Patent Publication Number: US-10325637-B2

Title: Flexible point-to-point memory topology

Description:
This application relates to U.S. Ser. No. 15/090,818, filed Apr. 5, 2016, which is incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to memory generally and, more particularly, to a method and/or apparatus for implementing a flexible point-to-point memory topology. 
     BACKGROUND OF THE INVENTION 
     Several types of memory modes are used on conventional motherboards, depending on how many dual inline memory modules (DIMMs) are installed. A single-channel (asymmetric) mode provides single-channel bandwidth operations when only one DIMM is installed or when the memory capacities of more than one DIMM are unequal. A dual-channel (interleaved) mode offers higher memory throughput and is enabled when the memory capacities of both channels are equal. A triple-channel mode provides triple-channel interleaving, which can reduce overall memory latency by accessing the DIMM memory sequentially. A quad-channel mode can be enabled when four (or multiples of four) DIMMs with identical capacity and speed are installed. Regardless of the memory mode used, each DIMM can operate only on a single channel. 
     Communication between the DIMMs and the motherboard can be done using differential uni-directional signaling or single-ended bidirectional signaling. When differential uni-directional signaling is used, more pins are needed to meet capacity demands for a given memory technology node, resulting in higher cost. Scaling capacity in point-to-point configurations has a direct impact on bandwidth. This is a disadvantage with respect to fly-by topologies where the bandwidth is constant for one DIMM or Many DIMMs. However, existing technology (channels, I/O&#39;s, etc.) are capable of running double data rate (DDR) data (DQ) signals at significantly higher data rates in point-to-point topologies. 
     It would be desirable to implement a flexible point-to-point memory topology. 
     SUMMARY OF THE INVENTION 
     The present invention concerns an apparatus including a plurality of memory devices and a control circuit. The control circuit may be configured to enable a plurality of access modes for the plurality of memory devices. In a one-channel mode, all of the memory devices are accessed using a single selectable channel. In a two-channel mode, a first portion of the plurality of memory devices is accessed using a first channel and a second portion of the plurality of memory devices is accessed using a second channel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will be apparent from the following detailed description and the appended claims and drawings in which: 
         FIG. 1  is a diagram illustrating a memory system populated using dual-inline-memory modules (DIMMs) in accordance with an example embodiment of the invention; 
         FIG. 2  is a diagram illustrating a two-channel mode of a dual-in-line-memory module (DIMM) in accordance with an example embodiment of the invention; 
         FIG. 3  is a diagram illustrating a single-channel modes of a dual-in-line-memory module (DIMM) in accordance with an example embodiment of the invention; 
         FIG. 4  is a diagram illustrating system memory slots populated by a dual-in-line-memory module (DIMM) operating in the two-channel mode and a bridge card; 
         FIG. 5  is a diagram illustrating system memory slots populated by dual-in-line-memory modules (DIMMs) operating in the single-channel modes; 
         FIG. 6  is a diagram illustrating various communication buses of a dual-in-line-memory module (DIMM) in accordance with an example embodiment of the invention; 
         FIG. 7  is a diagram illustrating a duplex data buffer in accordance with an example embodiment of the invention; 
         FIG. 8  is a diagram illustrating a duplex data buffer in accordance with another example embodiment of the invention; 
         FIGS. 9A-9C  are diagrams illustrating various operating modes of a duplex registered clock driver and a duplex data buffer in accordance with example embodiments of the invention; and 
         FIG. 10  is a diagram illustrating an example implementation of a duplex data buffer in accordance with an example embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the invention include providing a method and/or apparatus for implementing a flexible point-to-point memory topology that may (i) keep signals point-to-point, (ii) reduce signal line loading to enable faster speeds, (iii) use longer burst lengths, (iv) send commands in multiple cycles, (v) reduce command/address signals, (vi) reduce pin count, (vii) utilize duplex registered clock drivers and duplex data buffers, (viii) provide multi-channel bandwidth with one or more memory modules installed, (ix) allow a single memory module to service two channels independently, and/or (x) be implemented as one or more integrated circuits. 
     Referring to  FIG. 1 , a diagram of a memory system is shown in accordance with an embodiment of the present invention. In an example, circuits  100   a - 100   n  may be implemented as memory modules (or boards). For example, the memory modules  100   a - 100   n  may be implemented as dual in-line memory modules (DIMMs). In some embodiments, the memory modules  100   a - 100   n  may be implemented as double data rate fourth generation (DDR4) dual in-line memory modules (DIMMs). The memory modules  100   a - 100   n  may comprise a block (or circuit)  102 , a number of blocks (or circuits)  104   a - 104   n , a plurality of memory devices  106 , and/or various other blocks, circuits, pins, connectors and/or traces. The circuit  102  may be implemented as a duplex registered clock driver (RCD). In an example, the duplex RCD circuit  102  may be implemented as a duplex DDR4 RCD circuit. The circuits  104   a - 104   n  may be configured as duplex data buffers. In an example, the memory devices  106  may be implemented a synchronous dynamic random access memory (SDRAM) devices. The type, arrangement and/or number of components of the memory modules  100   a - 100   n  may be varied to meet the design criteria of a particular implementation. 
     The memory modules  100   a - 100   n  are shown connected to a block (or circuit)  20 . The circuit  20  may implement a memory controller. The memory controller  20  may be located in another device, such as a computing engine. Various connectors/pins/traces  60  may be implemented to connect the memory modules  100   a - 100   n  to the memory controller  20 . In some embodiments, the connectors/pins/traces  60  may comprise a 288-pin configuration. In an example, the memory controller  20  may be a component on a computer motherboard. In another example, the memory controller  20  may be a component of a microprocessor. In yet another example, the memory controller  20  may be a component of a central processing unit (CPU). 
     In an example, some of the connectors/pins/traces  60  may be part of the memory modules  100   a - 100   n  and some of the connectors/pins/traces  60  may be part of the motherboard and/or memory controller  20 . The memory modules  100   a - 100   n  may be connected to a computer motherboard (e.g., by pins, traces and/or connectors  60 ) to transfer data between components of a computing device and the memory modules  100   a - 100   n . The motherboard topology and number of memory modules  100   a - 100   n  installed generally determines whether the memory modules  100   a - 100   n  may operate in a single(one)-channel mode or a two-channel mode. In the two-channel mode, the slot in which the particular memory modules  100   a - 100   n  are installed may determine a particular configuration of each memory module when operating in the single-channel mode (e.g., described in more detail below in connection with  FIGS. 9A-9C ). In an example, the memory controller  20  may be implemented on a northbridge of a motherboard and/or as a component of a microprocessor (e.g., an Intel CPU, an AMD CPU, an ARM CPU, etc.). The implementation of the memory controller  20  may be varied according to the design criteria of a particular implementation. 
     In various embodiments, the memory modules  100   a - 100   n  may implement DDR4 SDRAM memory modules. In an example, the DDR4 SDRAM memory modules  100   a - 100   n  may have a memory module density of 512 gigabyte (GB), one terabyte (TB), or higher per module (e.g., compared to 128 GB per dual in-line memory module (DIMM) in DDR3). The DDR4 SDRAM memory modules  100   a - 100   n  may operate at voltages of 1.2-1.35 volts (V) with a frequency between 800-1600 megahertz (MHZ) (e.g., compared to 1.5-1.65V at frequencies between 400-1067 MHz in DDR3). In some embodiments, the memory modules  100   a - 100   n  may be implemented as low voltage memory and operate at 1.05V. For example, the low voltage embodiments of the memory modules  100   a - 100   n  may implement 35% power savings compared to DDR3 memory. The DDR4 SDRAM memory modules  100   a - 100   n  may transfer data at speeds of 2.13-4.26 giga-transfers per second (GT/s) and higher (e.g., compared to 0.8-2.13 GT/s in DDR3). The operating parameters of the memory modules  100   a - 100   n  may be varied according to the design criteria of a particular implementation. 
     In an example, the memory modules  100   a - 100   n  may be compliant with the DDR4 specification titled “DDR4 SDRAM”, specification JESD79-4A, November 2013, published by the Joint Electron Device Engineering Council (JEDEC) Solid State Technology Association, Arlington, Va. Appropriate sections of the DDR4 specification are hereby incorporated by reference in their entirety. 
     The memory modules  100   a - 100   n  may be implemented as DDR4 load reduced DIMMs (LRDIMMs). The duplex data buffers  104   a - 104   n  may allow the memory modules  100   a - 100   n  in a DDR4 LRDIMM configuration to operate at higher bandwidth and/or at higher capacities compared to DDR4 RDIMM (e.g., 2400 or 2666 MT/s for DDR4 LRDIMM compared to 2133 or 2400 MT/s for DDR4 RDIMM at 384 GB capacity). For example, compared to DDR4 RDIMM configurations, the DDR4 LRDIMM configuration of the memory modules  100   a - 100   n  may allow improved signal integrity on data signals, and/or better intelligence and/or post-buffer awareness by the memory controller  20 . 
     Referring to  FIG. 2 , a diagram of a memory module  100   i  is shown illustrating a two channel mode of a dual-in-line-memory module (DIMM) in accordance with an example embodiment of the invention. The memory module  100   i  may be representative of the memory modules  100   a - 100   n . The memory module  100   i  is shown operating in a two-channel mode in accordance with an example embodiment of the invention. In the two-channel mode, the memory module  100   i  is configured to service two channels (e.g., CH 0  and CH 1 ) independently. 
     In an example, the memory module  100   i  may comprise two rows (e.g., lower and upper) of five memory devices  106  on either side of the duplex RCD circuit  102 . A row of five duplex data buffers  104   a - 104   e  may be located below the two rows of memory devices  106  on the left side of the duplex RCD circuit  102 . Another row of five duplex data buffers  104   f - 104   n  may be located below the two rows of memory devices  106  on the right side of the duplex RCD circuit  102 . Each of the duplex data buffers  104   a - 104   n  may be configured to support two memory channels by implementing two data paths with the respective memory devices  106  located directly above. A first memory channel and data path of each duplex data buffer  104   a - 104   n  may communicate with the respective memory device(s)  106  in the lower row. A second memory channel and data path of each duplex data buffer  104   a - 104   n  may communicate with the respective memory device(s)  106  in the upper row. 
     In some embodiments, the memory devices  106  may be physically located on both sides (e.g., the front and back) of the circuit board of the memory module  100   i . When the memory devices  106  are located on both sides of the memory module  100   i , the first channel and data path of each duplex data buffer  104   a - 104   n  communicate with the memory devices  106  in the lower row on both the front and back sides and the second channel and data path of each duplex data buffer  104   a - 104   n  communicate with the memory devices  106  in the upper row on both the front and back sides. A capacity of memory on the memory module  100   i  may be varied according to the design criteria of a particular implementation. In an example where the memory module  100   i  is implemented as a DDP memory module, each memory channel/data path may communicate with two rows of memory devices  106 . 
     Referring to  FIG. 3 , a diagram is shown illustrating two dual-in-line-memory modules (DIMMs) operating in a single-channel mode in accordance with an example embodiment of the invention. In the single-channel mode, each memory module  100   i  is configured to service a particular channel (e.g., either CH 0  or CH 1 ) dependent upon the slot in which the memory module  100   i  is installed. When multiple memory modules  100   a - 100   n  are installed in a system, each of the memory modules  100   a - 100   n  may be configured to communicate using a separate channel. For example, in a system with two memory modules installed, a module  100   a  may be configured to communicate with the memory controller  20  using memory channel  1  (CH 1 ) and a memory module  100   n  may be configured to communicate with the memory controller  20  using memory channel  0  (CH 0 ). 
     When the memory modules  100   a - 100   n  are configured for single channel operation, a particular channel used to communicate is connected to one side of the duplex RCD circuit  102  and one side of each of the duplex data buffers  104   a - 104   n  ( 104 ). The unused portions of the data buffers  104  are placed in an idle state (e.g., indicated by cross hatching). For example,  FIG. 3  shows the memory module  100   a  having the memory channel CH 1  connected to the right hand data port of the duplex data buffers  104 , while the left hand data portions of the duplex data buffers  104  are idle. Command, address, and control signals of the memory channel CH 1  are presented to the right hand control port of the duplex RCD circuit  102 . For the memory module  100   n , signals related to the memory channel CH 0  are presented to the left hand side (portion) of the duplex data buffers  104  and to the left hand control port of the duplex RCD circuit  102 . The right hand side (portion) of the duplex data buffers  104  are set to the idle state (e.g., indicated by cross hatching). In general, the memory controller  20  is responsible for programming the memory modules  100   a - 100   n  for the appropriate configuration to match the memory topology of the motherboard. 
     Referring to  FIG. 4 , a diagram is shown illustrating a motherboard of a system with two memory slots populated by a single dual-in-line-memory module (DIMM) operating in the two channel mode and a bridge card. In one example, a single memory module in accordance with an example embodiment of the invention may be plugged into a slot  0  socket and a connection bridge dummy card may be plugged into a slot  1  socket. The memory module would be configured to operate in the two channel mode. The channel  1  bus to/from the host connects to one side of the slot  0  socket which is connected to the channel  1  portion of the duplex data buffers  104   a - 104   n  and the duplex RCD circuit  102 . For clarity, the channel portions of the duplex data buffers  104   a - 104   n  are represented by separate blocks (e.g., BUF 0 -BUF 4 ) adjacent to respective edges of the slot  0  socket. The channel  0  bus to/from the host connects to one side of the slot  1  socket. The connection bridge dummy card in slot  1  would pass the signals connected to the slot  1  socket to/from the channel  0  bus across to the slot  0  socket for connection to the channel  0  side of the duplex data buffers  104   a - 104   n  and the channel  0  control port of the duplex RCD circuit  102 . 
     Referring to  FIG. 5 , a diagram is shown illustrating the motherboard of the system with memory slots populated by two dual-in-line-memory modules (DIMMs), each operating in a respective one of the single channel modes. A first memory module is installed in slot  0  and a second memory module is installed in slot  1 . Each of the memory modules are configured for single channel operation. The memory module in slot  0  would receive channel  1  bus signals at the channel  1  side of the duplex data buffers  104   a - 104   n  and the channel  1  control port of the duplex RCD circuit  102 . The channel  0  duplex data buffer inputs and channel  0  control inputs of the duplex RCD circuit  102  would be connected to inactive lines and would be set into the idle state. The memory module in slot  1  would receive channel  0  bus signals at the channel  0  portion of the duplex data buffers and the channel  0  side of the duplex RCD circuit  102 . The channel  1  portion of the duplex data buffers would be in the idle state and connected with the inactive lines. 
     Referring to  FIG. 6 , a diagram is shown illustrating various communication buses of a dual-in-line-memory module (DIMM) in accordance with an example embodiment of the invention. Only the left hand side portion of a memory module  100  is illustrated for clarity. The right hand side portion is generally a mirror image of the left hand side. The memory controller  20  (not shown for clarity) may be configured to operate with two memory channels (e.g., CH 0  and CH 1 ). Each of the channels CH 0  and CH 1  may comprise a signal (e.g., CLK) and a number of control and address signals (e.g., ADDR/CMD), collectively annotated as CA/CTRL. When the memory module  100  is operating in the two-channel mode, the signals CA/CTRL of each of the channels CH 0  and CH 1  may be presented to respective ports of the duplex RCD circuit  102 . The memory controller  20  may generate and/or receive data signals (e.g., DQa-DQn) associated with each channel CH 0  and CH 1  that may be presented/received to/from respective channel ports (e.g., a port  111  and a port  112 , respectively) of each of the duplex data buffers  104   a - 104   n.    
     The duplex RCD circuit  102  may be configured to communicate with the memory controller  20 , memory devices  106 , and the duplex data buffers  104   a - 104   n . The memory devices  106  may be arranged in rows on the front and back of the memory module  100 . The rows of memory devices  106  on the front side may be designated as rank  0 . The rows of memory devices  106  on the back side may be designated as rank  1 . The duplex RCD circuit  102  may decode instructions received from the memory controller  20 . For example, the duplex RCD circuit  102  may receive commands that write register control words (RCWs). In various embodiments, the RCWs are implemented as configuration registers. In another example, the duplex RCD circuit  102  may receive commands for writing buffer control words (BCWs) to the duplex data buffers  104   a - 104   n . The BCWs may be used to configure operations and parameters of the duplex data buffers. The duplex RCD circuit  102  may be configured to train the memory devices  106 , the duplex data buffers  104   a - 104   n  and/or command and address lines connected to the memory controller  20 . For example, the RCWs may flow from the memory controller  20  to the duplex RCD circuit  102 . The RCWs may be used to configure the duplex RCD circuit  102 . The BCWs may flow from the memory controller  20  to the duplex RCD circuit  102  and from the RCD circuit  102  to the duplex data buffers  104   a - 104   n.    
     In an example, the duplex RCD circuit  102  may implement a 32-bit 1:2 command/address register. The duplex RCD circuit  102  may support one or more at-speed buses  110 . In an example, a BCOM bus  110   a  for channel  0  and BCOM bus  110   b  for channel  1  may be implemented between the duplex RCD circuit  102  and the duplex data buffers  104   a - 104   n . The duplex RCD circuit  102  may implement automatic impedance calibration. The duplex RCD circuit  102  may implement command/address parity checking. The duplex RCD circuit  102  may control register RCW readback. The duplex RCD circuit  102  may implement a 1 MHz inter-integrated circuit (I 2 C) bus (e.g., a serial bus). Inputs to the duplex RCD circuit  102  may be pseudo-differential using external and/or internal reference voltages. The clock outputs, command/address outputs, control outputs and/or data buffer control outputs of the duplex RCD circuit  102  may be enabled in groups and independently driven with different strengths. 
     The duplex RCD circuit  102  may receive the signal CLK and/or the signals ADDR/CMD from the memory controller  20 . Various digital logic components of the duplex RCD circuit  102  may be used to generate signals based on the signal CLK and/or the signals ADDR/CMD and/or other signals (e.g., RCWs). The duplex RCD circuit  102  may also be configured to generate a clock signal and address/command signals related to each channel for controlling accesses to the memory devices  106 . For example, the clock signal may be implemented in compliance with a signal Y_CLK in the DDR4 specification. The clock signal and/or the address/command signals related to each channel may be presented to each of the memory devices  106  associated with the respective memory channels (e.g., on a number of common busses). The duplex RCD circuit  102  may generate one or more data buffer control signals (e.g., DBC). The signals DBC may be presented to each of the duplex data buffers  104   a - 104   n . The signals DEC may be transmitted on a single common bus  110  (e.g., a data buffer control bus) or on respective buses  110   a  and  110   b  for each channel. 
     The duplex data buffers  104   a - 104   n  may be configured to receive data from the bus  110  (e.g., described below in connection with  FIG. 8 ) or from the buses  110   a  and  110   b  (e.g., described below in connection with  FIG. 7 ). The duplex data buffers  104   a - 104   n  may be configured to generate/receive data signals (e.g. DQ) to/from the channels CH 0  and CH 1 . The channels CH 0  and CH 1  may comprise traces, pins and/or connections between the memory controller  20  and the duplex data buffers  104   a - 104   n . The duplex data buffers  104   a - 104   n  may be configured to buffer data on the channels CH 0  and CH 1  for write operations (e.g., data transfers from the memory controller  20  to the corresponding memory devices  106 ). The duplex data buffers  104   a - 104   n  may be configured to buffer data on the channels CH 0  and CH 1  for read operations (e.g., data transfers from the corresponding memory devices  106  to the memory controller  20 ). The duplex data buffers  104   a - 104   n  generally limit the loading on the channel signals, allowing increased signaling speeds at a given memory technology node. 
     Each of the duplex data buffers  104   a - 104   n  may have a first input/output  111  that communicates respective data signals DQ to/from the memory controller  20  on the first channel CH 0 , a second input/output  112  that communicates respective data signals DQ to/from the memory controller  20  on the second channel CH 1 , a third input/output  113  that communicates respective data signals DQ to/from the memory devices  106  associated with the first channel CH 0 , and a fourth input/output  114  that communicates respective data signals DQ to/from the memory devices  106  associated with the second channel CH 1 . In an example, each of the input/outputs  111 ,  112 ,  113 , and  114  may be implemented as 4-bit wide data paths. Each of the duplex data buffers  104   a - 104   n  may also have a single connection to the bus  110  or respective connections to buses  110   a  and  110   b , allowing the duplex data buffers  104   a - 104   n  to be configured in the appropriate mode for the memory topology of the system in which the memory module  100  is installed. 
     Referring to  FIG. 7 , a diagram is shown illustrating a duplex data buffer  104   i  in accordance with an example embodiment of the invention. The duplex data buffer  104   i  may be representative of an example embodiment of the duplex data buffers  104   a - 104   n . The duplex data buffer  104   i  is shown having a first input/output  111 , a second input/output  112 , a third input/output  113 , and a fourth input/output  114 . 
     The first input/output  111  is configured for presenting/receiving the signals DQi CH 0  (e.g., the data signals DQ corresponding to memory channel CH 0 ) between the duplex data buffer  104   i  and the controller  20 . The second input/output  112  is configured for presenting/receiving the signals DQi CH 1  (e.g., the data signals DQ corresponding to memory channel CH 1 ) between the duplex data buffer  104   i  and the controller  20 . The third input/output  113  is configured for presenting/receiving the signals DQi CH 0  as memory input/output (MIO) signals corresponding to memory channel CH 0  between the duplex data buffer  104   i  and the respective memory devices (e.g., DRAM chips)  106 . The fourth input/output  114  is configured for presenting/receiving the signals DQi CH 1  as memory input/output (MIO) signals corresponding to memory channel CH 1  between the duplex data buffer  104   i  and the respective memory devices (e.g., DRAM chips)  106 . 
     The signals MIO are generally transmitted between the memory modules  106  and the respective duplex data buffer  104   a - 104   n . In an example, data (e.g., the signals DQi) from each channel of the memory controller  20  may be presented to the duplex data buffer  104   i , buffered in the duplex data buffer  104   i , then transmitted to the respective memory device(s)  106 . In another example, data from the respective memory device(s)  106  may be presented to the duplex data buffer  104   i , buffered in the duplex data buffer  104   i , and then transmitted on an appropriate memory channel to the memory controller  20 . 
     The duplex data buffer  104   i  is shown also receiving signals (e.g., DBC_CH 0 ) from the bus  110   a  at a first control port (e.g., DBC PORT  0 ) and signals (e.g., DBC_CH 1 ) from the bus  110   b  at a second control port (e.g., DBC PORT  1 ). The signals DBC_CH 0  and DBC_CH 1  may be presented to each of the duplex data buffers  104   a - 104   n  (e.g., using the data buffer control buses  110   a  and  110   b ). In an example, the signals DBC_CH 0  and DBC_CH 1  are illustrated comprising five signals transmitted over 9 pins/bits (e.g., a pair of signals BCK_T/BCK_C, a signal BCOM, a signal BCKE, a signal BODT and/or a signal BVREFCA). However, other numbers of pins/bits may be implemented accordingly to meet the design criteria of a particular application. The first control port of the duplex data buffer  104   i  is shown having an input  115  receiving the signals BCK_T/BCK_C, an input  117   a  receiving the signal BCOM, an input  117   b  receiving the signal BCKE, an input  117   c  receiving the signal BODT, and an input  117   d  receiving the signal BVREFCA, all corresponding to channel  0  (CH 0 ). The second control port of the duplex data buffer  104   i  is shown having an input  116  receiving the signals BCK_T/BCK_C, an input  118   a  receiving the signal BCOM, an input  118   b  receiving the signal BCKE, an input  118   c  receiving the signal BODT, and an input  118   d  receiving the signal BVREFCA, all corresponding to channel  1  (CH 1 ). 
     In various embodiments, the signals BCK_T/BCK_C may be implemented as a 2-bit signal representing a differential (e.g., true (T) and complementary (C) versions) clock signal for the duplex data buffers  104   a - 104   n . In various embodiments, the signal BCOM may be implemented as a 4-bit signal representing data buffer commands. The signal BCOM may be implemented as a unidirectional signal from the duplex RCD circuit  102  to the duplex data buffers  104   a - 104   n . In an example, the signal BCOM may be implemented at a single data rate (e.g., 1 bit per signal per clock cycle). However, a particular command may take a different number of clock cycles to transfer information. The signal BCKE may be a function registered dedicated non-encoded signal (e.g., DCKE). The signal BODT may be a function registered dedicated non-encoded signals (e.g., DODT). The signal BVREFCA may be a reference voltage for use with pseudo-differential command and control signals. 
     Each channel of the duplex data buffers  104   a - 104   n  may receive a set of duplex data buffer commands (e.g., for writing buffer control words (BCWs)) from the signals DBC. The buffer control words may be used to customize operation of the respective channel of the duplex data buffers  104   a - 104   n . The buffer control words may flow from the memory controller  20 , through the duplex RCD circuit  102 , to the duplex data buffers  104   a - 104   n . The buffer control words may be similar to register control words (RCWS) used for configuring the duplex RCD circuit  102 . Similar to commands for writing the register control words, the commands for writing the buffer control words may look like an MRS7 command, where the address lines are really the payload. 
     In embodiments where the buses  110 ,  110   a , and/or  110   b  comprise nine pins, the duplex RCD circuit  102  may do more than pass a buffer control word directly through to the duplex data buffers  104   a - 104   n . In one example, the duplex RCD circuit  102  may convert (e.g., multiplex) an MRS7 command format into a buffer control word in a BCOM format. The duplex RCD circuit  102  may map the 12 address bits of the MRS7 command into five separate data transfers, each 4 bits wide. The five data transfers may be set up back to back over one of the buses  110 ,  110   a , or  110   b . For example, 5 clock cycles plus a parity cycle may be used to complete the buffer command in the buffer control word. Once the buffer control word reaches the duplex data buffers  104   a - 104   n , the duplex data buffers  104   a - 104   n  may decode the buffer control word, write the buffer control word to a function space of the duplex data buffer, and complete the buffer command in the buffer control word. 
     A function of the signal BCOM may be to transmit the buffer control words. However, compliant with the JEDEC specification for DDR4 SDRAM, the duplex RCD circuit  102  may send all read/write commands and MRS information over the bus  110 , or buses  110   a  and  110   b  (e.g., to allow the duplex data buffers  104   a - 104   n  to keep track of what the memory devices  106  are doing). In some embodiments, different buffer commands may take a different number of cycles to transfer the information. 
     The duplex RCD circuit  102  may receive an MRS7 command from the memory controller  20  (e.g., from a host). For example, a host may want to change a parameter (e.g., typically on boot up of a computing device). The duplex RCD circuit  102  may check the MRS7 command to determine whether the address bit  12  is set to 1 (e.g., a logical one). In an example, when an address bit  12  of the MRS7 command is set to 1, the duplex RCD circuit  102  may recognize the command as a buffer command (e.g., a command that is not meant for the duplex RCD circuit  102 ). The duplex RCD circuit  102  may convert the command from the memory controller  20  to a buffer control word and send the buffer control word to the duplex data buffers  104   a - 104   n  via the bus  110 . The duplex data buffers  104   a - 104   n  may write the buffer control word to a function space to complete the command. 
     The duplex data buffers  104   a - 104   n  may be configurable. The buffer commands may allow the memory controller  20  to customize aspects of termination (e.g., ODT), signal strength on the DQ lines, and/or events (e.g., receiver timing, driver timing, etc.) in both directions (e.g., for both read and write operations). In some embodiments, some of the configurations of the duplex data buffers  104   a - 104   n  may be decided based on system level configurations. Generally, most of the configuration of the duplex data buffers  104   a - 104   n  may be decided during training steps. During training steps, host controllers (e.g., the memory controller  20 ) may test and compare results of various training steps to determine an optimal configuration. 
     In various embodiments, the bus  110  may be used to send commands/data to program configuration registers of the duplex data buffers  104   a - 104   n . The bus  110  may also send commands (e.g., data reads and/or data writes) that control data traffic through the duplex data buffers  104   a - 104   n . For example, some commands may optimize power consumption of the duplex data buffers  104   a - 104   n . In another example, read/write delays may be added per data line. 
     The duplex data buffers  104   a - 104   n  may implement dual multi-bit (e.g., 4-bit) bidirectional data registers with differential data strobes (e.g., DQS_T/DQS_C). The duplex data buffers  104   a - 104   n  may implement automatic impedance calibration. The duplex data buffers  104   a - 104   n  may implement BCOM parity checking. The duplex data buffers  104   a - 104   n  may implement control register (e.g., buffer control word) readback. 
     Referring to  FIG. 8 , a diagram is shown illustrating a duplex data buffer  104   i ′ in accordance with another example embodiment of the invention. In some embodiments, the duplex data buffer  104   i ′ may be used to implement the duplex data buffers  104   a - 104   n . The duplex data buffer  104   i ′ may be implemented similarly to the duplex data buffer  104   i , except that the duplex data buffer  104   i ′ implements a single control port (e.g. DBC PORT) through which both sides of the duplex data buffer  104   i ′ may be controlled. 
     In another example, the signals DBC may be implemented comprising the five signals BCK_T/BCK_C, BCOM, BCKE, BODT, and BVREFCA transmitted over 10 pins/bits (e.g., the signal BCOM may be implemented with 5 bits instead of 4 bits). However, other numbers of pins/bits may be implemented accordingly to meet the design criteria of a particular application. The control port DBC PORT of duplex data buffer  104   i ′ is shown having an input  115  receiving the signals BCK_T/BCK_C, an input  119   a  receiving the signal BCOM, an input  119   b  receiving the signal BCKE, an input  119   c  receiving the signal BODT, and an input  119   d  receiving the signal BVREFCA. The control port DBC PORT is used to program operation of both channels of the duplex data buffer  104   i ′. In various embodiments, the signal BCOM may be implemented as a 5-bit signal communicating data buffer commands. The signal BCOM may be configured to signal which port of the duplex data buffer  104   i ′ the signal BCOM is meant to control/program. The signal BCOM may be implemented as a unidirectional signal from the duplex RCD circuit  102  to the duplex data buffers  104   a - 104   n . In an example, the signal BCOM may be implemented at a single data rate (e.g., 1 bit per signal per clock cycle). However, a particular command may take a different number of clock cycles to transfer information. 
     Referring to  FIGS. 9A-9C , diagrams are shown illustrating various operating modes of the duplex registered clock driver  102  and the duplex data buffers  104   a - 104   n  in accordance with example embodiments of the invention. Referring to  FIG. 9A , a diagram is shown illustrating a two-channel mode (e.g., MODE  0 ). In the two-channel mode, an exemplary duplex RCD circuit  102   i  and an exemplary duplex data buffer  104   i  are configured to service both channel  0  and channel  1  independently. Referring to  FIG. 9B , a diagram is shown illustrating a first single-channel mode (e.g., MODE  1 ). In the first single-channel mode MODE  1 , the duplex RCD circuit  102   i  and the duplex data buffer  104   i  (representing the duplex data buffers  104   a - 104   n ) are configured to communicate data to/from all of the memory devices  106  of the memory module using only channel  0 . For example, the channel  1  bus side circuits are placed in the idle state and data to/from the memory devices  106  is routed to the channel  0  circuitry. Referring to  FIG. 9C , a diagram is shown illustrating a second single-channel mode (e.g., MODE  2 ). In the second single-channel mode MODE  2 , the duplex RCD circuit  102   i  and the duplex data buffer  104   i  are configured to communicate data to/from all of the memory devices  106  of the memory module using only channel  1 . For example, the channel  0  bus side circuits are placed in the idle state and the data to/from the memory devices  106  is routed to the channel  1  circuitry. 
     Referring to  FIG. 10 , a diagram is shown illustrating an example implementation of a duplex data buffer  104   i  in accordance with an example embodiment of the invention. The duplex data buffers  104   a - 104   n  may be implemented as illustrated by the duplex data buffer  104   i . In an example, the duplex data buffer  104   i  may comprise a block  200 , a block  202   a  and a block  202   b . The block  200  may implement a transaction control logic of the duplex data buffer  104   i . The blocks  202   a  and  202   b  may implement channel circuitry for the duplex data buffer  104   i . The block  200  may be configured to control operations of the duplex data buffer  104   i  based upon commands received via the bus  110  or the buses  110   a  and  110   b . The circuit  200  is generally enabled to configure the circuit  202   a  and  202   b  for operation in either the two channel mode or one of the single channel modes described above in connection with  FIGS. 9A-9C . The circuit  200  may be configured to implement the data buffer control (DBC) ports described above in connection with  FIGS. 7 and 8 . 
     In one example, the block  202   a  may be used to provide the channel  0  data path and the block  202   b  may be used to provide the channel  1  data path. In various embodiments, the block  202   a  may comprise a block (or circuit)  204   a , a block (or circuit)  206   a , a block (or circuit)  208   a , a block (or circuit)  210   a , a block (or circuit)  212   a , a block (or circuit)  214   a , a block (or circuit)  216   a , and a block (or circuit)  218   a . The block  202   b  may comprise a block (or circuit)  204   b , a block (or circuit)  206   b , a block (or circuit)  208   b , a block (or circuit)  210   b , a block (or circuit)  212   b , a block (or circuit)  214   b , a block (or circuit)  216   b , and a block (or circuit)  218   b . The blocks  204   a  and  204   b  may be implemented as receivers or buffers. The blocks  206   a  and  206   b  may be implemented as transmitters or buffer/drivers. The blocks  208   a ,  208   b ,  214   a , and  214   b  may be implemented as switches. The blocks  210   a ,  210   b ,  212   a , and  212   b  may be implemented as memories. The memories  210   a ,  210   b ,  212   a , and  212   b  may be configured as first-in first-out (FIFO) memories. In some embodiments, the memories  210   a ,  210   b ,  212   a , and  212   b  may be implemented as separate, independent memory blocks or devices. In some embodiments, the memories  210   a ,  210   b ,  212   a , and  212   b  may be implemented as portions of a single memory block or device. The blocks  216   a  and  216   b  may be implemented as transmitters or buffer/drivers. The blocks  218   a  and  218   b  may be implemented as receivers or buffers. 
     In one example, the input/output  113  may be connected to an input of the block  204   a  and the output of the block  206   a . The input/output  114  may be connected to an input of the block  204   b  and the output of the block  206   b . An output of the block  204   a  may be connected to an input of the block  210   a . An output of the block  204   b  may be connected to an input of the block  210   b . An input of the block  206   a  may be connected to an output of the block  208   a . An input of the block  206   b  may be connected to an output of the block  208   b . A first input of the block  208   a  may be connected to a first input of the block  208   b  and an output of the block  212   a . A second input of the block  208   a  may be connected to a second input of the block  208   b  and an output of the block  212   b.    
     An output of the block  210   a  may be connected to a first input of the block  214   a  and a first input of the block  214   b . An output of the block  210   b  may be connected to a second input of the block  214   a  and second input of the block  214   b . An output of the block  214   a  may be connected to an input of the block  216   a . An output of the block  218   a  may be connected to an input of the block  212   a . An output of the block  216   a  and an input of the block  218   a  may be connected to the input/output  111 . An output of the block  218   b  may be connected to an input of the block  212   b . An output of the block  216   b  and an input of the block  218   b  may be connected to the input/output  112 . 
     Although embodiments of the invention have been described in the context of a DDR4 application, the present invention is not limited to DDR4 applications, but may also be applied in other high data rate digital communication applications where different transmission line effects, cross-coupling effects, traveling wave distortions, phase changes, impedance mismatches and/or line imbalances may exist. The present invention addresses concerns related to high speed communications, flexible clocking structures, specified command sets and lossy transmission lines. Future generations of DDR can be expected to provide increasing speed, more flexibility, additional commands and different propagation characteristics. The present invention may also be applicable to memory systems implemented in compliance with either existing (legacy) memory specifications or future memory specifications. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the scope of the invention.