Buffer to multiply memory interface

[0002] The present disclosure relates to providing data buffers in an interface between a chipset and multiple ranks of memory modules.

[0003] Computer systems often contain one or more integrated circuit (IC) chips (chipset) that are coupled to memory modules using a memory interface. The memory interface provides communication between the IC chipset (e.g. the CPU) and the memory modules. The memory interface may-contain address bus lines, command signal lines, and data bus lines. Increasing demand for higher computer performance and capacity has resulted in a demand for a larger and faster memory. However, as the operating speed and the number of memory modules connected to the chipset increase, the increased capacitive loading may place a substantial limit on the amount and speed of memory.

[0004] Prior art designs, such as a registered dual in-line memory module (DIMM), have addressed the above-described difficulties by providing an address/command buffer in the address bus lines and the command signal lines to relieve the capacitive loading effects. Karabatsos (U.S. Pat. No. 5,953,215) describes a loading relief design for the data bus lines by providing FET switches in the interface between the chipset and the memory modules.

[0005] In the prior art design 100 of FIG. 1, the interface 108 between the chipset 102 and the memory modules 104 is unbuffered. In some embodiments, the memory modules 104 may be individually mounted on memory boards 106 as shown. In other embodiments, the memory modules 104 may be soldered directly onto the same motherboard as the chipset 102.

[0006] In the prior art design 100, the chipset 102 is configured to receive two supply voltages, 1.0 volt (low) and 1.5 volts (high). The high voltage is necessary on the chipset side to provide compatible driving voltage on the memory interface 108. Further, the pin count on the chipset 102 may be designed to be 2x in order to provide a particular memory access rate or frequency, such as ω).

[0007] Different aspects of the disclosure will be described in reference to the accompanying drawings wherein:

[0008] FIG. 1 shows a prior art design of an interface between a chipset and memory modules;

[0009] FIG. 2 illustrates an embodiment of an interface having a plurality of data buffers disposed between the chipset and the memory modules;

[0010] FIG. 3 shows a layout configuration of a data buffer;

[0011] FIG. 4 is a front view of the interface showing the details of memory boards having the memory modules;

[0012] FIG. 5 shows an alternative embodiment of the interface where each memory board contains two ranks of memory modules; and

[0013] FIG. 6 shows a method for buffering the data passed between the chipset and a multiple ranks of memory modules.

[0014] The inventors of the present disclosure recognized that none of the prior art designs offer isolation of supply voltages and interfaces coupled to the chipset and the memory modules. Buffering the address and command lines is relieves the capacitive loading effects, while providing the FET switches in the data lines offers a loading relief on those lines. However, neither design provides electrical isolation between the chipset and the memory data.

[0015] The differences in fabrication process between the chipset and the memory modules place additional burdens on the computer system design. For example, oxides on a memory chip are designed to be thick to provide capacitors with good retention characteristic. Thick oxides also keep leakage current low. However, a higher voltage (on the order of about 1.2 to 1.8 volts) must be supplied to build conducting channels beneath the oxides. The chipset (Central Processing Unit (CPU) or application specific integrated circuit (ASIC) design) fabrication process, on the other hand, promotes thinner oxides providing faster transistors. Therefore, the chipset may be operated at a lower voltage, typically less than 1.0 volt.

[0016] The present disclosure describes methods and systems for providing electrical isolation between the chipset and the memory data. The method includes providing at least one buffer in a memory interface between a chipset and memory modules. Each memory module includes a plurality of memory ranks. The buffer allows the memory interface to be split into first and second sub-interfaces. The first sub-interface is between the chipset and the buffer. The second sub-interface is between-the buffer and the memory modules. The method also includes interleaving output of the memory ranks in the memory modules, and configuring the at least one buffer to properly latch data being transferred between the chipset and the memory modules. The first and second sub-interfaces operate independently but in synchronization with each other.

[0017] Buffering provides isolation of voltages and interfaces coupled to each of the chipset and the memory modules. The isolation of voltages allows the chipset to be operated with a low operating voltage, which substantially precludes the need for the chipset to have a higher voltage common with a memory supply voltage. The memory module is then allowed to operate at voltages appropriate for its own operational purpose. The voltages may be independent of the operating voltage at the connecting system (chipset).

[0018] The isolation of the interfaces allows the inherently faster chipset interface to run at higher multiples of the memory interface rate. For example, the chipset to data buffer interface may run at twice the rate of the buffer to memory interface. This may allow the chipset to operate at twice the rate and access the same amount of data with half the number of data bus lines or pins. This provides computer system designers with a flexibility of utilizing a wider range of memory types and interfaces for a particular computer system. Further, by providing a data buffer on the memory module itself, the memory interface may be simplified by providing a short, fixed length stubs from the buffer to the memory module. In some configurations, the data buffer may be provided on the same motherboard as the chipset. An advantage provided by the electrical isolation that leads to the reduction in the number of pin count is illustrated in the design comparison between FIGS. 1 and 2.

[0019] In the illustrated embodiment 200 of FIG. 2, a plurality of data buffers 206 is disposed in the memory interface between the chipset 202 and the memory modules 204 to provide electrical isolation. For the illustrated embodiment, a multidrop bus 208 provides the interface between the chipset 202 and the data buffers 206. The interface between the chipset 202 and the data buffers 206 may be run at the same data access rate or frequency (ω) as before, but with half the pin count (x) of the prior art design. The interface between the data buffers 206 and the memory modules 204 still has 2x number of pins to provide the same data access rate as before. In practice, x is often selected to be 16 or 32. Moreover, the chipset 202 is configured to operate with only the low voltage (1.0 volt) as shown. The memory modules 204 are operated with only the high voltage (1.5 volts).

[0020] In the illustrated embodiment of FIG. 2, the data buffer 206 is provided on the same memory board 210 as the memory module 204. However, the data buffer 206 may be provided on the motherboard containing the chipset 202.

[0021] FIG. 3 shows a layout configuration of a data buffer 300, similar to the data buffer 206 of FIG. 2, in accordance with an embodiment of the present disclosure. The data buffer 300 includes three portions 302, 304, 306. The first portion 302 is a chipset input/output (I/O) port configured to send and receive data to and from the chipset through the multidrop bus 208. The first portion 302 operates at the same voltage (<1.0 volts) as the chipset. This allows compatibility of interface between the chipset and the data buffer 300. The second portion 304 is a core data path logic portion allowing for buffering of data between the chipset and the memory module. The third portion 306 is a memory I/O port configured to send and receive data to and from the memory module. The third portion operates at the same nominal voltage as the memory module (between 1.2 and 1.8 volts).

[0022] A front view of the memory interface showing the details of the memory boards 402, and highlighting the connections to the data buffers 404 is shown in FIG. 4. In this embodiment, the front view of the memory interface shows the isolation of the memory modules 406 from the chipset 408. With separation of the address and data bus lines shown, the reduction in the pin count can be ascertained. The memory modules in this and other embodiments may be of any memory types. However, in particular, the memory modules may be dynamic random access memories (DRAM), double data rate (DDR) DRAM, or quad data rate (QDR) DRAM. The quad data rate DRAM may be achieved by providing a pin count of 4xin the second sub-interface between the buffer and the memory module, and operating the first sub-interface between the buffer and the chipset at 4 times the rate of the second sub-interface (see FIG. 2).

[0023] FIG. 5 shows an alternative embodiment 500 of FIG. 2, having two ranks 502 of memory modules 504, 505. In the illustrated embodiment 500, the two ranks 502 are on the opposite sides of the memory board 506. However, in other embodiments, the two ranks 502 may be disposed on the same side of the memory board 506.

[0024] In some configurations, the two ranks 502 of the memory modules 504, 505 may be operated in an interleaved mode with the data buffers 508 using the same set of wires. The data from the memory module 504 may be interleaved with the data from the memory module 505 by connecting the output of the two modules 504, 505 in a wired-OR configuration. The outputs may be interleaved by providing a multiple ports 510 on each buffer as shown in FIG. 5. Data from the two modules 504, 505 is then sequentially read into the data buffers 508. Control logic in the data buffer may coordinate the transfer of data from the memory modules 504, 505 in an interleaved mode. Thus in this configuration, the two ranks 502 of the memory modules 504, 505 are bit-wise configured to double the bit numbers required on the data buffer to the chipset interface.

[0025] FIG. 6 shows a method for buffering the data passed between the chipset and a multiple ranks of memory modules that provide isolation of voltages and interfaces. The method includes providing at least one buffer in an interface between a chipset and the multiple ranks of memory modules at 600. The buffers allow the memory interface to be split into two interfaces. The first interface is between the chipset and the buffers. The second interface is between the buffers and the multiple ranks of memory modules. Outputs of the multiple ranks of memory modules are interleaved by wired-OR'ing the outputs of the multiple ranks of memory modules at 602. The buffer is then configured to properly latch the data being transferred between the chipset and the plurality of memory modules at 604. This allows the first and second interfaces to operate independently but synchronized with each other.

[0026] While specific embodiments of the invention have been illustrated and described, other embodiments and variations are possible. For example, although the figures show data buffers providing double (i.e. factor =2) the memory data access rate for a particular number of pin counts, the factor may be any feasible number that affords increased data access rate.

[0027] All these are intended to be encompassed by the following claims.