Abstract:
An existing trace array on a chip is used to store the locations of bit failures from the automatic self-testing of an SRAM array. If a system is having problems, a technician can trigger the automatic test and then scan the trace array, thereby locating a large number of errors on the SRAM array very quickly.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   This application relates to testing of static random access memory (SRAM). More specifically, the application relates to the use of existing on-chip arrays for testing of the SRAM, especially in the system environment. 
   2. Description of Related Art 
   SRAM, short for static random access memory, is a type of memory that is faster and more reliable than the more common DRAM (dynamic RAM). The term static is derived from the fact that the memory doesn&#39;t need to be refreshed like dynamic RAM. SRAM give a faster access time and a shorter cycle time, compared to DRAM, although it is more expensive. Due to its high cost, SRAM is often used only as a memory cache. 
   It is common for many chips made by International Business Machines, Inc. (IBM), such as microprocessor chips containing SRAM caches, to have an automatic built-in self-testing (ABIST) circuits. However, unless the chip is connected to the probes of an advanced testing engine (ATE), it is possible only to test to the first failure and then the test will stop for lack of a place to store information. Not only is the ATE an expensive machine (about a million dollars), but it is also not a device that can be taken into the field to test a customer&#39;s computer when problems develop. It would be desirable to have a method for testing the SRAM on the chip in the field. 
   SUMMARY OF THE INVENTION 
   The invention uses an existing trace array on the microprocessor chip to store the locations of bit failures from the automatic self-test of the SRAM array. If a system is having problems, a technician can trigger the automatic self-test and then scan the trace array, thereby locating a large number of errors on the SRAM array very quickly. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  depicts a data processing system in which a microprocessor chip containing SRAM memory can be installed according to an embodiment of the invention. 
       FIG. 2  depicts a block diagram of a data processing system in which a microprocessor chip containing SRAM memory can be installed according to an embodiment of the invention. 
       FIG. 3  depicts a block diagram of a portion of a microprocessor chip containing SRAM memory on which the present invention can be implemented. 
       FIG. 4  depicts a flowchart showing the actions used to implement an exemplary embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With reference now to the figures and in particular with reference to  FIG. 1 , a pictorial representation is shown of a data processing system in which a microprocessor chip containing SRAM memory can be implemented, in accordance with a preferred embodiment of the present invention. A computer  100  is depicted which includes system unit  102 , video display terminal  104 , keyboard  106 , storage devices  108 , which may include floppy drives and other types of permanent and removable storage media, and mouse  110 . Additional input devices may be included with personal computer  100 , such as, for example, a joystick, touchpad, touch screen, trackball, microphone, and the like. Computer  100  can be implemented using any suitable computer, such as an IBM eServer computer or IntelliStation computer, which are products of International Business Machines Corporation, located in Armonk, N.Y. Although the depicted representation shows a computer, other embodiments of the present invention may be implemented in other types of data processing systems, such as a network computer. Computer  100  also preferably includes a graphical user interface (GUI) that may be implemented by means of systems software residing in computer readable media in operation within computer  100 . 
   With reference now to  FIG. 2 , a block diagram of a data processing system is shown in which the present invention can be implemented. Data processing system  200  is an example of a computer, such as computer  100  in  FIG. 1 , in which a microprocessor chip containing SRAM memory according to the present invention may be located. Data processing system  200  employs a peripheral component interconnect (PCI) local bus architecture. Although the depicted example employs a PCI bus, other bus architectures such as Accelerated Graphics Port (AGP) and Industry Standard Architecture (ISA) may be used. Processor  202  and main memory  204  are connected to PCI local bus  206  through PCI bridge  208 . PCI bridge  208  also may include an integrated memory controller and cache memory for processor  202 . Additional connections to PCI local bus  206  may be made through direct component interconnection or through add-in connectors. In the depicted example, local area network (LAN) adapter  210 , small computer system interface SCSI host bus adapter  212 , and expansion bus interface  214  are connected to PCI local bus  206  by direct component connection. In contrast, audio adapter  216 , graphics adapter  218 , and audio/video adapter  219  are connected to PCI local bus  206  by add-in boards inserted into expansion slots. Expansion bus interface  214  provides a connection for a keyboard and mouse adapter  220 , modem  222 , and additional memory  224 . SCSI host bus adapter  212  provides a connection for hard disk drive  226 , tape drive  228 , and CD-ROM drive  230 . Typical PCI local bus implementations will support three or four PCI expansion slots or add-in connectors. 
   An operating system runs on processor  202  and is used to coordinate and provide control of various components within data processing system  200  in  FIG. 2 . The operating system may be a commercially available operating system such as Windows XP, which is available from Microsoft Corporation. An object oriented programming system such as Java may run in conjunction with the operating system and provides calls to the operating system from Java programs or applications executing on data processing system  200 . “Java” is a trademark of Sun Microsystems, Inc. Instructions for the operating system, the object-oriented programming system, and applications or programs are located on storage devices, such as hard disk drive  226 , and may be loaded into main memory  204  for execution by processor  202 . 
   Those of ordinary skill in the art will appreciate that the hardware in  FIG. 2  may vary depending on the implementation. Other internal hardware or peripheral devices, such as flash read-only memory (ROM), equivalent nonvolatile memory, or optical disk drives and the like, may be used in addition to or in place of the hardware depicted in  FIG. 2 . Also, the processes of the present invention may be applied to a multiprocessor data processing system. 
   For example, data processing system  200 , if optionally configured as a network computer, may not include SCSI host bus adapter  212 , hard disk drive  226 , tape drive  228 , and CD-ROM  230 . In that case, the computer, to be properly called a client computer, includes some type of network communication interface, such as LAN adapter  210 , modem  222 , or the like. As another example, data processing system  200  may be a stand-alone system configured to be bootable without relying on some type of network communication interface, whether or not data processing system  200  comprises some type of network communication interface. As a further example, data processing system  200  may be a personal digital assistant (PDA), which is configured with ROM and/or flash ROM to provide non-volatile memory for storing operating system files and/or user-generated data. 
   The depicted example in  FIG. 2  and the above-described examples are not meant to imply architectural limitations. For example, data processing system  200  also may be a notebook computer or hand held computer in addition to taking the form of a PDA. Data processing system  200  also may be a kiosk or a Web appliance. 
     FIG. 3  depicts a microprocessor chip  300 . Chip  300  contains microprocessor circuitry  301 , as well as SRAM cache arrays and associated circuitry, according to an exemplary embodiment of the invention. Two large SRAM cache arrays  302 ,  304  are connected to respective automatic, built-in self-testing circuits  308 ,  310 . In this embodiment, each array has its own dedicated ABIST circuitry. Alternatively, each test circuit can test a number of arrays of SRAM cells, with circuits present to select an array to be tested. ABIST engines  308 ,  310  are designed to test the SRAM cells in a strict order, which does not vary. This allows the address of a failure to be calculated by knowing the number of computer cycles that have elapsed since the start of the test. The output of testing circuits  308 ,  310  are respective fail signals  322 ,  324 , which are fed into multiplexor  312 , along with an input from the normally traced data  320  of the prior art. Multiplexor  312  will forward a selected input signal to be stored in trace array  314 . Typically, when a self-test is to be performed, a clock signal is sent to the trace array. From that point on, a signal is sent whenever a bit fails. When the fail signal is received, a count of the computer cycles since the last signal is recorded in trace array  314 . Later, when trace array  314  is read, the cycle information is used to calculate addresses of bit failures. Trace array  314  can be read through the action of scan controller  316 , which can itself be contacted through JTAG slave  318  from external JTAG scan interface  330 . 
   Trace array  314  contains only a small amount of storage. In the exemplary embodiment above, trace array  314  contains space to store 256 bit failures. Therefore, if the number of failures goes above this number, the testing will be unable to record further failures and will be aborted. 
   JTAG is a standard for a testing architecture, originally developed by the Joint Test Action Group and later adopted by The Institute of Electrical and Electronics Engineers (IEEE) as the Standard Test Access Port and Boundary-Scan Architecture. This standard defines the chip access port as containing four required leads: (1) test mode select, (2) test clock input, (3) test data input, and (4) test data output, plus an optional lead: (5) test reset. 
     FIG. 4  depicts a flowchart showing the actions used to test a chip, according to an exemplary embodiment of the invention. A signal to the test-mode-select pin on the chip triggers a self-test of the SRAM (step  402 ). When triggered, the ABIST circuit sets a pointer to point to the first bit to be tested and sends a start signal (step  404 ) over signal lines  322 ,  324 . The ABIST circuit tests the current bit by writing a value to that bit, then reading the bit back (step  406 ). If an error occurs (step  408 ), i.e., the value read is not the same as the value written, the self-test circuit sends an error signal on a respective signal line  322 ,  324 . At the trace array, a value, which indicates the number of cycles since the last signal, is saved in the next available location in the trace array (step  410 ). The pointer is then incremented by 1 to point to the next bit (step  412 ). A determination is made whether the test is completed, i.e., whether all cells have been checked (step  414 ). As mentioned above, the trace array has a limited number of locations for entries, e.g. 256; the number of errors found can be compared to this number (step  416 ). If the number of errors reaches the number of entries, there is no more storage space for errors, so the testing is completed. Otherwise, the testing continues until the entire array has been checked. When the self-test completes, a scan of the trace array is performed (step  440 ). As the values in the trace array are received, post-processing can be used to calculate the address when failures have occurred. The number of errors and the pattern of their locations can then be used as an aid in diagnosing problems with the chip. Given the capability to store self-testing information on the chips itself, a technician does not need to have the expensive testing equipment of the prior art in order to determine the incidence and pattern of memory errors, but can perform the test anywhere, such as at a customer site, greatly improving the flexibility of the testing capabilities. 
   The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.