Abstract:
A memory module adapted for installation in an open memory socket on a mainboard of a computer. The memory module includes a substrate with an edge connector comprising pins along an edge of the substrate, and at least one memory package mounted to the substrate and containing a memory die electrically connected to input/output leads located along the perimeter of the memory package and through which data signals are transmitted to and from the memory die. Data signal lines electrically connect a plurality of the input/output leads of the memory package to a plurality of the pins of the edge connector. Termination resistors individually electrically connect each of the data signal lines to ground, a supply voltage, or a reference voltage of the memory package so as to reduce noise and signal reflections through the data signal lines.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/522,171, filed Aug. 25, 2004. 

   BACKGROUND OF THE INVENTION 
   The present invention generally relates to memory modules for installation in computer memory subsystems. More particularly, this invention relates to a memory module equipped with termination resistors located directly on or immediately adjacent the I/O pins of memory packages of the module to reduce noise and signal reflections. 
   The computer industry has moved to higher speed grades not only in the field of processor technology but also relating to all peripheral devices including the system memory. The latter has become the main bottleneck in the overall system performance in that, with increasing clock rates, the central processors are starved for data and more and more cycles are wasted idly because of the lack of data and instructions to be processed. 
   Memory clock and data frequency are limited primarily by two different factors, the first being the core and I/O design of the actual memory IC and the second being the interfacing with the rest of the system logic. In the case of the first, consumer-class graphics boards have shown that speed grades of 700-800 MHz data rate are within realistic expectations of high-grade components. However, one must bear in mind that this is achieved using a point to point connectivity without socketed interfaces. 
   One particular obstacle in the case of system memory is the interfacing of the memory devices themselves with the rest of the system. Such interfacing typically involves soldering memory components (e.g., packages) onto a substrate, such as a printed circuit board (PCB), forming what is commonly termed a memory module, and routing the signals in and out of the memory module through edge connectors on the PCB to a system board (mainboard or motherboard) using, for example, a dual in-line memory module (DIMM) socket (slot). This kind of interfacing necessarily involves the transition of all signals through a variety of interfaces that include both soldered and pressure contacts. Each transition from one medium to the next generates signal reflections that fill the bus with noise. If the noise levels reach a critical threshold, they will interfere with the signal integrity of the actual data, addresses, and commands and induce errors on several levels. Such degradation of signal quality can be of particularly concern with higher signal frequencies or with digital signals that exhibit shorter rise times. Where lower power supply voltages are employed, electrical resonances and reflections can also be of concern because lower noise is required to avoid detrimentally affecting signal integrity and electrical system reliability. 
   Suffice it to say that the amount and level of signal reflections are inversely correlated to the quality of the interface. Consequently, a major hurdle toward reaching clean signals is the socket interface formed by the edge connections of the memory module and the DIMM slot of the system board. The most common approach to reducing noise and increasing signal integrity as well as providing termination of the actual signal has been the use of termination resistors in signal paths. In the first generation DDR (or DDR-SDRAM—double data rate synchronous DRAM) architecture, termination is provided on the mainboard by means of a parallel resistor to the termination voltage (VTTI). One of the improvements of second generation DDR (DDR2) is the presence of the so-called on-die-termination (ODT). Briefly, ODT moves the point at which termination occurs as close to its source as possible—that is, the point of signal origin, which is the die itself. Because of this measure, the input/output (I/O) frequency can be increased since the improved termination scheme ameliorates shortcomings of the system PCB design and bus interface. 
   The most effective method for reducing noise and signal reflections is their elimination as close to their source as possible. As such, on-board (mainboard) termination is inferior to on-die termination, especially since the termination of all memory slots is shared and, by extension, the noise is shared between all modules in the system before it reaches the termination resistors. However, current die and package designs do not leave enough room to implement efficient termination in the package. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides an approach to reducing noise and signal reflections through the use of termination resistors directly on or immediately adjacent the I/O pins of the memory components, and terminating the resistors to ground, supply voltage, or reference voltage. 
   More particularly, this invention involves a memory module adapted for installation in an open memory socket on a mainboard of a computer. The memory module includes a substrate with an edge connector comprising pins along an edge of the substrate, at least one memory package mounted to the substrate and containing a memory die electrically connected to input/output leads located along the perimeter of the memory package and through which data signals are transmitted to and from the memory die, data signal lines electrically connecting a plurality of the input/output leads of the memory package to a plurality of the pins of the edge connector, and termination resistors individually electrically connecting each of the data signal lines to ground, a supply voltage, or a reference voltage of the memory package so as to reduce noise and signal reflections through the data signal lines. According to a preferred aspect of the invention, neither the memory die nor the mainboard requires termination resistors, though it should be understood that mainboard termination resistors are no hindrance to the use of the current invention. 
   In view of the above, the termination is not on-die, though it occurs as close as possible to the die and will not allow contention of the bus with noise. Another benefit in this approach is the minimum time until signal and noise are terminated, which frees the bus immediately after the actual signal is propagated, whereas, a noticeable delay is encountered in the case of an on-board termination on the mainboard. 
   According to an optional aspect of the invention, the termination resistors can be implemented in the form of a termination to the VRef value to help both high and low signal voltages to return to the crossover point much faster because of an additional push-pull mechanism for the high-low voltage swing. According to another optional aspect of the invention, the resistors can be terminated to the supply voltage (VDD or VSS) to account for asymmetries in the voltage swing, or a virtual termination voltage could be used that differs from VRef and can be adjusted above or below VRef to optimally compensate for asymmetries in the high-low voltage swing of the memory components used. 
   A shortcoming of static termination schemes is the constant passive current draw to the termination sink, which occurs regardless of whether the termination is at the level of the supply voltage, the ground or any termination voltage. In addition, termination of the signal itself will also weaken it and, therefore, have disadvantages in that it would require higher drive strength of the output buffers. For either reason, it can be advantageous to implement a switch to turn On or Off the termination depending on whether the devices are active or not. This type of dynamic termination is implemented in the on-die Termination of the DDR2 and later designs and reduces the passive power dissipation of the memory subsystem. Because of such benefits, the present invention encompasses an embodiment in which the chip select signal that selects the active physical bank within the memory subsystem (or another suitable signal) is used as a control signal to turn the termination On or Off. 
   In view of the above, the present invention provides for the elimination or at least significant reductions in signal reflections directly adjacent to the die, which results in faster termination of signals and freeing up the bus. Because the invention can be implemented by modifying only the memory module, such as the PCB, and does not require modifications to the mainboard or memory die, the invention offers the further advantage of backward compatibility with existing systems. According to optional aspects of the invention, the above advantages can be obtained while also adjusting for asymmetries of the memory component electrical characteristics. 
   Other objects and advantages of this invention will be better appreciated from the following detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic layout of a computer mainboard with on-board termination resistors to ground in accordance with the prior art. 
       FIGS. 2 and 3  provide a schematic comparison of mainboard termination and on-die-termination in accordance with prior art practices. 
       FIG. 4  shows a schematic representation of termination resistors terminating I/O lines on a memory module PCB in accordance with an embodiment of the present invention. 
       FIG. 5  shows a schematic representation terminating the output of a memory DRAM package with an on-package integrated resistor network that can be tied either to the termination voltage, to ground, or to the supply voltage in accordance with another embodiment of the present invention. 
       FIG. 6  shows a schematic representation of an on-package termination of I/O pins to the reference voltage (VTT) in accordance with a third embodiment of the present invention. 
       FIG. 7  shows a schematic representation of an on-package termination of I/O pins to the reference voltage (VTT), which can be turned On or Off using the Chip Select signal as a control signal in accordance with a fourth embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As previously discussed, the problem of signal reflections in memory subsystems of computers has typically been addressed in the past by adding termination resistors on the mainboard. Such an approach is depicted in  FIG. 1 , which shows a schematic layout of a computer mainboard  10 , including a CPU socket  12  and a chipset containing a memory controller  14  connected to memory slots (sockets)  16  with data lines  24 . Memory modules  18 , such as single in-line memory modules (SIMM&#39;s) or dual in-line memory modules (DIMM&#39;s), are represented as being installed in memory slots  16 . Also shown is a graphics interface  20  (AGP or PCIe or similar) and expansion slots  22 . In accordance with conventional mainboard termination approaches to reducing signal reflection, each of the data lines  24  is schematically portrayed as being individually terminated to ground with a termination resistor  26 . In practice, the positions of the termination resistors  26  will depend on the specific layout of the mainboard  10 . 
     FIG. 2  is a schematic representation of the termination approach of  FIG. 1 , and in combination with  FIG. 3  provides a schematic comparison of mainboard termination and the more recent on-die-termination (ODT) approach. In  FIG. 2 , signal reflections  30  are represented as occurring between a pair of memory components, identified in  FIG. 2  as DRAM packages  28 , one of which is denoted as being active while the other is in stand-by. In contrast, the on-die-termination approach of  FIG. 3  is represented as eliminating inter-component reflections by placing termination resistors  26  directly on the dies within the packages  28 . Integration of resistor termination directly onto the memory die as represented in  FIG. 3  has been adopted in second and third generation DDR (DDR2 and DDR3, respectively). The on-die-termination approach depicted in  FIG. 3  is dynamic, meaning that it can be turned on and off depending on load as opposed to the static termination used on the mainboard of  FIGS. 1 and 2 . In addition, on-die-termination techniques allow for changes in the resistor value depending on the overall system memory density during memory initialization on boot-up. However, while providing the above advantages, the approach taken in  FIG. 3  cannot be retrofitted to existing dies without substantial changes and costs to integrate such on-die-termination into existing designs. 
     FIGS. 4 and 5  schematically represent two approaches by which signal reflections can be eliminated or reduced and the above-noted disadvantages of the prior art overcome by placing termination resistors at or immediately adjacent the I/O leads of memory component packages. As such, termination is integrated onto the memory module PCB or the memory package, as opposed to the mainboard (on which the module is installed) or the memory die (contained within the memory package). In this manner, the signals are terminated long before they reach the mainboard. Aside from elimination of ringing and interferences between the reflected signals and those generated by either the memory packages or the memory controller, an additional benefit is that signal reflections and other interface-generated noise never reach the mainboard traces (e.g., lines  24  in  FIG. 1 ) that are shared among memory modules on a per channel basis. Therefore, an advantage of the current invention is that a signal reflection from, for example, DIMM 0, that causes some undesired stray signals on the bus is not broadcasted toward any other memory modules, for example, DIMM 1, DIMM 2, etc., where they could interfere with signal integrity. Finally, in addition to providing advantages with respect to better signal integrity, the present invention also enables memory modules to remain backwards-compatible with previous memory subsystems and memory dies, making higher frequency operation possible beyond the original scope of the DDR design since termination of stray signals occurs faster and the bus is freed up faster. 
   In  FIG. 4 , a memory package  32 , such as a DRAM, SDR SDRAM, or DDR SDRAM chip, is schematically represented on part of a PCB  18  of a memory module, such as a SIMM or DIMM. For clarity, only the features of the package  32  and PCB  18  necessary for those skilled in the art to gain an understanding of the invention are shown. Data signal lines  36  electrically connect I/O leads (pins)  38  of the package  32  to I/O pins of an edge connector  34  on the PCB  18 . Parallel termination resistors  40  are represented as being located on the PCB  18  immediately adjacent the I/O leads  38  of the package  32 . As used herein, the resistors  40  are said to be immediately adjacent the I/O leads  38  on the basis that the resistors  40  are physically located between the package  32  and any adjacent memory component, as opposed to being located on the PCB  18  between the package  32  and the edge connector  34 . More particularly, the resistors  40  are depicted in  FIG. 4  as being connected to the signal lines  36  as close as physically possible to the leads  38 , with the result that the resistors  40  are generally aligned along the edges of the package  32  with some of the resistors  40  being between adjacent signal lines  36 . Instead of being placed alongside the package  32 , it is also within the scope to terminate the termination resistors  40  to a common termination voltage beneath the package  32  to ensure equal path lengths for all signal lines to the termination device. The resistors  40  may be thick-film resistors or discrete resistors mounted to the PCB  18  or integral parts of a customized PCB  18 . Suitable resistance values for the resistors  40  are in a range of about 20 to about 150 ohms, more preferably about 60 to about 110 ohms. 
   As with  FIG. 4 ,  FIG. 5  also depicts only those features necessary to gain an understanding of the invention.  FIG. 5  schematic represents the placement of parallel termination resistors  50  as part of an integrated resistor network formed directly on the exterior of a memory package  42 . As such, the resistors  50  are said to be immediately adjacent the I/O leads  38  on the basis that the resistors  50  are physically located on the package  42 , though not on the memory die (not shown) within the package  42 . Similar to  FIG. 4 , I/O leads  48  of the package  42  are electrically connected via signal lines  46  to I/O pins of an edge connector  44  of the PCB  18 . Because the resistors  50  are located on the package  42 , they can be tied either to ground, the termination voltage (VTT), or the supply voltage (VSS) of the package  42 . As with the embodiment of  FIG. 4 , the resistors  50  may be thick-film resistors or discrete resistors mounted to the package  42  or integrated into the PCB design, and suitable and preferred resistance values for the resistors  50  are in a range of about 20 to about 150 ohms and about 60 to about 110 ohms, respectively. 
   From the above, those skilled in the art will appreciate that the embodiments of  FIGS. 4 and 5  are able to provide memory module manufacturers with such additional benefits as the possibility of selectively tuning the termination values to match the detailed characteristics of the memory components used. Moreover, the current invention allows a much higher degree of flexibility in the design in that, in addition to termination of I/O signals, address and command signals can also be terminated using the on-module and on-package termination approaches of this invention. 
   According to an alternative approach of the invention, termination resistors can be implemented in the form of a termination to the VRef value to help both high and low signal voltages to return to the crossover point much faster because of an additional push-pull mechanism for the voltage swing. Such an approach is represented in  FIG. 6 , in which on-package termination of a DRAM package  52  is represented with I/O leads  58  of the package  52  being terminated to the reference voltage VTT. For this purpose, a conducting plane  54  is shown connected to the VTT supply  56 , and each I/O lead  58  is individually connected to the plane  54  with a resistor  60 . A suitable resistance value for the resistors  60  is about 100 ohms resistor, with a suitable range being about 20 to about 150 ohms. With this same approach, the termination resistors  60  could be terminated to supply voltage (VDD or VSS) or ground to account for asymmetries in the signal swing. Another alternative is to terminate the resistors  60  to a virtual termination voltage that differs from VRef and can be adjusted above or below VRef to optimally compensate for asymmetries in the high-low voltage swing of the memory components used. The bias achieved this way allows precise matching of high and low times (with reference to the crossover point) by shifting the termination voltage to a positive or negative bias. Since the highest attainable frequency is limited by the shorter of the two clock hemi-cycles, distributing 50% each to high and low voltage swing would allow the module to run at the highest frequency attainable by the package  52 . 
   According to an additional aspect of the invention, the termination drain can be dynamically coupled or decoupled to the termination resistors  60  as shown in  FIG. 7 , where a switch  62  is implemented using the chip select signal as control signal for opening or closing of the connection to the termination voltage, thus turning On and Off the termination according to whether the physical bank of memory is active or not. It should be understood that other signals such as, for example, the write-enable signal could be used to differentiate the termination according to Read or Write. 
   While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.