Abstract:
Memory module using partially defective synchronous memory devices, such as SDRAM components. Multiple partially defective SDRAM components are configured to provide a reliable and nondefective memory module that takes advantage of the manner in which defective cells are localized on each SDRAM component.

Description:
This application includes subject matter related to application Ser. No. 09/035,629, filed concurrently herewith on Mar. 5, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the use partially defective synchronous memory chips. More particularly, the present invention relates to the configuration of defective SDRAM components to create a nondefective memory module or array. 
     BACKGROUND AND SUMMARY OF THE DISCLOSURE 
     As is well known in the art, during the production of monolithic memory devices from silicon wafers, some of the memory storage cells can become defective and unreliable. The defective cells can be the result of a number of causes, such as impurities introduced in the process of manufacturing the monolithic memory device from the silicon wafer, or localized imperfections in the silicon substrate itself. 
     Often, while some memory cells are defective, many other cells on the same memory chip are not defective, and will work reliably and accurately. In addition, it is often the case that the defective cells are localized and confined to particular outputs from the memory device. The remaining, nondefective outputs can be relied upon to provide a consistent and accurate representation of the information in the storage cell. 
     Techniques have been developed for salvaging the non-defective portions of defective asynchronous memory technologies (e.g., DRAM). Asynchronous memory technologies are relatively slow devices that operate in response to control signals generated by a memory controller, rather than in response to the system clock. The control signals allow the asynchronous memory device to operate at a speed that is much slower than the system clock, and that ensures reliable read and write memory operations. 
     Synchronous memory devices such as SDRAM, on the other hand, are much faster devices that operate on the system clock. SDRAM is an improvement over prior memory technologies principally because SDRAM is capable of synchronizing itself with the microprocessor&#39;s clock. This synchronization can eliminate the time delays and wait states often necessary with prior memory technologies (e.g., DRAM), and it also allows for fast consecutive read and write capability. 
     However, no attempts have been made to salvage non-defective portions of synchronous memory. Some people skilled in the art may believe that the use of techniques for salvaging defective memory devices would not work with higher-speed synchronous memory devices such as SDRAM because they operate at much higher speeds than previous memory devices, such as asynchronous DRAM. For SDRAM, it may be believed that the rate at which the clock input cycles and the load on the device driving the inputs (e.g., the clock and the address) to the SDRAM devices would make reliable input transitions unattainable. 
     The present invention addresses the problem of salvaging partially defective synchronous memory devices. In one embodiment of the present invention, multiple partially defective SDRAM components are configured to provide a reliable and nondefective memory module. Such an embodiment takes advantage of the manner in which defective cells are localized on each memory chip, and combines multiple memory chips to provide a memory bus that is of the desired width and granularity. In addition, it is possible with such an embodiment to provide a computer system in which the main memory is synchronized with the system clock, and is constructed, at least in part, from partially defective memory chips. 
     The nature of the present invention as well as other embodiments of the present invention may be more clearly understood by reference to the following detailed description of the invention, to the appended claims, and to the several drawings herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a prior art computer system using a wait state control device with DRAM memory chips. 
     FIG. 2 is a block diagram of a computer system employing SDRAM memory chips. 
     FIG. 3 is a block diagram of a partially defective SDRAM component. 
     FIG. 4 is a memory map showing the localized nature of defective memory cells in one embodiment of the present invention. 
     FIG. 4A is a memory map showing defective memory cells corresponding to a defect that differs from that of FIG.  4 . 
     FIG. 5 is an embodiment of the present invention using six partially defective SDRAM components to make a 64-bit memory module. 
     FIGS. 6 and 7 are embodiments of the present invention using sixteen defective SDRAM components where four bits in each of the eight bit memory cells are defective. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a block diagram of a prior art computer system comprising a microprocessor  16 , a memory controller  14 , and main memory  12 . In the system shown, main memory  12  is made up of dynamic random access memory (DRAM). Also shown in FIG. 1 is a wait state control device  18  and a system clock  20 . As is well known in the art, due to differences in speed between the processor  16  and the DRAM  12 , it is often necessary to insert “wait states” when the processor carries out a memory operation involving the DRAM  12 . Typically, the DRAM  12  is slower than the processor  16 , so one or more additional states are added to the microprocessor&#39;s memory access cycle to ensure that the memory  12  is given a sufficient amount of time to carry out the memory (read/write) operation. 
     In addition, the clock  20  in the system of FIG. 1 is not a direct input to the DRAM  12 . Instead, as is well known in the art, control signals are derived from the clock, and the DRAM  12  is operated through the use of these control signals. The signals presented to the DRAM device  12  change relatively slowly compared to the rate at which the clock changes. 
     FIG. 2 shows a block diagram of a computer system in one embodiment of the present invention, where the computer system comprises a clock  20 , a processor  16 , a memory controller  22 , and main memory  24 . Often, the clock  20  operates at 66 MHz or 100 MHz, but it may operate at any speed. Unlike FIG. 1, the main memory in FIG. 2 is made up of one or more SDRAM chips, and the SDRAM memory is synchronized with the clock  20 , which means that it operates synchronously with the clock  20 . This synchronization can eliminate some or all of the wait states normally necessary with DRAM devices, and it also allows for fast consecutive read and write capability. Unlike FIG. 1, in FIG. 2 the clock  20  is provided as an input to the memory  24 . Thus, in FIG. 2, at least some of the inputs to the memory  24  may change at a rate approaching or equal to the rate of the clock  20 . 
     FIG. 3 is a block diagram of a partially defective SDRAM component  26  having twelve address inputs A0 to A11, and eight data outputs DQ 0  to DQ 7 . The component  26  is a 1024 K×83×2 SDRAM. The “8” in this description represents the eight output lines, meaning the data width is 8 bits wide (the granularity may also be eight bits). The “1024K” is the addressable space in each bank of memory within the SDRAM, and the “2” indicates that there are two such 1024K banks of memory within this component. Generally, components such as that described in FIG. 3 are mounted on SIMMs (Single In-line Memory Modules) or DIMMs (Dual Inline Memory Modules), but any other appropriate packaging technology could be used to practice one or more of the inventions described herein. 
     In operation, the SDRAM component  26  is addressed by using a multiplexed row and column address, as is well known in the art. The twelve address inputs on the memory component are first presented with an eleven bit row address on A0 to A10. After the row address has been presented to the SDRAM  26 , an nine bit column address is presented to the SDRAM  26  on address inputs A0 to A8. Thus, the full address is twenty bits wide, thereby making a 1024K address space based on the row and column addresses. The SDRAM  26  has two of these 1024K banks of memory addressable with the row and column addresses. The particular 1024K bank within the SDRAM component is selected by an additional row address bit, which is presented to the SDRAM with the row address on address input A11. 
     The SDRAM component shown in FIG. 3 is partially defective in the sense that some of the DQ outputs do not consistently present valid or accurate data. In the particular SDRAM shown in FIG. 3, data outputs DQ 2  to DQ 5  are defective, whereas data outputs DQ 0 , DQ 1 , DQ 6 , and DQ 7  are not defective. Thus, these latter DQ outputs can be relied upon for accurate and consistent data, whereas the data outputs DQ 2  to DQ 5  cannot. 
     FIG. 4 is a memory map of the SDRAM component of FIG. 3, showing the portions of memory that are defective. As can be seen from FIG. 4, in the particular SDRAM component of FIG. 3, the defects are such that every addressable eight bit memory location has both reliable and unreliable (or unused) DQ outputs, and they are consistently arranged within each addressable octet. 
     This result may follow from the nature of the defect, where certain DQ outputs always present valid data, whereas other DQ outputs may not be reliable, and may occasionally present bad data. Defects in the silicon or impurities introduced in the manufacturing process will often result in defects like those illustrated in FIG.  4 . 
     FIG. 5 is a schematic diagram of a memory module in one embodiment of the present invention where multiple partially defective SDRAM components are combined to create a nondefective 512K×64×2 memory module. The edge connector  52  is connected to each of the partially defective 512k×16×2 SDRAM components  54  to  59 . Each of the SDRAM components are defective in a manner similar to that shown in FIG.  4 . The SDRAM components  54 ,  55 ,  57 , and  58  each have four defective or unused DQ outputs (i.e., DQ 0  to DQ 3 ), and the remaining twelve DQ outputs are not defective. The SDRAM components  56  and  59  have eight unreliable or unused DQ outputs (DQ 0  to DQ 7 ), and eight reliable and nondefective DQ outputs. By using the twelve nondefective DQ outputs from SDRAM components  54 ,  55 ,  57 , and  58  and by using the eight nondefective DQ outputs from SDRAM components  56  and  59 , a 512K×64×2 memory module can be constructed from the six partially defective SDRAM components as shown in FIG.  5 . 
     In a manner similar to that described in connection with FIG. 3, the SDRAM components in FIG. 5 are addressed by first presenting an eleven bit row address followed by an eight bit column address. Thus, the memory address is nineteen bits wide. An additional bit is presented at address input A11 with the eleven bit row address to select one of the two 512K memory banks within each SDRAM component. 
     FIG. 6 is a schematic of another embodiment of the present invention, where sixteen partially defective 1024K× 8 ×2 SDRAM components  72  to  87  are used to create a 1024K×64×2 memory module. Each of the partially defective SDRAM components in FIG. 6 has four unreliable or unused outputs (DQ 0  to DQ 3 ) and four nondefective outputs (DQ 4  to DQ 7 ). Using the four nondefective outputs from each of the sixteen SDRAM components provides a 64 bit quad word data path. 
     The SDRAM components of FIG. 6 are addressed by first presenting an eleven bit row address followed by a nine bit column address. Thus, the memory address is twenty bits wide. An additional bit is presented at address input A11 with the eleven-bit row address to select one of the two 1024K memory banks within each SDRAM component. 
     It should be understood that the present invention does not necessarily require any particular arrangement for the defective DQ outputs. For example, in FIG. 6, the defective DQ outputs need not be the same for each component  72 - 87 , and the defective outputs may not be consecutive or symmetric. As can be seen from FIG. 6, the components  72  and  80  make up the low order byte of data in the 64-bit quad word. It is possible that component  72  may have only three defective outputs, thereby allowing five bits in the low order byte to be taken from component  72 , and only three bits from component  80 . Any other combination would also be appropriate. Similarly, the defective outputs in component  80  could come in any combination, and need not be DQ 0 , DQ 1 , DQ 2 , DQ 3 . Rather, the defective outputs could be DQ 1 , DQ 4 , DQ 6 , and DQ 7 , or any other combination. 
     FIG. 7 is a schematic of another embodiment of the present invention, where sixteen partially defective 1M×8×2 SDRAM components  92  to  107  are used to create a 1M×64×2 memory module. Each of the partially defective SDRAM components in FIG. 7 have four unreliable or unused outputs and four nondefective outputs. This embodiment differs from that in FIG. 6 in that the outputs DQ 0  to DQ 3  are nondefective, whereas outputs DQ 4  to DQ 7  are defective. The four nondefective outputs from each of the sixteen SDRAM components provides a 64 bit quad word data path. 
     Although the SDRAM components in FIG. 7 are 1M×8×2 components having two banks of 1M×8 bit memory, it is possible that they could be 2M×8 bit components having only a single bank of memory. In such an embodiment, the components are addressed by first presenting a twelve bit row address to the address inputs A0 to A11, followed by a nine bit column address, which is presented at address inputs A0 to A8. Thus, the full address is 21 bits wide, thereby providing a 2M address space, and the SDRAM components have (or are treated as having) only a single bank of memory. 
     It is also possible that the memory components have more than two banks of memory. In some more modem devices, two bank select lines (e.g., BA 0  and BA 1 ) are used to select one of four banks of memory in a particular component or module. (Often, but not necessarily, such select signals are presented to the component with the row address.) As one skilled in the art would recognize, the present invention is applicable to memory components of this nature, and is applicable generally to memory components having any number of banks of memory. 
     Although the present invention has been shown and described with respect to preferred embodiments, various changes and modifications that are obvious to a person skilled in the art to which the invention pertains, even if not shown or specifically described herein, are deemed to lie within the spirit and scope of the invention and the following claims.