US 7395476 B2
A buffer device for testing a memory subsystem. The buffer device includes a parallel bus port adapted for connection to a slow speed bus and a serial bus port adapted for connection to a high speed bus. The high speed bus operates at a faster speed than the slow speed bus. The buffer device also includes a bus converter having a standard operating mode for converting serial packetized input data received via the serial bus port into parallel bus output data for output via the parallel bus port. The buffer device also includes an alternate operating mode for converting parallel bus input data received via the parallel bus port into serial packetized output data for output via the serial bus port. The serial packetized input data is consistent in function and timing to the serial packetized output data.
1. A method for creating test data for testing a packetized cascade memory subsystem, the method comprising:
receiving test data at a bus converter, the test data in a parallel bus data format and received via a slow speed bus;
converting the test data into a serial packetized data format at the bus converter, the converting the test data resulting in converted test data; and
transmitting the converted test data from the bus converter to the memory subsystem via a high speed bus, wherein the high speed bus operates at a faster speed than the slow speed bus,
wherein the bus converter includes a standard operating mode for converting serial packetized input data received via the high speed bus into parallel bus output data for output to the slow speed bus and an alternate operating mode for the converting the test data into the serial packetized data format, and wherein the serial packetized input data is consistant in function and timing to the converted test data.
2. The method of
3. The method of
4. The method of
5. The method of
receiving test result data from the memory subsystem via the high speed bus, wherein the test result data from the memory subsystem via the high speed bus, wherein the test result data is responsive to the test data and is in the serial packetized data format;
converting the test result data into the parallel bus data format resulting in converted test result data; and
transmitting the converted test result data to a tester device via the slow speed bus.
6. The method of
The invention relates to testing a memory subsystem and in particular, to providing a high speed test interface to a memory subsystem.
Computer memory subsystems have evolved over the years, but continue to retain many consistent attributes. Computer memory subsystems from the early 1980's, such as the one disclosed in U.S. Pat. No. 4,475,194 to LaVallee et al., of common assignment herewith, included a memory controller, a memory assembly (contemporarily called a basic storage module (BSM) by the inventors) with array devices, buffers, terminators and ancillary timing and control functions, as well as several point-to-point busses to permit each memory assembly to communicate with the memory controller via its own point-to-point address and data bus.
FIG. 2, from U.S. Pat. No. 5,513,135 to Dell et al., of common assignment herewith, depicts an early synchronous memory module, which includes synchronous dynamic random access memories (DRAMs) 8, buffer devices 12, an optimized pinout, an interconnect and a capacitive decoupling method to facilitate operation. The patent also describes the use of clock re-drive on the module, using such devices as phase lock loops (PLLs).
FIG. 3, from U.S. Pat. No. 6,510,100 to Grundon et al., of common assignment herewith, depicts a simplified diagram and description of a memory system 10 that includes up to four registered dual inline memory modules (DIMMs) 40 on a traditional multi-drop stub bus channel. The subsystem includes a memory controller 20, an external clock buffer 30, registered DIMMs 40, an address bus 50, a control bus 60 and a data bus 70 with terminators 95 on the address bus 50 and data bus 70.
As shown in
As the density and speed of memory subsystems increases, it becomes more difficult to test memory subsystems with standard test devices. In general, there is a lack of low cost industry test capability for very high speed memory assemblies. Existing mainstream memory device and module testers are capable of operating at data rates of about 200 to 500 megabytes per second (Mb/s) with extensions to 1 gigabyte per second (Gb/s) per pin possible on some test systems, and often at a very high cost. Although higher speed testers are expected in the future, they are not expected to keep pace with rapid performance improvements on future memory modules.
Other known testing solutions include using a built in self test (BIST) mode and/or using a transparent mode. BIST is a pre-programmed or programmable sequence and pattern generator, in conjunction with an error checking capability. BIST is implemented in many new designs, but it is limited in test coverage and flexibility due to the die size and power. The programmability is limited due to logic complexity. In addition, timing and voltage adjustments are also limited. Therefore, while BIST is often used for testing memory subsystems, it is often supplemented with other testing methods for thorough test coverage. Transparent mode refers to the capability of having the automated test equipment (ATE) provide address, command, clocks and data at a conventional speed (e.g., 400 Mb/s data) and the memory module passing the information, unmodified and unchecked, to the DRAMs located on the memory module. The use of the transparent mode for testing does not result in testing the memory subsystem at full operating speed.
Based on the lack of available high speed testers and the limited capability of known test features (e.g., BIST, transparent mode), an alternate method of testing that can be utilized to test high speed devices at a relatively low cost would be useful.
Exemplary embodiments of the present invention include a buffer device for testing a memory subsystem. The buffer device includes a parallel bus port adapted for connection to a slow speed bus and a serial bus port adapted for connection to a high speed bus. The high speed bus operates at a faster speed than the slow speed bus. The buffer device also includes a bus converter having a standard operating mode for converting serial packetized input data received via the serial bus port into parallel bus output data for output via the parallel bus port. The buffer device also includes an alternate operating mode for converting parallel bus input data received via the parallel bus port into serial packetized output data for output via the serial bus port. The serial packetized input data is consistent in function and timing to the serial packetized output data.
Additional exemplary embodiments include a method for testing a packetized cascade memory subsystem. The method includes receiving test data at a bus converter. The test data is in a parallel bus data format and received via a slow speed bus. The test data is converted into a serial packetized data format, resulting in converted test data. The converted test data transmitted to the memory subsystem via a high speed bus. The high speed bus operates at a faster speed than the slow speed bus.
Still further exemplary embodiments of the present invention include a storage medium encoded with machine-readable computer program code for testing a packetized cascade memory subsystem, the storage medium including instructions for causing a computer to implement a method. The method includes receiving test data at a bus converter. The test data is in a parallel bus data format and received via a slow speed bus. The test data is converted into a serial packetized data format, resulting in converted test data. The converted test data transmitted to the memory subsystem via a high speed bus. The high speed bus operates at a faster speed than the slow speed bus.
Referring now to the drawings wherein like elements are numbered alike in the several FIGURES:
Exemplary embodiments of the present invention utilize the same buffer devices contained in a packetized cascade memory subsystem for testing the packetized cascade memory subsystem. When used in the packetized cascade memory subsystem in a standard operating mode, a bus converter within the buffer device receives serial packetized data (i.e., in a serial bus packetized data input format) from a high speed bus (e.g., 1.6 Gb/s and 3.2 Gb/s) and converts the data into parallel bus data (i.e., in a parallel bus memory data output format) at a slower speed (e.g., 400 Mb/s and 800 Mb/s) for communicating with memory devices (e.g., SDRAMs and DDR2s). The high speed bus implements a packetized multi-transfer interface. When used in an alternate operating mode for testing, the bus converter within the buffer device converts slower speed parallel bus data (i.e., in a parallel bus memory data input format) received from testing equipment into serial packetized data (i.e., in a serial bus packetized data output format) for transmission on a high speed bus. The serial packetized data is used as input for testing the memory subsystem via the high speed bus. In this manner, test data may be created by standard testing equipment and converted by the buffer device for use in testing a packetized cascade memory subsystem that includes the buffer device and a high speed bus.
Exemplary embodiments of the present invention provide a high speed test interface to a memory subsystem, such as the one depicted in
Although point-to-point interconnects permit higher data rates, overall memory subsystem efficiency must be achieved by maintaining a reasonable number of memory modules 806 and memory devices per channel (historically four memory modules with four to thirty-six chips per memory module, but as high as eight memory modules per channel and as few as one memory module per channel). Using a point-to-point bus necessitates a bus re-drive function on each memory module, to permit memory modules to be cascaded such that each memory module is interconnected to other memory modules, as well as to the memory controller 802.
An exemplary memory structure includes two unidirectional busses between the memory controller 802 and memory module 806 a (“DIMM #1”) as well as between each successive memory module 806 b-d (“DIMM #2”, “DIMM #3” and “DIMM #4”) in the cascaded memory structure. The downstream memory bus 904 is comprised of twenty-two single-ended signals and a differential clock pair. The downstream memory bus 904 is used to transfer address, control, write data and bus-level error code correction (ECC) bits downstream from the memory controller 802, over several clock cycles, to one or more of the memory modules 806 installed on the cascaded memory channel. The upstream memory bus 902 is comprised of twenty-three single-ended signals and a differential clock pair, and is used to transfer read data and bus-level ECC bits upstream from the sourcing memory module 806 to the memory controller 802. Using this memory structure, and a four to one data rate multiplier between the DRAM data rate (e.g., slow speed or 400 to 800 Mb/s per pin) and the unidirectional memory bus data rate (e.g., high speed or 1.6 to 3.2 Gb/s per pin), the memory controller 802 signal pincount, per memory channel, is reduced from approximately one hundred and twenty pins to about fifty pins.
The memory controller 802 interfaces to the memory modules 806 via a pair of high speed busses (or channels). The downstream memory bus 904 (outbound from the memory controller 802) interface has twenty-four pins and the upstream memory bus 902 (inbound to the memory controller 802) interface has twenty-five pins. The high speed channels each include a clock pair (differential), a spare bit lane, ECC syndrome bits and the remainder of the bits pass information (based on the operation underway). Due to the cascaded memory structure, all nets are point-to-point, allowing reliable high-speed communication that is independent of the number of memory modules 806 installed. Whenever a memory module 806 receives a packet on either bus, it re-synchronizes the command to the internal clock and re-drives the command to the next memory module 806 in the chain (if one exists).
With the aforementioned downstream bus width of twenty-two bits, and the burst of eight, a full frame can constitute up to one hundred and seventy-six unique bits, depending on the assignment or use of these bits and the actual wires on the bus. This width is more than adequate to provide the approximately one hundred and twenty memory signals defined as being required by the memory module in
Also as shown in
The seventy-two bits of data written to the DDR2 memory devices 1004 are shown as m_dq(0:71) 1210, and are shown arriving at the memory devices 1004 one full memory clock after the write command is decoded, as a DDR signal relative to the m_clk 1208. In an exemplary embodiment of the present invention, the data, or m_dq(0:71) 1210 is single ended. The nine DDR data strobes (m_dqs_p) 1212 are also shown, as single ended signals, switching one quarter of a clock cycle prior to the DDR2 SDRAMs 1004, thereby ensuring that the strobe switches approximately in the center of each valid write data bit. In an exemplary embodiment of the present invention, the m_dqs_p 1212 is differential. This diagram demonstrates a burst of four data bits to the memory devices 1004 (wd0 through wd3) with seventy-two bits of memory data being provided to the memory devices every memory clock cycle. In this manner, the data rate of the slower memory modules 806 is matched to the high-speed memory bus that operates at four times the speed of the memory modules.
The command bit field is shown as c0 through c7 and consists of sixty-four bits of information provided to the module over eight transfers. The ECC bit field (ecc0 through ecc3) consists of thirty-two bit positions over eight transfers but is actually formatted in groups of sixteen bits. Each sixteen bit packet consists of four transfers over each of the four wires, and provides the bus level fault detection and correction across each group of 4 bus transfers. The spare bit position may be used to logically replace any of the twenty-one wires, also defined as bitlanes, used to transfer bits in the command, data and ECC fields, should a failure occur in one of those bitlanes that results in errors that exceed a system-assigned failure threshold limit. Using this exemplary embodiment of the present invention provides that out of the one hundred and seventy-six possible bit positions, one hundred and sixty-eight are available for the transfer of information to the memory module 806, and of those one hundred and sixty-eight bit positions, thirty-two bit positions are further assigned to providing ECC protection on the bus transfers themselves, thereby allowing a total of one hundred and thirty-six bit positions to be used for the transfer of information to the memory module 806.
Exemplary embodiments of the present invention include the buffer device 1002 described above including an alternate operating mode to be used in testing high speed cascaded memory subsystems, such as those described above, with a slow speed testing device. As described above, the standard operating mode of the buffer device 1002 includes the receipt, ECC correction and bus conversion from a high speed (e.g., 1.6 Gb/s to 3.2 Gb/s) serial packetized bus to a slower (e.g., 400 to 800 Mb/s) parallel DDR2 memory bus. The alternate operating mode includes having the parallel bus act as the “master” and initiating a bus conversion from the parallel bus to the serial packetized bus when placed in the second operating mode. In the second mode, address, command, clock and data are received on the parallel bus and the buffer device 1002 outputs a properly formatted series of packetized frames which can be used to operate a downstream buffered memory module 806.
Also included in
In addition to using the standard and alternate operating modes in the buffer device 1002 for testing, exemplary embodiments of the present invention allow an enhanced test capability where the buffer device 1002 may be utilized in a stand alone fashion, upstream from the last memory module 806 under test. The buffer device 1002 can forward, on its parallel bus, an exact copy of the information initially sent out by the tester, on the tester parallel bus, to the first parallel to serial bus converter device. The ATE tester device will then be able to compare the information sent out by the tester to the information received by the tester (a pre-defined number of clock counts later) and to determine if any errors have occurred. This capability provides a means of performing low cost bit error testing, of inducing bus errors, of verifying bus error detection, and of permitting full testing of the upstream memory port on the memory module 806 under test at full speed.
In an exemplary embodiment of the present invention, commands and data that have been sent out from the ATE device 1402 will be sent back to the ATE device 1402 via the second buffer device 1002 to verify the integrity of the memory subsystem. For example, a correctable error may be injected and propagated from the ATE device 1402 to the first buffer device 1002 and converted from low speed to packetized high speed. The data with the error is then sent to the first memory module 806 on the downstream memory bus 904. The error is then detected at the buffer device 1002 located on the first memory module 806 (e.g., a status condition is set to indicate an error has been detected) and the data is forwarded on to the second memory module 806. The second memory module 806 also detects an error and logs a status. Eventually, the data with the error arrives back at the second buffer device 1002 and is converted from packetized high speed to low speed. The ATE device 1402 then verifies the command that was originally sent out to the first buffer device 1002.
By implementing the logic associated with standard and alternate operating modes of the buffer device 1002 described herein, it is possible to utilize existing ATE devices to fully test memory modules 806, at speed and at a low cost. This capability further reduces the added cost of having a different high speed interface because the buffer devices 1002 with high speed interfaces can be utilized for their own testing. In addition, exemplary embodiments of the present invention may be utilized for testing the cascaded memory subsystems described herein. The operating mode may be selected via software and/or hardware and memory modules may be shipped with both operating modes or with the alternate operating mode disabled. The alternate operating mode would be enabled for users who perform tests on the memory modules.
As described above, the embodiments of the invention may be embodied in the form of computer-implemented processes and apparatuses for practicing those processes. Embodiments of the invention may also be embodied in the form of computer program code containing instructions embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or any other computer-readable storage medium, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. The present invention can also be embodied in the form of computer program code, for example, whether stored in a storage medium, loaded into and/or executed by a computer, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. When implemented on a general-purpose microprocessor, the computer program code segments configure the microprocessor to create specific logic circuits.
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another.
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