Patent Publication Number: US-2023152840-A1

Title: Register clock driver with chip select loopback

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of PCT International Application No. PCT/CN2021/130922, filed on Nov. 16, 2021. The entire content of PCT International Application No. PCT/CN2021/130922 is incorporated by reference herein. 
    
    
     BACKGROUND OF THE SPECIFICATION 
     The present disclosure relates to memory devices. More specifically, the present disclosure relates in some embodiments to register clock drivers of a double data-rate (DDR) random access memory (RAM) module. 
     Power consumption and efficiency is increasingly important in both fixed and portable computing devices to reduce power costs and ensure a long battery life. Portable computing devices, such as, e.g., laptops, notebooks, netbooks, or other computing devices that may need to rely on battery power, often have stringent requirements for power consumption and efficiency. In such computing devices, each component typically needs to be optimized to reduce power consumption. In an aspect, double data rate fifth generation (DDR5) memory modules can be used in these computing devices. When compared to its predecessors (e.g., DDR3 and DDR4 memory modules), DDR5 memory modules have reduced power consumption, improved bandwidth, and improved efficiency (e.g., faster). In an aspect, DDR5 memory modules can incorporate on-board voltage regulators in order to reach higher speeds. 
     SUMMARY 
     In an embodiment, an apparatus in a memory module is generally described. The apparatus can include a receiver configured to receive a chip select signal for selecting one or more memory ranks of the memory module. The apparatus can further include a logic circuit coupled to the receiver. The apparatus can further include an output driver coupled to the logic circuit. The logic circuit can be configured to decode the chip select signal to generate an output signal for selecting the one or more memory ranks of the memory module. The output driver can be configured to select the one or more memory ranks using the output signal. The apparatus can further include a loopback circuit configured to sample the chip select signal from one or more of a first sampling point between the receiver and the logic circuit and a second sampling point between the logic circuit and the output driver. 
     In another embodiment, an apparatus comprising a memory module is generally described. The memory module can include a plurality of memory ranks and a register clock driver (RCD). The RCD can be coupled to the plurality of memory ranks. The RCD can include a receiver configured to receive a chip select signal for selecting one or more memory ranks among the plurality of memory ranks. The RCD can further include a logic circuit coupled to the receiver. The RCD can further include an output driver coupled to the logic circuit. The logic circuit can be configured to decode the chip select signal to generate an output signal for selecting the one or more memory ranks of the memory module. The output driver can be configured to select the one or more memory ranks using the output signal. The RCD can further include a loopback circuit configured to sample the chip select signal from one or more of a first sampling point between the receiver and the logic circuit and a second sampling point between the logic circuit and the output driver. 
     In another embodiment, a method for operating a memory module is generally described. The method can include receiving, by a register clock driver (RCD) of a memory module, a chip select signal for selecting one or more memory ranks of the memory module. The method can include sampling, by the RCD, the chip select signal from one or more of a first sampling point between a receiver of the RCD and a logic circuit of the RCD and a second sampling point between the logic circuit of the RCD and an output driver of the RCD. 
     The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. In the drawings, like reference numbers indicate identical or functionally similar elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram of an example memory system according to an embodiment of the disclosure. 
         FIG.  2    is a block diagram illustrating an example memory module of the memory system of  FIG.  1    according to an embodiment of the disclosure. 
         FIG.  3    is a block diagram of an example memory module including a register clock driver with a chip select loopback circuit according to an embodiment of the disclosure. 
         FIG.  4    is a diagram illustrating details of a register clock driver with a chip select loopback circuit according to an embodiment of the disclosure. 
         FIG.  5    is a circuit diagram of a loopback circuit according to an embodiment of the disclosure. 
         FIG.  6    is a flowchart of an example process  600  that may implement a register clock driver with chip select loopback according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     A register clock driver (RCD) is a device is used on a memory module, such as DDR5 memory modules. The RCD can buffer the command address (CA) bus, chip select signals, and clock signals between a host controller and the DDR5 memory module. When RCD is used on a load-reduction dual in-line memory module (LRDIMM), the RCD can also create a buffer communications (BCOM) bus to control the data buffers of LRDIMM. LRDIMM can support relatively higher densities than other memory modules, and includes a memory buffer chip. In an aspect, LRDIMM can be used in, for example, servers, and the memory buffer chip of the LRDIMM can reduce and minimize the load on the server&#39;s memory bus. For example, the LRDIMM can use the memory buffer chip to consolidate the electrical loads of the ranks on the LRDIMM to a single electrical load, allowing them to have up to, for example, eight ranks on a single DIMM module. Thus, using LRDIMMs can allow systems to be configured with the largest possible memory footprints. 
     However, LRDIMM usage of the memory buffer chip, instead of registers like other DIMMs, can penalize signal integrity, especially as the speed of DDR5 memory modules increases (e.g., the higher the speed, the poorer the signal integrity). In one example, the disclosed embodiments may integrate a loopback or feedback circuit having hardware components to form feedback paths for chip select signals of a RCD. The feedback paths can drive the chip select signals of the RCD back to a host controller for debugging, testing, or training. Further, in one embodiment, the feedback path for the chip select signals can utilize a portion of other existing feedback paths reserved for other internal signals of the RCD that may be fed back to the host controller. Furthermore, the feedback circuit integrated in the RCD can sample chip select signals from different locations within the RCD such that the host controller can determine which component or processing stage of the RCD may be causing problems. Still further, integrating the feedback circuit in the RCD allows the RCD to undergo testing of chip select signals at any time, including during normal operations of the memory module. By being able to test the chip select signals any time, signal integrity of the chip select signals may be maintained to improve functionality of the memory module. 
       FIGS.  1  and  2    illustrate an example embodiment of a memory system  10 . The memory system  10  includes memory modules  20   1 ,  20   2  . . .  20   N , also referred to herein collectively or individually as memory module(s)  20 , connectors  70  and a memory controller  80 . 
     With reference to  FIG.  1   , in one example embodiment, the memory modules  20  may comprise dual in-line memory modules (DIMMs). In some embodiments, the memory modules  20  may be implemented as double data rate fifth generation (DDR5) SDRAM modules. While described and illustrated herein as having a particular type, arrangement and number of components, in other embodiments, memory modules  20  may comprise any other type, arrangement or number of components. 
     An example memory module  20  comprises circuitry blocks  30   1 ,  30   2 ,  30   3 ,  30   4 ,  30   5  . . .  30   P-4 ,  30   P-3 ,  30   P-2 ,  30   P-1  and  30   P , circuitry blocks  40   1 ,  40   2 , . . .  40   M-1  and  40   M , a registered clock driver (RCD)  50 , a PMIC  60 , connectors  70  and any other blocks, circuits, pins, connectors, traces or other component typically found in a memory module. In some embodiments, circuitry blocks  30   1 ,  30   2 ,  30   3 ,  30   4 ,  30   5  . . .  30   P-4 ,  30   P-3 ,  30   P-2 ,  30   P-1  and  30   P  may be configured as data buffers and will also be referred to herein collectively or individually as data buffers(s)  30 . In some embodiments, circuitry blocks  40   1 ,  40   2 , . . .  40   M-1  and  40   M  may be configured as memory devices and will also be referred to herein collectively or individually as memory device(s)  40 . While described herein as data buffers  30  and memory devices  40 , circuitry blocks  30  and  40  may also or alternatively be utilized for any other purpose by the memory modules  20 . 
     In some embodiments, data buffers  30  and memory devices  40  comprise synchronous dynamic random-access memory (SDRAM) devices, chips or modules. In some embodiments, data buffers  30  and memory devices  40  also or alternatively comprise any other types of memory devices such as, e.g., SRAM, DRAM, MROM, PROM, EPROM and EEPROM. The data buffers  30 , memory devices  40  or both may be physically located on one side or both sides (e.g., the front and back) of the memory module  20 . 
     PMIC  60  is configured to perform power management for the memory module  20 . For example, the PMIC  60  may be configured to scale up or scale down voltages, perform DC-DC conversions or perform other similar power management operations. In some embodiments, PMIC  60  may comprise low-dropout regulators (LDOs), DC-DC converters such as, e.g., buck or boost converters, pulse-frequency modulation (PFM), pulse-width modulation (PWM), power field-effect transistors (FETs), real-time clocks (RTCs) or any other circuity that may typically be found in a PMIC. 
     Connectors  70  may comprise, for example, pins, traces or other connections that are configured to connect the memory modules  20  to other components of a computing system such as, e.g., a memory controller  80 , motherboard, or other components. In some embodiments, the connectors  70  may comprise, e.g., a 288-pin configuration or any other pin configuration. 
     In some embodiments, the memory module  20  comprises the connectors  70 . In other embodiments, a motherboard, memory controller  80  or any other component of a computing device comprises the connectors  70 . In another embodiment, one or more of the connectors  70  may be part of the memory module  20  and one or more of the connectors  70  may be part of the motherboard, memory controller  80  or other component of the computing device. 
     The memory modules  20  may be connected to the motherboard, memory controller  80  or other component of the computing device, e.g., by connectors  70 , to transfer data between components of the computing device and the memory modules  20 . For example, in an embodiment that implements a UDIMM, the connectors  70  may comprise a 64-bit bus, a 72-bit bus or a bus comprising any other number of bits. 
     The memory modules  20  are shown connected to the memory controller  80  of the computing device. In an example embodiment, the memory controller  80  may be implemented as a component of a computer motherboard, or main board, of the computing device, e.g., on a northbridge of the motherboard. In another example, the memory controller  80  may be implemented as a component of a microprocessor of the computing device. In yet another example, the memory controller  80  may be implemented as a component of a central processing unit (CPU) of the computing device. In other embodiments, the memory controller  80  may be implemented as a part of any other component of the computing device. 
     In some embodiments, the memory modules  20  are implemented as DDR5 SDRAM memory modules. As an example, the memory modules  20  may comprise a memory module density of 128 gigabyte (GB), 512 GB, one terabyte (TB), or higher per module. Memory modules  20  may operate with a frequency of about 1.2 to about 3.2 giga-Hertz (GHz) and a data rate range of about 3.2 GT/s to about 4.6 GT/s and in some cases a data rate up to about 8 GT/s or more. In some embodiments, the memory modules  20  may alternatively comprise smaller or larger densities, operate at lower or higher frequencies and operate at lower or higher data rates. 
     With reference now to  FIG.  2   , a block diagram illustrating an example memory module  20  of  FIG.  1    is shown. The memory module  20  may be representative of the memory modules  20 A- 20 N. The memory module  20  is shown communicating with the memory controller  80 . The memory controller  80  is shown as part of a circuit  90  such as, e.g., a motherboard, main board or other component of a computing device that communicates with the memory module  20 . 
     The memory module  20  comprises one or more groupings of circuits  22   1 ,  22   2 ,  22   3 ,  22   4 ,  22   5  . . .  22   Q-4 ,  22   Q-3 ,  22   Q-2 ,  22   Q-1  and  22   Q , also referred to herein collectively or individually as data paths  22  of the memory module  20 . In the example shown, the memory module  20  may comprise five data paths  22 , e.g., data paths  22   1 ,  22   2 ,  22   3 ,  22   4  and  22   5 , on one side of the RCD  50  and five data paths  22 , e.g., data paths  22   Q-4 ,  22   Q-3 ,  22   Q-2 ,  22   Q-1  and  22   Q , on the other side of the RCD  50 . In other embodiments, memory module  20  may comprise other arrangements having a greater or smaller number of data paths  22  on each side of the RCD  50 . 
     The data paths  22  may each comprise a respective memory channel  42   1 ,  42   2 ,  42   3 ,  42   4 ,  42   5  . . .  42   R-4 ,  42   R-3 ,  42   R-2 ,  42   R-1  and  42   R , also referred to herein collectively and individually as memory channel(s)  42 . Each memory channel  42  may comprise one or more of the memory devices  40 . For example, memory channel  42   1  may comprise memory devices  40   1  through  40   S , while memory channel  42   R  may comprise memory devices  40   T  through  40   M . 
     The memory controller  80  is configured to generate a variety of signals including a clock signal (CLK), control signals (ADDR and CMD) and command signals. One or more of the CLK, ADDR and CMD signals may be provided to the RCD  50 , e.g., via one or more buses  23 . 
     Signals from the memory controller  80  may also be transmitted from the memory controller  80  to the PMIC  60  via a bus  24 , also referred to herein as a host interface bus  24 . In some embodiments, host interface bus  24  is bi-directional and is configured to communicate commands or other data between PMIC  60  and memory controller  80  or other components of the memory module  20 . The host interface bus  24  may implement an I 2 C protocol, an I 3 C protocol or any other protocol. 
     A data bus  72  may be connected between the memory controller  80  and the data paths  22 , e.g., with data buffers  30 , and may comprise connectors  70 , e.g., traces, pins and other connections, between the memory controller  80  and the data paths  22 . 
     The memory controller  80  may generate or receive data signals, e.g., DQa-DQn, and data strobe signals, e.g., DQSa-DQSn, that may be presented to or received from the data bus  72 . Portions of the signals DQa-DQn and DQSa-DQSn may be presented to or received from respective data paths  22 . In the example shown, each of the signals DQa-DQn may have a corresponding signal DQSa-DQSn. In some embodiments, one DQS signal may strobe multiple DQ signals, e.g., one DQS signal for four DQ signals in some embodiments. 
     The RCD  50  is configured to communicate with the memory controller  80 , the data buffers  30 , the memory channels  42  and the PMIC  60 . The RCD  50  is configured to decode instructions, e.g., control words, received from the memory controller  80 . For example, the RCD  50  may be configured to receive and decode register command words (RCWs). In another example, the RCD  50  may be configured to receive and decode buffer control words (BCWs). The RCD  50  is configured to train one or more of the data buffers  30 , memory devices  40  and the command and address lines between the RCD  50  and the memory controller  80 . For example, the RCWs may flow from the memory controller  80  to the RCD  50  and be used to configure the RCD  50 . 
     In some embodiments, the RCD  50  may implement a command/address register, e.g., a 32-bit 1:2 command/address register. The RCD  50  may support an at-speed bus, e.g., a unidirectional buffer communications (BCOM) bus between the RCD  50  and the data buffers  30 . In some embodiments, the RCD  50  may implement one or more of automatic impedance calibration, command/address parity checking, control register RCW readback, a serial bus such as, e.g., a 1 MHz inter-integrated circuit (I 2 C) bus, and a 12.5 MHz inter-integrated circuit (I 3 C) bus. Inputs to the RCD  50  may be pseudo-differential using one or more of external and internal voltages. The clock outputs, command/address outputs, control outputs and data buffer control outputs of the RCD  50  may be enabled in groups and independently driven with different strengths. 
     The RCD  50  is configured to receive the CLK, ADDR and CMD signals or other signals such as, e.g., RCWs and BCWs, from the memory controller  80  and to utilize various digital logic components to generate corresponding output signals based on the CLK, ADDR and CMD signals. For example, the RCD  50  is configured to generate corresponding signals such as, e.g., CLK′, ADDR′ and CMD′ signals based on the received CLK, ADDR and CMD signals. The CLK′, ADDR′ and CMD′ signals may be presented to the memory channels  42 . For example, the CLK′ signals may be transmitted from the RCD  50  to the memory channels  42  on a common bus  25  and the ADDR′ and CMD′ signals may be transmitted from the RCD  50  to the memory channels  42  on a common bus  26 . The RCD  50  is also configured to generate one or more data buffer control (DBC) signals that are transmitted to the data buffers  30 , for example, on a common bus  27 , also referred to herein as a data buffer control bus  27 . 
     The data buffers  30  are configured to receive commands and data from the data buffer control bus  27  and to generate data, receive data or transmit data to and from the data bus  72 . Each data path  22  also comprises bus  28  between its data buffer  30  and memory channel  42  that is configured to carry the data between the data buffer  30  and memory channel  42 . For example, as seen in  FIG.  2   , data path  22   1  comprises a bus  28  between data buffer  30   1  and memory channel  42   1 . 
     The data buffers  30  are configured to buffer data on the buses  72  and  28  for write operations, e.g., data transfers from the memory controller  80  to the corresponding memory channels  42 , and read operations, e.g., data transfers from the corresponding memory channels  42  to the memory controller  80 . 
     In some example embodiments, the data buffers  30  exchange data with the memory devices  40  via the buses  28  in small units, e.g., 4-bit nibbles. In other embodiments, larger or smaller sizes of data transfer may alternatively be utilized. In some cases, the memory devices  40  may be arranged into multiple sets, e.g., two sets. For example, for a two set/two memory device implementation, e.g., memory devices  40   1  and  40   2 , each set may contain a single memory device  40 , e.g.,  40   1  or  40   2 ) with each memory device  40  being connected to the respective data buffers  30  through an upper nibble and a lower nibble. For two set/four memory device implementation, each set may contain two memory devices  40 . The first set may be connected to the respective data buffers  30  through the upper nibble and the second set may be connected to the respective data buffers  30  through the lower nibble. For two set/eight memory device implementation, each set may contain four of the memory devices  40 . The first set of four memory devices  40  may connect to the respective data buffers  30  through the upper nibble and the second set of four memory devices may connect to the respective data buffers  30  through the lower nibble. Other numbers of sets, other numbers of memory devices per set and other data unit sizes may alternatively be utilized. 
     Memory module  20  may also comprise an interface  29  that is configured to enable communication between the RCD  50  and the PMIC  60 . For example, the interface  29  may utilized as part of a register clock driver/power management integrated circuit interface, e.g., an RCD-PMIC interface. The interface  29  is configured to support one or more signals or connections that may be bidirectional or unidirectional. 
       FIG.  3    is a block diagram of an example  300  that can implement a register clock driver with a chip select loopback circuit according to an embodiment of the disclosure. The system  300  can include a host device  302  and a memory module  304 . The memory module  304  can be one of the memory modules  20 , and the host device  302  can be the memory controller  80  shown in  FIG.  1    and  FIG.  2   . In one or more embodiments, the host device  302  can be, for example, a part of a circuit, a motherboard, a main board, a processor or processor core, a central processing unit (CPU), or other component of a computing device that communicates with the memory module  304 . The system  300  can be implemented in a computing device such as a desktop computer, a laptop computer, a server, a benchmark testing device, etc. In an embodiment, the memory module  304  can be, for example, a dual in-line memory modules (DIMMs). In some embodiments, the memory module  304  may be implemented as a double data rate fifth generation (DDR5) load-reduction dual in-line memory module (LRDIMM) including synchronous dynamic random-access memory (SDRAM) devices. 
     The memory module  304  can include a register clock driver (RCD)  306  and a plurality of memory ranks. In the example shown in  FIG.  3   , the memory module  304  can include four memory ranks labeled as Rank  0 , Rank  1 , Rank  2 , Rank  3 . In one or more embodiments, the memory module  304  may include less than four ranks, or more than four ranks up to, for example, eight ranks. In some embodiments, the RCD  306  may implement a command/address register, and may support a unidirectional buffer communications (BCOM) bus between the RCD  306  and data buffers in the memory module  304 . 
     The RCD  306  may be configured to receive a plurality of input signals  308  from the host device  302 . The input signals  308  can include control signals such as address signals (e.g., bank address signals, row address signals, column address signals, gated column address strobe signals, chip-select signals, parity signals), and command signals (e.g., refresh, pre-charge, etc.), and data signals (e.g., data to be written to the memory devices among the memory module  304 ). 
     The RCD  306  can be configured to operate in loopback and pass-through modes. The pass-through mode allows the RCD  306  to decode the input signals  308  and proceed to read and/or write to the memory ranks of the memory module  304  based on the input signals  308 . In one embodiment, the RCD  306  can include one or more logic circuit configured to receive the input signals  308  and generate a set of output control signals that can be transmitted to appropriate memory devices among the memory module  304 . Further, the RCD  306  may be configured to decode instructions from the input signals  308 . For example, the RCD  306  may be configured to receive and decode register command words (RCWs) and buffer control words (BCWs) received from the host device  302 . The pass-through mode allows input signals  308  to be processed by the logic circuit of the RCD  306  and output to one or more memory ranks among the memory module  304 . 
     The loopback mode allows the RCD  306  to feed back samples of one or more signals among the input signals  308 , or among other internal signals in the RCD  306  (e.g., signals being exchanged between components within RCD  306 ), to the host device  302 , or another device, for testing, debugging and training purposes. In one embodiment, the loopback mode of the RCD  306  can be activated by enabling a loopback circuit  310  in the RCD  306 . The loopback circuit  310  may be configured to sample signals among the input signals  308  or internal signals of the RCD  306 , and transmit the sampled signals to the host device  302 . In one or more embodiments, the pass-through mode and the loopback mode can be activated individually or simultaneously. 
     In as aspect, sampling external signals (e.g., signals provided to the RCD  306  by another device outside of RCD  306 , or provided by the RCD  306  to another device), and sampling output signals of the RCD  306 , can allow the host device  302  to determine whether errors are present or absent within the RCD  306 . However, such sampling techniques may not allow the host device  302  to determine which part, or processing stage, or component, of the RCD  306  caused the errors. The integration of the loopback circuit  310  in the RCD  306  can allow the loopback circuit  310  to sample internal signals in the RCD  306  from one or more sampling points within the RCD  306 . The sampled signals from the one or more sampling points inside the RCD  306  can allow the host device  302  to identify specific processing stage or component in the RCD  306  that may be causing errors. In one embodiment, a chip select signal among the input signals  308  can be sampled at multiple sampling points in the RCD  306  to determine whether any error relating to the chip select signal is present or not (e.g., the chip select signal being incorrect, or undesirable delays, etc.) in response to being processed by different processing stages in the RCD  306 . In another embodiment, the loopback circuit  310  may use existing internal loopback lines in the RCD  306  for sampling the chip select signals in order to reduce the number of hardware or traces that may be used for integrating the chip select signal loopback feature. 
       FIG.  4    is a diagram illustrating details of the example register clock driver (RCD)  306  of  FIG.  3    according to an embodiment of the disclosure. In the example shown in  FIG.  4   , the RCD  306  can include the loopback circuit  310 , a receiver  404 , a logic circuit  406 , and an output driver  408 . The receiver  404  can be configured to receive the input signals  308  (see  FIG.  3   ) from the host device  302 . In one embodiment, the receiver  404  can include hardware or circuit components such as buffers and amplifiers. The input signals  308  can pass through these components in the receiver  404 , and can be outputted as intermediate signals  407 . In an aspect, the intermediate signals  407  can deviate from an expected output from the receiver  404  due to variations such as process and temperature variations of the components in the receiver  404 . The receiver  404  can buffer and send the intermediate signals  407  to the logic circuit  406 . 
     The logic circuit  406  can be configured to decode the intermediate signals  407  to generate another set of intermediate signals  409 , and send the intermediate signals  409  to the output driver  408 . In an embodiment, the input signals  308  can include a chip select signal  412  for selecting one or more memory ranks in the memory module  304  (see  FIG.  3   ). The intermediate signals  407  can include a processed version (e.g., buffered, amplified, etc.) of the chip select signal  412 . The logic circuit  406  can decode the intermediate signal  407  including the processed chip select signal  412  and generate intermediate signals  409  that represent voltages that can be applied to the memory module  304  for activating the selected memory ranks. The logic circuit  406  can send the intermediate signals  409  to the output driver  408 . The output driver  408  can output a voltage  430  to activate the memory ranks being selected in the chip select signal  412 . 
     The loopback circuit  310  can be configured to sample the chip select signal  412  at different sampling points inside the RCD  306 . For example, the loopback circuit  310  can sample the chip select signal  412  by obtaining a copy of the intermediate signal  407 , labeled as sampled signal  422 , from a sampling point  414  between the receiver  404  and the logic circuit  406 . The loopback circuit  310  can also sample the chip select signal  412  by obtaining a copy of the intermediate signal  409 , labeled as sampled signal  424 , from another sampling point  416  between the logic circuit  406  and the output driver  408 . The loopback circuit  310  can send the sampled signals  422 ,  424  to the host device  302 . 
     In an embodiment, the host device  302  can store a copy of the chip select signal  412 . The host device  302  can compare the chip select signal  412  with the sampled signals  422 ,  424 , to determine differences among the chip select signal  412  and the sampled signal  422 ,  424 . In one embodiment, the host device  302  can simulate operations of the receiver  404  and the logic circuit  406 . The simulations can result in generation of simulated signals  434 ,  436 , that may be simulated version of the intermediate signals  407 ,  409 , respectively. The host device  302  can compare the intermediate signal  407  with the simulated signals  434  to identify if there are differences between the intermediate signal  407  with the simulated signals  434 . If a difference between the intermediate signal  407  and the simulated signals  434  exceeds a predefined difference (that may be stored in the host device  302 ), the host device  302  can determine that there may be errors in the receiver  404 . The host device  302  can generate a flag and output the flag on, for example, a user interface in a display connected to the host device  302  to notify a user that there may be errors in the receiver  404  and/or to indicate that the receiver  404  may need to be reconfigured to maintain signal integrity of the chip select signal  412  and/or future chip select signals. 
     If a difference between the intermediate signal  409  and the simulated signals  436  exceeds a predefined difference (that may be stored in the host device  302 ), the host device  302  can determine that there may be errors in the logic circuit  406 . The host device  302  can generate a flag and output the flag on, for example, a user interface in a display connected to the host device  302  to notify a user that there may be errors in the logic circuit  406  and/or to indicate that the logic circuit  406  may need to be reconfigured to maintain signal integrity of the chip select signal  412  and/or future chip select signals. 
     In one embodiment, the host device  302  may sample the voltage  430  at the output of the RCD  306 . The host device  302  may simulate the voltage  430 , and compare the sampled voltage with the simulated voltage. If there are no differences between the intermediate signals  407 ,  409  and the simulated signals  434 ,  436 , or if the differences do not exceed predefined differences stored in the host device  302 , but there is a difference between the sampled and simulated versions of the voltage  430 , then the host device  302  can determine that there may be errors in the output driver  408  and the output driver  408  may need to be reconfigured to maintain signal integrity of the chip select signal  412  and/or future chip select signals. 
     In one embodiment, if there is no difference between the intermediate signal  407  and the simulated signals  434 , or if the difference between the intermediate signal  407  and the simulated signals  434  does not exceed the predefined difference stored in the host device  302 , but there is a difference between the intermediate signal  409  and the simulated signals  436 , then the host device  302  can determine that there may be errors in the logic circuit  406 , but not in the receiver  404 . 
     In one embodiment, if there is no difference between the intermediate signal  409  and the simulated signals  436 , or if the difference between the intermediate signal  409  and the simulated signals  436  does not exceed the predefined difference stored in the host device  302 , but there is a difference between the intermediate signal  407  and the simulated signals  434 , then the host device  302  can determine that there may be errors in the receiver  404 , but not in the logic circuit  406 . 
     Further, in one embodiment, the host device  302  can send an enable signal  440  to the loopback circuit  310  to enable or disable the loopback circuit  310 . The enable signal  440  can be, for example, a binary signal such that a binary zero can disable the loopback circuit  310  and a binary one can enable the loopback circuit  310  (or vice versa). In response to disabling the loopback circuit  310 , the RCD  306  can operate in a normal or pass-through mode where the RCD can buffer and decode the input signals  308  for controlling, reading, and/or writing to the memory ranks of the memory module  304 . Note that when the loopback circuit  310  is disabled, the connections between the sampling point  414  and the loopback circuit  310 , and between the sampling point  416  and the loopback circuit  310 , can be opened or disconnected such that signals are not being sampled from the samplings points  414 ,  416 . In response to enabling the loopback circuit  310 , the connections between the sampling point  414  and the loopback circuit  310 , and between the sampling point  416  and the loopback circuit  310 , can be closed or connected such that the loopback circuit  310  can sample signals from the sampling points  414 ,  416 . In one embodiment, the pass-through mode and the loopback mode can be activated simultaneously such that the loopback circuit  310  can sample signals during normal operations of the RCD  306 . 
     By integrating the loopback circuit  310  in the RCD  306 , one or more test points can be inserted in the RCD  306  to maintain signal integrity. For example, chip select signals can be sampled from more than one sampling points inside the RCD  306  as shown in  FIG.  4   . The different sampling points in the RCD  306  can provide information on which specific parts or stages of the RCD  306  may have errors and may need attention. Further, the integration of the loopback circuit  310  allows dedicated ports of the RCD  306  to be assigned to the sampling points (e.g.,  414 ,  416 ), such that testing can be performed periodically to improve and maintain signal integrity during normal operations. 
       FIG.  5    is a circuit diagram of the loopback circuit  310  according to an embodiment of the disclosure. The loopback circuit  310  can include a plurality of circuit components to facilitate sampling and selection of sampled signals that can be fed back to the host device  302  (see  FIG.  4   ). In the example shown in  FIG.  5   , the loopback circuit  310  can include a plurality of multiplexers  502 ,  504 ,  506 ,  508 , and  510 . The multiplexer  502  can be configured to receive a plurality of command/address signals (DCA0_A to DCA6_A), and a parity signal or parity bit (DPAR_A) sampled from one or more sampling points in the RCD  306  (see  FIG.  3   ,  FIG.  4   ). The sampled signals DPAR_A and DCA0_A to DCA6_A can correspond to a first channel of the memory module  304  and the RCD  306  (“Channel A”). The multiplexer  504  can be configured to receive a plurality of command/address signals (DCA0_B to DCA6_B), and a parity signal or parity bit (DPAR_B) sampled from one or more sampling points in the RCD  306 . The sampled signals DPAR_B and DCA0_B to DCA6_B can correspond to a second channel of the memory module  304  and the RCD  306  (“Channel B”). A select signal  520 , labeled as RX_LOOPBACK_SEL, can be provided by the RCD  306  and/or the host device  302  (see  FIG.  3   ,  FIG.  4   ) for selecting one of the sampled signals received by the multiplexers  502 ,  504 , to be outputted to the multiplexer  510 . In one embodiment, the select signal  520  can be a three-bit signal having a first bit, a second bit, and a third bit. 
     The multiplexer  506  can be configured to receive sampled chip select signals (DCS0_A) from a sampling point in the RCD  306 , where the sampled chip select signal DCS0_A corresponds to Channel A. The multiplexer  508  can be configured to receive sampled chip select signals (DCS0_B) from a sampling point in the RCD  306 , where the sampled chip select signal DCS0_B corresponds to Channel B. The sampled chip select signals received by the multiplexers  502 ,  504  can be one or more of the signals sampled from sampling points  414 ,  416 , shown in  FIG.  4   . In one embodiment, one of the bits in the select signal  520  can be used for selecting one of the input sampled signals received by the multiplexers  506 ,  508 , to be outputted to the multiplexer  510 . 
     The multiplexer  510  can be configured to receive outputs from the multiplexers  502 ,  504 ,  506 ,  508 . The multiplexer  510  can also receive sampled external signals such as DLBD_A and DLBD_B that may be sampled externally, such as at sampling points outside of the RCD  306 , between the RCD  306  and the host device  302 , and/or between the RCD  306  and the memory ranks of the memory module  304  (see  FIG.  3   ). The multiplexer  510  can also receive sampled clock signals that may be internal or external to the RCD  306  (e.g., an internal clock signal labeled as Internal QCK in  FIG.  5   ). A select signal  522 , labeled as RX_LOOPBACK_CTRL, can be provided by the RCD  306  and/or the host device  302  for selecting one of the sampled signals received by the multiplexer  510  to be outputted as sampled signals  422 ,  424  shown in  FIG.  4   . In one embodiment, the select signal  522  can be a three-bit signal having a first bit, a second bit, and a third bit. 
     In one embodiment, the select signal  522  can control the selection of the sampled chip select signals to be outputted by the multiplexer  510 . For example, the third bit (or the least significant bit) in the select signal  520  can be used for selecting the sampled signals in the multiplexers  506 ,  508 . As such, the select signal  520  being ‘000’, ‘010’, ‘100’, ‘110’ can cause a binary ‘0’ to be transmitted to the selection pins of the multiplexers  506 ,  508 . In an example, the select signal  520  being ‘000’ can cause the multiplexer  502  to select DCA0_A, the multiplexer  504  to DCA0_B, and the binary ‘0’ causes the multiplexers  506 ,  508  to select the chip select signals DCS0_A and DCS0_B, respectively. Therefore, the select signal  520  being ‘000’ can cause the multiplexers  502 ,  504 ,  506 ,  508  to output DCA0_A, DCA0_B, DCS0_A, DCS0_B, respectively. The select signal  522  can select one of the outputs DCA0_A, DCA0_B, DCS0_A, DCS0_B, such that even though the third bit of select signal  520  is shared by multiple multiplexers, the multiplexer  510  can output a desired sampled signal. By utilizing existing bits in select signals for other sampled internal signals, the loopback circuit  310  can provide sampling of chip select signals internally without having to design and assign new control word bits for chip select sampling and loopback. 
     In one embodiment, the multiplexers  502 ,  504 ,  506 ,  508 ,  510  can be considered as one group of loopback block within the loopback circuit  310 . The loopback circuit  310  can include more than one loopback block identical to the example shown in  FIG.  5    that may receive different types of sampled signals to be fed back to the host device  302 . For example, the multiplexers  502 ,  504 ,  506 ,  508 ,  510  shown in  FIG.  5    can be configured for sampling the command/address signals, parity signals, chip select signals from inside the RCD  306  and external data signals DLBD_A and DLBD_B. The loopback circuit  310  can include another loopback block having a similar arrangement of multiplexers  502 ,  504 ,  506 ,  508 ,  510  that can be configured for sampling the command/address signals, parity signals, chip select signals from inside the RCD  306  and external clock signals (e.g., instead of DLBD_A and DLBD_B, the multiplexer  510  may receive external clock signals). 
       FIG.  6    is a flowchart of an example process  600  that may implement a register clock driver with a chip select loopback circuit according to an embodiment of the disclosure. The process  600  can include one or more operations, actions, or functions as illustrated by one or more of blocks  602 ,  604 , and/or  606 . Although illustrated as discrete blocks, various blocks can be divided into additional blocks, combined into fewer blocks, eliminated, in different order, or performed in parallel, depending on the desired implementation. 
     The process  600  can be implemented by a register clock driver (RCD) of a memory module. The process  600  can being at block  602 . At block  602 , the RCD can receive a chip select signal for selecting one or more memory ranks of the memory module. In one embodiment, the memory module can be a DDR5 load-reduction dual in-line memory module (LRDIMM) including synchronous dynamic random-access memory (SDRAM) devices. 
     The process  600  can proceed from block  602  to block  604 . At block  604 , the RCD can sample the chip select signal from a first sampling point between a receiver of the RCD and a logic circuit of the RCD. The process  600  can proceed from block  604  to block  606 . At block  606 , the RCD can sample the chip select signal from a second sampling point between the logic circuit of the RCD and an output driver of the RCD. The process  600  can proceed from block  606  to block  608 . At block  608 , the RCD can send the sampled chip select signals to a memory controller. In one embodiment, the RCD may perform the operations in one of block  604  and block  606 . In another embodiment, the RCD may perform operations in both blocks  604 ,  606 , individually or simultaneously. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements, if any, in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The disclosed embodiments of the present invention have been presented for purposes of illustration and description but are not intended to be exhaustive or limited to the invention in the forms disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiments were chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.