is compensated to account for the round trip flight time on the primary channel (t TR ), the primary channel flight time does not appear in the expression for total latency. More specifically, the round-trip latency between the master device  210  and the stick channel  275  (i.e., node N) is equal to X+Y cycles. By selecting X and Y to add to a whole number of clock cycles, the round-trip latency between the master device  210  and the stick channel  275  is effectively aligned with a clock for the primary channel (CTM  240  in the embodiment of  FIG. 2 ). That is, the round-trip time from the master device  210  to a given stick channel is aligned on a clock cycle boundary. As discussed below, this latency alignment simplifies timing in the memory system significantly, allowing more efficient bandwidth utilization on the primary channel and stick channels than is achieved with the above-described prior art techniques. Referring to  FIG. 2 , for example, by choosing X to be 2.5 clock cycles and Y to be 1.5 clock cycles (the timing shown in  FIGS. 3A and 3B ), the roundtrip latency between master device  210  and any one of stick channels  275 A,  275 B and  275 C is aligned with every fourth clock cycle of CTM  240 . Consequently, the master device  210  may use the four clock cycles which follow a transmission to any of memory devices  260 B– 260 I to transmit or receive other information on the primary channel  215 . 
     FIG. 5  illustrates the scaleability of the above-described latency alignment technique and the manner in which programmable latency registers may be used in conjunction with latency-aligning transceivers to establish a flat response latency over an entire memory system. Memory system  700  includes a number of transceivers (T 1 –T 5 ) that each serve as bi-directional repeaters for respective stick channels ( 775 A– 775 E). Transceivers T 1 , T 3  and T 5  are each coupled to the primary channel  715  and include latency alignment circuitry that aligns the round-trip latency between the master device and stick channels  775 A,  775 C and  775 E, respectively, to an integer number of clock cycles, N. Transceivers T 2  and T 4  are hosted by stick channels  775 A and  775 C, respectively, and include latency alignment circuitry that aligns the round-trip latency between the respective masters (T 1  and T 3 ) for their host channels and stick channels  775 B and  775 D to the integer number of clock cycles, N. In one embodiment, N is equal to four so that the round-trip latency between master device  210  and stick channel  775 A is four clock cycles and the round-trip latency between master device  210  and stick channel  775 B is eight clock cycles. More generally, the latency from the master device  210  to a given stick channel is M×N, where M is the number of transceivers that must be crossed to reach the stick channel, and N is the latency-aligned, round-trip time from a master of a given host channel to a stick channel that is coupled to the host channel through a single transceiver. 
   Note that no matter how many transceivers must be crossed in the memory system of  FIG. 5 , the overall round-trip time between master device  210  and any stick channel in the memory system is aligned with the transmit clock of master device  210  (e.g., CFM  250  in  FIG. 2 ). This enables construction of memory systems having large numbers of memory devices (“MEM” in  FIG. 5 ) without loss of determinism in system timing. The intervals between command and response transmissions are well defined and may therefore be used for command and response pipelining. 
   Another benefit of the above-described latency-aligning tranceivers is that they may be used in conjunction with programmable-latency memory devices to provide a memory system with flat latency response. That is, the response latency of all memory devices may be made substantially equal, regardless of their proximity to the master device  210 . Referring to  FIG. 5 , for example, memory devices hosted by stick channels  775 A,  775 C and  775 E may be programmed to delay their outputs by four clock cycles so that the overall response latency for all memory devices in the memory system is substantially equal (with sub-clock cycle variance due to relative positions of memory devices on their stick channels). Expressed analytically, the total response delay perceived by the master device  210  is:
 
(N×M)+t-stick TR +t DEVLAT +t DEV     —     PROG ,
 
   where t DEV     —     PROG  is the number of additional cycles of delay programmed within a given memory device, M is the number of transceivers that must be crossed to reach the stick channel that hosts the target memory device, and N is the latency-aligned, round-trip time from a master of a host channel to a stick channel coupled to the host channel through a single transceiver. Thus, to provide a flat response latency throughout the memory system, the delay time (t DEV     —     PROG ) for each memory device in the memory system may be set as follows: 
   
     
       
             
             
             
           
         
             
                 
                 
             
             
                 
               No. Transceivers Separating Memory 
                 
             
             
                 
               Device From Master Device 210 
               t DEV     —     PROG   
             
             
                 
                 
             
           
           
             
                 
               M 
               0 
             
             
                 
               M-1 
               N 
             
             
                 
               M-2 
               2N 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               1 
               (M-1) × N 
             
             
                 
               0 
               M × N 
             
             
                 
                 
             
           
        
       
     
   
   In this way, the total response latency will be substantially the same for each memory device in the memory system, regardless of the number of memory devices or stick channels in the memory system. 
     FIG. 6  is a block diagram of a transceiver according to one embodiment. The transceiver  220  receives the CTM  240  and CFM  250  clock signals from the master device. The transceiver  220  further receives host channel  410 . Host channel  410  transmits address and data information from the master device to the transceiver  220 . For one embodiment, host channel  410  is a parallel bus, having multiple conductors. For another embodiment, host channel  410  is a serial communication path. For another embodiment, host channel  410  may include multiple buses, such as an address bus and a separate data bus, or even multiple control paths. 
   The transceiver  220  acts as a slave device toward the master device  210  and includes a slave interface  420  to receive data and control signals from the master device via host channel  410 . To the master device, the transceiver  220  appears to be a memory device. Requests from the master device arrive at the transceiver in the CFM  250  timing domain, and responses are sent back to the master in the CTM  240  timing domain. The master device  210  does not need to be modified to interact with the transceiver. 
   On the stick channel  490 , the transceiver  220  functions as a master device, providing a master interface  430  to retransmit the requests/commands from the master device to the memory devices (or transceivers) coupled to stick channel  490 , and to forward responses from the memory devices to the master device via the slave interface  420  and host channel  410 . The memory devices perceive no difference in system operation resulting from the presence of transceiver  220  and therefore require no design modification. 
   The transceiver  220  provides the clock-from-transceiver (CFT)  290  and clock-to-transceiver (CTT)  280  signals to the memory devices and transceivers coupled to channel  490 . In one embodiment, CTE  270  is routed to the end of the stick channel where it is folded back to provide CTT  280 . As discussed above, CTT  280  is folded back away from the transceiver  220  to provide CFT  290 . 
   Data is transmitted to devices coupled to stick channel  490  in the CFT  290  clock domain and received from devices coupled to stick channel  490  in the CTT  280  clock domain. 
   For one embodiment, the transceiver  220  includes a stick transceiver  440  and a host transceiver  450 . The stick transceiver  440  transmits and receives data on the stick channel  490 . The host transceiver  450  transmits and receives data on the host channel  410 . 
   The transceiver  220  further includes a first synchronizing unit  460 . The synchronizing unit  460  synchronizes data transmitted from the memory channel to the stick channel to the CFT  290 . For one embodiment, the transceiver  220  may also include a second synchronizing unit  470  for synchronizing signals transmitted from the stick channel  490  to the host channel  410  with CTM  240 . For one embodiment, the second synchronizing unit  470  may be omitted if the CTT clock is synchronized with one of the clocks on the memory channel (e.g., in an embodiment in which the stick clocks CTT and CFT are synchronized with CTM  240 ). 
   The transceiver  220  further includes an isolation unit  480  that operates to prevent the transceiver  220  from repeating signals onto either the host channel  410  or the stick channel  490 . For one embodiment, the isolation unit  480  asserts an isolate signal  595  to force both sets of bus driver circuits into a high-impedance (non-driving) state. Using the isolate feature, the transceiver  220  can effectively split a memory system into two partitions. In normal operation (not isolated), the transceiver  220  passes packets between the two partitions and the channel functions normally. When the transceiver&#39;s isolation unit  480  is enabled, the two partitions become electrically isolated and, if desired, each individual section can operate independently. This may be advantageous in certain graphics applications, for example with a frame buffer and normal (code and data) DRAMs sharing a single channel partitioned by a transceiver. 
   The transceiver  220  further includes a power logic  485  for turning off the transceiver  220  when it does not need to transmit. In one embodiment, power logic  485  merely turns off the stick transceiver  440 , so that signals received via host channel  410  are not retransmitted on stick channel  490 . Circuitry may be provided to interpret incoming addresses to determine whether they decode to memory devices coupled to stick channel  490  (or downstream stick channels). Stick transceiver  440  may then be selectively enabled and disabled depending on whether memory devices coupled to stick channel  490  are being addressed. For example, if a certain amount of time passes (or transactions detected) without memory devices coupled to stick channel  490  being addressed, power unit  485  may disable stick transceiver  440  to save power. Alternatively, transceiver  220  may power down stick transceiver  440  and other circuitry within transceiver  220  in response to a power-save command received on the host channel  410 . Also, in alternative embodiments, transceiver  220  may remain fully enabled at all times and power unit  485  may be omitted altogether 
   For one embodiment the transceiver  220  does not interpret incoming transmissions on the host channel and therefore does not respond to commands. That is, the transceiver  220  cannot be “addressed” by a master device (e.g., device  210  of  FIG. 2 ). Consequently, in this embodiment the transceiver  220  does not include registers which may be read or written by a master device. In alternative embodiments, the transceiver  220  include command interpretation circuitry for parsing packetized commands or other transmissions received on the host channel. In these embodiments, the transceiver  220  may perform timing adjustments or other operations in response to commands from a master device. For example, the transceiver  220  may perform output driver calibration or other signal parameter calibration operations in response to commands from the master device. Also, instead of calibration, the transceiver  220  may receive control parameters from the master device and install them in appropriate registers to provide master-specified signal adjustments (e.g., adjustments to slew rate, drive strength, receive and transmit timing, equalization, reference voltage adjustment, clock duty cycle correction and so forth). Moreover, as discussed above, the transceiver  220  may enter a power-saving state in response to commands received on the host channel. 
     FIG. 7  illustrates the synchronization and transceiver logic of a transceiver  220  according to one embodiment. The transceiver  220  receives a host channel  570  that couples the transceiver  220  to a master device along with signal lines for clock signals CTM  240  and CFM  250 . Though not shown, the transceiver  220  may also include isolation circuitry and power saving circuitry as described above in reference to  FIG. 6 . 
   The transceiver  220  also receives signal lines for clock signals CTE  580 , CTT  585  and CFT  590  along with a stick channel  575  that couples the transceiver  220  to memory devices and/or other transceivers. 
   The transceiver  220  includes a phase locked loop (PLL)  510  which performs a clock recovery function, generating a buffered output  512  in phase alignment with CFM  250 . This recovered version of CFM  250  is input to the primary receiver  515  where it is used to time reception of signals from the host channel  570 . The transceiver  220  also includes PLL  525  to generate a recovered version of CTM  240  (i.e., buffered output  527 ) for clocking primary transmitter  520 . A PLL  550  is used to generate CTE  580  for the stick channel such that CTT  585  arrives at the transceiver 180 degrees out of phase with CTM  240 . This inverted version of CTM  240  is designated “stick clock” in  FIG. 7 . PLL  545  is also used to generate a clock signal  529  that is 180 degrees out of phase with CTM  240  (i.e., in phase with the stick clock) for clocking the secondary receiver  540 . The 180 degree phase offset between CTM  240  and the stick clock permits the latency between reception of signals in secondary receiver and retransmission of the signals at the primary transmitter  520  to be aligned on half-clock cycle boundaries (e.g., 1.5 clock cycles as shown in  FIG. 3A ). 
   Because transceiver  220  receives data from the host channel  570  in response to edges of CFM  250  and then retransmits the data on the stick channel in response to edges of CTM  240 , the time required to cross the transceiver in the direction of the stick channel (t LAT(PS) ) is compensated by the amount of time by which CFM  250  lags CTM  240 . That is, t LAT(PS)  is equal to the number of cycles of CTM  240  that transpire during the transceiver crossing, less t TR . By contrast, data crossing the transceiver in the direction of the host channel  570  is both received and retransmitted in response to clock edges aligned with edges of CTM  240  (StickClk being an inverted version of CTM  240 ). That is, t LAT(SP)  is equal to the number of cycles of CTM  240  consumed crossing the transceiver without compensation for t TR . This asymmetry between t LAT(PS)  and t LAT(SP)  results in a bidirectional transceiver crossing time that includes compensation for t TR , thus causing the round-trip latency between the master device and a given stick channel to be aligned to the CTM  240  clock. 
   Transceiver  220  also includes a re-timing circuit  530  that delays the data transfer between the primary receiver  515  and the secondary transmitter  535  when t TR  becomes so small that half clock cycle boundary may be crossed. More specifically, re-timing circuit  530  determines the phase difference (t TR ) between the recovered versions of CTM  240  and CFM  250  and selects between a delayed and a non-delayed path for transferring data from primary receiver  515  to secondary transmitter  535 , ensuring that the overall t LAT(PS)  is a fixed number of clock cycles less t TR . 
     FIG. 8  is a diagram of a transceiver that includes circuitry for preventing a latch-up condition. Latch-up occurs when data received from a first channel and transmitted to the second channel is detected on the second channel, and promptly retransmitted to the first channel. This feedback latches the device into a state. 
   Portions of the transceiver have been omitted from  FIG. 8  for simplicity. Only the primary receiver  515 , primary transmitter  520 , secondary transmitter  535 , secondary receiver  540 , and re-timer  530  are shown. 
   A latch-up prevention logic  610  is placed between primary receiver  515  and primary transmitter  520 . A similar latch-up prevention logic  620  is placed between secondary transmitter  535  and secondary receiver  540 . The latch-up prevention logic  610  receives an input from the primary receiver  515  and from the secondary receiver  540 . The output of the latch-up prevention logic  610  is coupled to a disable logic (DL)  630  in the primary transmitter  520 . Similarly, the latch-up prevention logic  620  receives an input from the secondary receiver  540  and the primary receiver  515 . The output of the latch-up prevention logic  620  is coupled to a disable logic (DL)  640  in the secondary transmitter  535 . Pin  680  is coupled to the host channel  570  (not shown), while pin  690  is coupled to stick channel  575  (not shown). 
   When the primary receiver  515  receives data from the host channel  570 , it sends a disable signal through node  517  to the latch-up prevention logic  610 . The latch-up prevention logic  610  sends a disable signal to the primary transmitter&#39;s disable logic  630 . The disable logic  630  prevents the primary transmitter  520  from transmitting information received from the secondary transceiver  540  for a period of time. The disable signal is also sent to the disable logic (DL)  625  of latch-up prevention logic  620 . The disable signal turns off the latch-up prevention logic  620 . The data received by the primary receiver  515  is transmitted, through the secondary transmitter  535  to the stick channel. When the secondary receiver  540  receives the same data from the stick channel, the latch-up prevention logic  620  is already disabled, preventing the turning off of the secondary transmitter  535 . Furthermore, the primary transmitter  520  is already disabled, preventing the retransmission of the data to the host channel. In this manner, the latch-up prevention logic  610  prevents the system latch up. 
   The latch-up prevention logic  610 ,  620  releases their transmitter,  520  and  535  respectively, after the entire data is transmitted by the primary receiver  515 . 
   Similarly, if data is first received on the stick channel by the secondary receiver, latch-up prevention logic  620  disables secondary transmitter  535  through disable logic  640 . The disable signal further disables latch-up prevention logic  610  through disable logic  615 . Using the above-described latch-up prevention logics, the danger of latch-up is avoided. 
   For one embodiment, the latch-up prevention logic  610  may be implemented as an AND gate and an inverter, such that the output of the secondary receiver  540  is inverted, and coupled as an input to an AND gate. The other input to the AND gate is the logic from the primary receiver  515 . In this way, only when the output of the primary receiver  515  is on, while the output of the secondary receiver  540  is off, does the latch-up prevention logic  610  output its disable signal. 
   Although the exemplary embodiments of latency-aligning receivers and systems and methods for incorporating latency-aligning receivers have been described in terms of memory systems. It will be appreciated that the concepts and principles disclosed are not limited to memory systems, but rather may be applied in any system where it is desirable to increase the number of devices attached to a communication path without overloading the communication path or complicating system timing. More generally, though the invention has been described with reference to specific exemplary 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 as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.