Wavelength division multiplexed memory module, memory system and method

[0001] This invention relates to computer systems, and, more particularly, to a computer system having a memory hub coupling several memory devices to a processor or other memory access device.

[0002] Memory modules are in common use in computer systems in the form of double in-line memory modules (“DIMMs”). An example of a conventional DIMM-type memory module 10 is shown in FIG. 1. The memory module 10 includes a circuit board substrate 14 on which several memory devices 20, typically dynamic random access memories (“DRAMs”), are mounted. In the DIMM-type memory module 10 shown in FIGS. 1, 8 memory devices 20 are mounted on each side of the substrate 14. Terminals 24 are formed along an edge of the substrate 14, which mate with slotted connectors (not shown) typically mounted on a computer system mother-board. The terminals 24 are electrically coupled to the power and signal terminals on the memory devices 20. The terminals 24 in the DIMM-type memory module 10 shown in FIG. 1 are on each side of the substrate 14. Also mounted on the substrate 14 may be a register 26 that stores command and address signals applied to the memory module 10 through the terminals 24 responsive to a clock signal that is also applied to the memory module 10 through the terminals 24. The register 26 then applies the command and address signals to the memory devices 20. Memory modules having a register 26 operating in this manner are known as “registered DRAM modules.” However, it should be understood that memory modules often do not include the register 26, and they may include components in addition to those shown in FIG. 1.

[0003] As the speed of computer systems continues to increase, the operating speed of memory devices has increased in a corresponding manner. A portion of a computer system 30 shown in FIG. 2 includes three memory modules 10a,b,c coupled to a system controller 32 though a common data bus 34, address bus 36 and command bus 38. The system controller 32 initiates a memory operation by coupling a memory request in the form of a memory command and a memory address (generally in the form of a row address and a column address) to all of the memory modules 10 through the command bus 38 and the address bus 36, respectively. If the memory operation is a write operation, the system controller 32 will also couple write data to the memory modules 10 through the data bus 34. To prevent all of the memory modules 10 from responding to the memory request, the system controller 32 also generally applies a unique chip select or similar select signal to each of the memory modules 10. A unique select signal is thus applied to each of the memory modules 10 so that only the memory module 10 receiving the select signal responds to the memory request.

[0004] The bandwidth of data between the system controller 32 and the memory modules 10 could be increased by simultaneously accessing all of the 16 memory devices 20 (FIG. 1) in each of the modules 10. For example, if the 16 memory devices 20 included in the memory module 10 could be divided into 2 sets or “ranks” of 8 memory devices and both of the ranks could be accessed at the same time, data could be read from the ranks at a rate that is 2 times faster than the rate at which data can be read from each rank of the memory devices 20. Unfortunately, data can be accessed in conventional memory modules 10 only one rank of 6 memory devices 20 at a time. As the operating speed of memory devices continue to increase, the bandwidth of data coupled from the memory modules 10 threatens to be limited by the bandwidth of the data bus 34 coupled between the system controller 32 and the memory modules 10.

[0005] Another factor that limits the operating speed of computer systems using the system controller 32 coupled to the memory modules 10 through the buses 34-38 is the need to allow for a settling time between writing data to a memory module 10 and reading data from a memory module 10. When the system controller 32 outputs data to the memory modules, the data signals are reflected from various locations, such as the junction between the data bus 34 and terminals 24 (FIG. 1) on the substrates 14 of the modules 10. Therefore, signal induced noise is present on the data bus for a considerable period after data have been written to the memory modules 10. Signal induced noise is generated on the data bus for the same reason in a read operation when one of the memory modules 10 couples data onto the data bus 34 for transfer to the system controller 32. This noise must be allowed to settle before data are subsequently written to or read from the memory modules 10 or else the noise may be mistakenly interpreted as read or write data. The need to provide for a settling time can markedly reduce the effective memory bandwidth of computer systems and other devices using memory modules.

[0006] There is therefore a need for a computer system architecture and memory module that permits a higher bandwidth of data transfer to and from memory modules and that does not require settling times between a memory accesses.

[0007] A plurality of memory modules are optically coupled to a controller. Each of the memory modules includes a plurality of memory devices and an optical memory hub. The optical memory hub in each memory module is electrically coupled to the memory devices in the module, and includes an optical input/output port coupled to an optical input/output port of the controller. The optical memory hub in each memory module receives optical input signals and applies corresponding electrical signals to the memory devices in the module. The optical memory hub in each memory module also receives electrical signals from the memory devices in the module and transmits corresponding optical output signals. The optical memory hub in each of the memory modules receives or transmits at least some optical signals at a wavelength that is different from the wavelength of at least some optical signals received or transmitted by the optical memory hubs in a plurality of the other memory modules.

[0015] FIG. 3 is a block diagram of a portion of a computer system according to one example of the present invention. A controller 40, such as a system controller or a memory controller, is coupled to 4 memory modules 44a-d though an optical communication path 50, although a greater or less number of modules 44 may be coupled to the controller 40. The controller 40 and the memory modules 44a-d are coupled to the optical communication path 50 through optical input/output ports 56, 62a-d, respectively. The controller 40 may communicate with the memory modules 44 in any of a variety of communications protocols, but they preferably communicate using optical signal packets that contain data, address and command information. The optical communication path 50 may be one or more optical waveguides, such as optical fibers or waveguides mounted on printed circuit boards, free space, or some other optical coupling medium that allows light to be transmitted between the controller 40 and the memory modules 44. The controller 40 can transmit and receive light at any of a plurality of wavelengths λ. The memory modules 44 preferably can also transmit and receive light at any of a plurality of wavelengths λ, as will be explained in greater detail below. The memory modules 44 also preferably have electrical terminals 58 through which at least power may be applied to the memory modules 44.

[0016] As further shown in FIG. 3, each memory module 44 includes a wave-division multiplexed optical hub 70 having an optical port 72 optically coupled to the communication path 50. The optical hub 70 can receive light at any of a plurality of wavelengths λ, and it converts the received light into corresponding electrical signals that are applied to a bus system 78. The optical hub 70 also receives electrical signals from the bus system 78, and it converts the received electrical signals to corresponding optical signals, preferably at any of a plurality of wavelengths λ. These optical signals are applied to the optical port 72 and coupled to the controller 40 through the optical communication path. The bus system 78 couples the optical hub 70 to several memory devices 80, which may be, for example, synchronous random access memories (“SDRAMs”). The bus system 78 may include, for example, separate command, address and data buses, although it may alternatively include a greater or lesser number of buses. For example, a single bus may be used to couple one or more packets containing command, address and data bits between the optical hub 70 and the memory devices 80.

[0017] With further reference to FIG. 3, each memory module 44 may also include a volatile or non-volatile memory device 84, such as an electrically erasable programmable read only memory (“EEPROM”), for storing parameters indicative of the operation of the memory module 44. For example, the memory device 84 may store information specifying the wavelength λ the memory module 44 will use to communicate with the controller 40 through the optical communication path 50. The memory device 84 is preferably electrically coupled to the optical hub 70 through lines 88. However, the non-volatile memory device 84 may also be programmed through a serial bus (not shown) of the type that is typically used to program EEPROMs in memory modules. The parameters of the memory module 44 are read by the controller 40 through a bus 52, which may be either a serial bus or a parallel bus. The controller 40 may read the parameters from the memory device 84 at power-up, for example, to allow the controller 40 to configure itself in accordance with the parameters of the memory module 44.

[0018] Although the optical communication path 50 may include a single communication link through which command, address and data signals are coupled, it preferably includes several communication links operating in conjunction with each other. For example, a first communication link may be used to couple the data signals between the controller 40 and the memory module 44, and a second communication link may be used to couple command and address signals from the controller 40 to the memory module 44. In either case, the command, data and address signals are preferably coupled through a conventional communication protocol, such as by sending data packets, time-division multiplexing, etc. Whatever communication protocol is used, wavelength division multiplexing is used to allow the controller 40 to selectively communicate with the memory modules. A variety of wavelength division multiplexing protocols may be used to allow communication between the controller 40 and the memory modules 44. For example, with reference to FIG. 4, a first wavelength λ₁ is used to communicate between the controller 40 and the optical hub 70 in the first memory module 44a, a second wavelength λ₂ is used to communicate between the controller 40 and the optical hub 70 in the second memory module 44b, a third wavelength λ₃ is used to communicate between the controller 40 and the optical hub 70 in the third memory module 44c, and a fourth wavelength λ₄ is used to communicate between the controller 40 and the optical hub 70 in the fourth memory module 44d. Using this protocol, the controller 40 may simultaneously communicate with two or more memory modules 44, and, in doing so, may be simultaneously performing two or more memory operations of the same type or of different types. For example, the controller 40 may be simultaneously reading data from the second memory module 44b using light at the second wavelength λ₂, and reading data from the fourth memory module 44d using light at the fourth wavelength λ₄. As a further example, the controller 40 may be writing data to the first memory module 44a using light at the first wavelength λ₁ at the same time the controller 40 is reading data from the third memory module 44c using light at the third wavelength λ₃. Use of this protocol is most appropriate where several memory modules 44 are coupled to the controller since there is no advantage to using this protocol of wavelength division multiplexing where a single memory module 44 is coupled to the controller 40.

[0019] In another aspect of the invention, the communications protocol shown in FIG. 5 can be used. In the protocol, read data are coupled from a plurality the memory modules 44 using optical signals having a first wavelength λ₁, and write data are coupled to a plurality the memory modules 44 using optical signals having a second wavelength λ₂. In order to allow the controller 40 to selectively direct memory requests to only one of the memory modules 44, the command and address signals are coupled to each of the memory modules 44 at different wavelengths. More specifically, as shown in FIG. 5, command and address signals are coupled to the first memory module 44a at a third wavelength λ₃, command and address signals are coupled to the second memory module 44b at a fourth wavelength λ₄, command and address signals are coupled to the third memory module 44c at a fifth wavelength λ₅, and command and address signals are coupled to the fourth memory module 44d at a sixth wavelength λ₆.

[0020] The advantage of the communication protocol shown in FIG. 5 over the protocol shown in FIG. 4 is that the protocol of FIG. 5 allows the controller 40 to read from one memory module, e.g., memory module 44b, at the same time that the controller 40 is writing to another memory module, e.g., memory module 44c. However, unlike the protocol shown in FIG. 4, the protocol shown in FIG. 5 does not allow read data to be coupled from two different memory modules 44 at the same time. The protocol shown in FIG. 5 is best suited to systems having relatively few memory modules 44 and/or processors or other memory access devices because there is less likely to be data bus conflicts in such systems.

[0021] Although two communications protocols have been shown in the drawings and explained herein, it should be understood that other communications protocols may be used. For example, instead of coupling commands and addresses to the memory modules 44 using optical signals having different wavelengths for each memory module, the same wavelength, may be used for a plurality the memory modules 44, and some other means may be used to selectively enable only one of the memory modules 44 to respond to the commands and addresses. For example, the memory modules 44 may have non-overlapping addresses so that the address signals coupled to the memory modules 44 uniquely identify each module 44. Other communication protocols that may be used will be apparent to one skilled in the art.

[0022] Although the wavelength division multiplexing protocol that is used by the controller 40 may be fixed, the controller 40 may dynamically select a wavelength division multiplexing protocol based on a variety of operating conditions. For example, the controller 40 may select a protocol based on the number of memory modules 44 in a system. For a larger number of memory modules, the wavelength division multiplexing protocol shown in FIG. 4 might be selected. For a smaller number of memory modules, the wavelength division multiplexing protocol shown in FIG. 5 might be selected. The controller 40 might also select a wavelength division multiplexing protocol based on the nature of the software application currently being executed in a computer system. For example, a graphics intensive application like video games might select one type of wavelength division multiplexing protocol while a computationally intensive application like spreadsheets might select a different wavelength division multiplexing protocol.

[0023] There are also a variety of techniques that can be used to designate the wavelength λ of the optical signals each memory module 44 can receive and the wavelength λ of the optical signals each memory module 44 can transmit. Each memory module 44 can, of course, be fabricated to receive and transmit optical signals having specific wavelengths λ. It would then be necessary to ensure that memory module operating at different wavelengths were present in a computer system or other electronic system. It would be less desirable for two memory modules 44 operating at the same wavelength λ to be present in a system. It is preferable to make all of the memory modules 44 identical, and to program them for different wavelengths after the modules 44 have been installed in a computer system or other electronic system.

[0024] One technique for programming each of several memory modules to operate at respective wavelengths λ (for either read data, write data, commands or addresses) is to apply at least one wavelength identifying signal to the memory module 44 based on which connector receives the memory module. For example, as shown in FIG. 6, each of four memory modules 90a-d is plugged into a respective slotted connector 94a-d. A number of wavelength identifying terminals 96 are provided at the edge of each of the modules 90. Each of the connectors 94 has one of several terminals 98 coupled to a predetermined voltage, such as ground, based on the location of the connector 94. Thus, the first connector 94a has its first terminal 98 coupled to ground, the second connector 94b has its second terminal 98 coupled to ground, the third connector 94c has its third terminal 98 coupled to ground, and the fourth connector 94d has its fourth terminal 98 coupled to ground. Each of the memory modules 44 includes a detection circuit 100 coupled to the wavelength identifying terminals 96 and to the optical hub 70. Using conventional circuitry, the detection circuitry 100 can detect which wavelength identifying terminal 96 is coupled to ground and cause the optical hub 70 in each of the memory modules 90 to transmit and receive optical signals at wavelengths corresponding to the connector 94 in which the memory module 90 is inserted. Other encoding schemes could also be used, such as by binary coding the terminals coupled to ground.

[0025] Another approach to programming memory modules is to program the memory modules upon power-up or at some other time. For example, a respective select signal can be applied sequentially to each of the memory modules 44 (FIG. 3) while to controller 40 outputs an optical signal having the wavelength λ that the memory module 44 will transmit and/or receive. For example, an optical signal having a wavelength of λ₁ may be transmitted by the controller 40 while the controller 40 applies a select signal to the first memory module 44a. The controller 40 then outputs an optical signal having a wavelength of λ₂ while applying a select signal to the second memory module 44b, etc. Thereafter the first memory module 44a will transmit and receive optical signals having the first wavelength λ₁, and the second memory module 44b will transmit and receive optical signals having the first wavelength λ₂. Other means of programming the memory modules 44 to receive and transmit light at respective wavelengths will be apparent to one skilled in the art. For example, as previously mentioned, however, the memory modules 44 may also be programmed through a serial bus (not shown) of the type that is typically used to program EEPROMs in memory modules.

[0026] A computer system 110 using the controller 40 and memory modules 44 of FIG. 3 according to one example of the invention is shown in FIG. 7. The computer system 110 includes a processor 114 for performing various computing functions, such as executing specific software to perform specific calculations or tasks. The processor 114 includes a processor bus 118 that normally includes an address bus, a control bus, and a data bus. The computer system 110 includes a system controller 120 that is coupled to the processor bus 118. The system controller 120 also includes the controller 40, which is, in turn, optically coupled to memory modules 44a-d through the optical communication path 50. However, it will be understood that the controller 40 may be external to the system controller 120 and coupled to it or some other component in the computer system 110, such as the processor 114. In addition, the computer system 110 includes one or more input devices 130, such as a keyboard or a mouse, coupled to the processor 114 through the system controller 120 to allow an operator to interface with the computer system 110. Typically, the computer system 110 also includes one or more output devices 134 coupled to the processor 114 through the system controller 120, such output devices typically being a printer or a video terminal. One or more data storage devices 140 are also typically coupled to the processor 114 through the system controller 120 to allow the processor 114 to store data or retrieve data from internal or external storage media (not shown). Examples of typical storage devices 140 include hard and floppy disks, tape cassettes, and compact disk read-only memories (CD-ROMs). The processor 114 is also typically coupled to cache memory 144, which is usually static random access memory (“SRAM”).

[0027] From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.