Patent Publication Number: US-8990488-B2

Title: Memory buffer with one or more auxiliary interfaces

Description:
RELATED APPLICATIONS 
     This application is a Continuation of U.S. utility application Ser. No. 13/359,877, filed Jan. 27, 2012, still pending, which claimed the benefit of provisional patent application No. 61/473,889 to Haywood, filed Apr. 11, 2011. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to memory systems, and more particularly to memory systems which include a memory buffer that serves as an interface between a host controller and the RAM chips residing on a DIMM. 
     2. Description of the Related Art 
     Traditional computer systems, such as servers, workstations, desktops and laptops, all use pluggable memory which can be inserted into slots on the computer&#39;s motherboard as needed. The most common form of pluggable memory is the DIMM. Historically, DIMMs contain multiple RAM chips—typically DRAM—each of which has a data bus width of 4 or 8 bits. Typically, eight or nine 8-bit DRAM chips (or twice as many 4-bit DRAM chips) are arranged in parallel to provide each DIMM with a total data bus width of 64 or 72 bits; the data bus, typically referred to as the ‘DQ’ bus, is connected to a host controller. Each arrangement of 64 or 72 data bits using DRAM chips in parallel is termed a ‘rank’. 
     A command/address (CA) bus also runs between the host controller and each DIMM; the CA and DQ busses together form a ‘system’ bus. With a basic unbuffered DIMM, the CA bus is connected to every DRAM on the DIMM. As a result, there is a high electrical load on the CA bus, given by the product of the number of DRAMs times the number of ranks. For the DQ bus, the number of electrical loads is equal to the number of ranks. 
     A buffering device is employed to reduce loading in a ‘load reduction’ DIMM (LR-DIMM), an example of which is illustrated in  FIG. 1 . An LR-DIMM  10  containing multiple DRAM chips  12  uses a logic device  14  to buffer the DQ and CA signals between the DRAMs and a host controller  16 . Logic device  14  may be, for example, a single device such as the iMB (isolating Memory Buffer) from Inphi Corporation. Memory systems of this sort are described, for example, in co-pending U.S. patent application Ser. Nos. 12/267,355 and 12/563,308, which are incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a memory system which includes a memory buffer having one or more auxiliary interfaces, which serve to expand the functionality of the memory buffer. 
     The present memory system includes a memory buffer having an interface arranged to buffer data and/or command bytes being written to or read from a plurality of RAM chips (typically DRAM) residing on a DIMM by a host controller. The memory buffer includes at least one additional interface arranged to buffer data and/or command bytes between the host controller or RAM chips and one or more external devices coupled to the at least one additional interface. For example, the memory buffer may include a SATA interface and be arranged to convey data between the host controller or RAM chips and FLASH memory devices coupled to the SATA interface. The additional interfaces may include, for example, a SATA interface, an Ethernet interface, an optical interface, and/or a radio interface. The memory buffer may reside on the DIMM along with the RAM chips with which it interfaces, or be separate from the DIMM. 
     These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a known memory system which includes a DIMM containing a memory buffer. 
         FIG. 2   a  is a block diagram of a memory system which includes a memory buffer in accordance with the present invention. 
         FIG. 2   b  is a block diagram of a memory system which includes a memory buffer in accordance with the present invention, in which the memory buffer resides on a DIMM. 
         FIG. 3  is another possible embodiment of a memory system containing a memory buffer in accordance with the present invention. 
         FIG. 4  is an embodiment of a memory system which includes two memory buffers in accordance with the present invention. 
         FIG. 5  is another possible embodiment of a memory system which includes memory buffers in accordance with the present invention. 
         FIG. 6  is an embodiment of a memory system which includes a plurality of memory buffers in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present memory system includes one or more memory buffers, each of which is arranged to buffer data and/or command bytes being written to or read from the RAM chips residing on a DIMM by a host controller. Note that the RAM chips are referred to hereinafter as DRAM, though the invention is not limited to any specific memory chip type. The present memory system expands the functionality of the memory buffer in such a system by incorporating one or more additional interfaces which are arranged to provide respective functions. For example, the memory buffer might include a serial ATA (SATA) interface, such that in addition to serving as a buffer between a host controller and the DRAM, the memory buffer also serves as a buffer between the host controller and/or DRAM and external devices capable of interfacing with a SATA interface. 
     One possible embodiment of this concept is shown in  FIG. 2   a . A memory buffer  20  interfaces with a host controller  22  via a host interface  23 , and operates as a buffer for the command and data lines connected to the DRAM residing on one or more DIMMs  24  via a DIMM interface  25 . Memory buffer  20  also includes a SATA interface  26 , such that memory buffer  20  also operates as a buffer between host controller  22  and/or DIMM  24  and any devices connected to and capable of being controlled via the SATA interface. For example, FLASH memory devices  28  might be connected to SATA interface  26  via a controller  30 , such that data from host controller  22  or DIMM  24  can be written to the FLASH memory via SATA interface  26 . In general, the external devices may be connected directly to a memory buffer&#39;s additional interfaces, or be coupled to them via intervening controller devices such as controller  30 , as needed. 
     The approach described herein is applicable to any memory system that employs a memory buffer which serves as an interface between the individual memory chips on a DIMM and a host controller, as long as the system is capable of providing full control of the memory chips to the memory buffer. Memory systems to which the present system might be adapted include systems in accordance with the DDR3, load-reduced DIMM (LRDIMM), registered or unregistered DIMM (RDIMM, UDIMM), non-volatile DIMM (NV-DIMM), or any DDR interface standard specifications. 
     A NV-DIMM system operates to write RAM data to FLASH memory in the event of a power loss. As such, a NV-DIMM system is well-suited for use with the present system: a SATA interface might be included within a memory buffer which interfaces with one or more a NV-DIMMs (per the arrangement shown in  FIG. 2 ); then, data stored in the DRAMs can be efficiently written to the FLASH memory if power is lost. 
     A SATA interface is merely one example of an interface that could be added to a memory buffer as described above. Other possible interfaces include an Ethernet interface, optical interface, and/or a radio interface, as well as interfaces capable of interfacing with additional known or future memory element types including, but not limited to, FLASH and DRAM. 
     Memory buffer  20  may be an IC that is separate and distinct from the DIMM ICs  24 , as shown in  FIG. 2   a . In this case, the DIMMs may be standard DIMMs—i.e., with no modifications needed to operate with the present memory system. 
     Alternatively, the memory buffer may reside on the DIMM containing the DRAM with which it interfaces. This is illustrated in  FIG. 2   b . Here, a DIMM  32  includes a memory buffer  33  as described herein. The DIMM includes a plurality of DRAM chips  34 , and memory buffer  33  includes an interface  35  arranged to buffer data and/or command bytes being written to or read from the DRAM by a host controller  36  via an interface  37 , and further includes at least one additional interface  38  arranged to buffer data and/or command bytes between host controller  36  and/or DRAM  34  and one or more external devices coupled to the at least one additional interface. 
     When the memory buffer resides on the DIMM as shown in  FIG. 2   b , it is likely that the DIMM will need to deviate from a standard form in order to accommodate either the memory buffer and/or additional interfaces  37 . The deviation from a standard form may also require additional I/O connectivity, to accommodate the data being handled by additional interfaces  37 . This may require, for example, that additional I/O pins be added to the DIMM&#39;s main connector, or that the DIMM include one or more connectors in addition to the DIMM&#39;s main connector. 
     As noted above, the approach described herein is applicable to any memory system that employs a memory buffer which serves as an interface between the individual memory chips on a DIMM and a host controller, as long as the system is capable of providing full control of the memory chips to the memory buffer. In previous memory systems which employ a memory buffer, the buffer is typically a ‘slave’ device, arranged such that most of the commands it receives from the host controller are simply passed through to the DIMM interface—i.e., the memory buffer does not control the DIMM interface of its own accord. However, to use the additional interfaces, the memory buffer has to have full control of the DRAM chips so that it can read and write between the DRAM and the additional interfaces, as well as preferably do general housekeeping and power management. 
     Control can be given to the memory buffer in several ways or under several conditions. For example, the change of control can occur when the memory system power fails, is indeterminate, and/or is restored. Full control of the DRAM might also be given to the memory buffer via direct or indirect command from the host controller. A direct command would take the form of a special command or signal sent by the host to the buffer. An indirect command could occur when, for example, control is transferred to the memory buffer when the host is not going to access the DRAM for a predetermined about of time, thereby providing some opportunistic access to the memory buffer during this downtime. 
     Multiple interfaces of the same type, or of different types, can be incorporated into the memory buffer as needed. An exemplary embodiment illustrating the addition of two such interfaces is shown in  FIG. 3 . Here, a memory buffer  40  interfaces with a host controller  42  via a host interface  43 , and operates as a buffer for the command and data lines connected to one or more DIMMs  46  via a DIMM interface  47 . Memory buffer  40  also includes two SATA interfaces  48 ,  50  such that memory buffer  40  may also operate as a buffer between host controller  42  or DIMMs  46  and external devices capable of interfacing with a SATA interface, such as FLASH memory devices  52 ,  54  connected to SATA interfaces  48 ,  50  via one or more controllers  56 ,  58 . This sort of configuration can be implemented with memory buffer  40  being separate from DIMMs  46  as illustrated in  FIG. 3 ; alternatively, memory buffer  40  could be implemented on the DIMM containing the DRAM with which it interfaces. 
     A memory system which includes a memory buffer per the present system might also include an interface which enables two or more host controllers to be ‘cross-connected’, so as to enable memory and/or data sharing between hosts. An exemplary embodiment is shown in  FIG. 4 . Memory buffers  60  and  62  interface with host controllers  64 ,  66  via respective host interfaces  68 ,  70 . Each buffer also includes a DIMM interface  72 ,  74  which operates as a buffer for the command and data lines connected to DIMMs  76 ,  78 . Memory buffer  60  also includes a cross-connect interface  80 , which is arranged to buffer data and/or command bytes between host controller  64  or DIMMs  76  and external devices connected to interface  80 . Similarly, memory buffer  62  includes a cross-connect interface  82  arranged to buffer data and/or command bytes between host controller  66  or DIMMs  78  and external devices connected to interface  82 . When cross-connect interfaces  80  and  82  are coupled together as shown, memory buffers  60  and  62  can operate as buffers between each other, as well as between host controllers  64  and  66 , enabling data and/or command bytes to be exchanged between the DIMMs and/or host controllers via the cross-connect interfaces. As with the other embodiments described herein, this sort of configuration can be implemented with memory buffers  60  and  62  being separate from DIMMs  76  and  78  as illustrated in  FIG. 4 ; alternatively, memory buffers  60  and  62  could be implemented on the DIMMs containing the DRAM with which they interface (DIMMs  76  and  78 , respectively). 
     Another possible embodiment is shown in  FIG. 5 , in which memory buffers per the present system enable two or more DIMMs and/or host controllers to be ‘cross-connected’ via a switch. Memory buffers  90  and  92  interface with host controllers  94 ,  96  via respective host interfaces  98 ,  100 . Each memory buffer also includes a DIMM interface  102 ,  104  which operates as a buffer for the command and data lines connected to one or more DIMMs  106 ,  108 . Each memory buffer also includes an ingress and an egress cross-connect interface ( 110  and  112  for buffer  90 ,  114  and  116  for buffer  92 ), which are connected to a switch  118 . The switch can be arranged to enable unidirectional communication (in either direction), or bidirectional communication, between the respective cross-connect interfaces—thereby enabling data and/or command bytes to be exchanged between the DIMMs and/or host controllers via the cross-connect interfaces. 
     Another possible embodiment is shown in  FIG. 6 . Here, a common host (CPU  120 ) is connected to multiple memory buffer chips  122  via memory channels  124 ; each memory buffer chip is in turn coupled to one or more DIMMs ( 2  DIMMs in the example shown in  FIG. 6 ). Each memory buffer has been adapted as described herein to include one or more additional interfaces; here, each buffer includes an ingress interface and an egress interface, which can be connected to each other and to additional external devices as needed. 
     A memory buffer as described herein may also include a direct memory access (DMA) controller capable of writing data to or reading data from the DIMMs containing the DRAM with which the memory buffer interfaces, and conveying data between the DIMM and one or more of the memory buffer&#39;s additional interfaces. For example, if the memory buffer includes a SATA interface, the DMA controller may be arranged to exchange data between the DIMMs and the SATA interface. 
     Some memory buffers includes a self-test engine capable of reading data from the DIMM and comparing it with one or more data patterns; for example, the MemBIST (Memory Built-In Self Test) engine is incorporated into the iMB memory buffer from Inphi Corp. One function of a self-test engine of this sort might be to test the integrity of a DIMM&#39;s DRAM by reading data from it and comparing it against an expected pattern stored in the memory buffer; an ‘error’ occurs if the data does not match the expected pattern, in which case the offending DRAM address is logged and the test continues. By changing the polarity of the error signal generated by the self-test engine, it is possible to change the operation performed by the self-test engine from ‘test’ to ‘search’. In this case, when a pattern read from DRAM matches a pattern stored or inputted into the memory buffer, the DRAM address of the “found” data is logged and the search continues. This search capability could be extended so that there are multiple possible match patterns, as well as “don&#39;t care” bits, which would enable searches to be executed for multiple patterns in parallel with wild cards. 
     Note that a memory buffer as described herein could be implemented as a single integrated circuit (IC), or with a multiple chip chipset with various functions spread among several ICs. For example, a memory system based on the DDR4 standard employs DIMMs which include 9 separate data buffer chips arranged close to the connector contacts and which provide an interface between the connector and the DRAMs, and a central control element which functions as the register section of the DIMM and includes an extra interface to control the data buffers. For this type of chipset implementation, implementing an auxiliary port as described herein requires a path from the data buffers to the central controller. 
     The embodiments shown in the figures and described above are merely exemplary. The present system encompasses any memory system which employs a memory buffer that serves as an interface between the individual memory chips on a DIMM and a host, and which includes at least one additional interface which enables the memory buffer to serve as an interface between the host and/or memory chips and additional external devices. 
     The embodiments of the invention described herein are exemplary and numerous modifications, variations and rearrangements can be readily envisioned to achieve substantially equivalent results, all of which are intended to be embraced within the spirit and scope of the invention as defined in the appended claims.