Multiple I/O select memory

Abstract: A semiconductor memory device architecture by which a given number of discrete components can provide a memory module of increased capacity. A memory module (50) includes a plurality of discrete memory circuits (52) each organized to provide an individual data string having a length that is an integer multiple of four bits. The memory circuits (52) are arranged to provide a combined data string having a length equal to the sum of the individual data string lengths and each circuit includes a signal line connected to control transfer of individual data strings. A different data pin is associated with each bit of the combined data string to transfer a datum for output from the module. Each signal line is connected to a control pin to receive an external signal for initiating transfer of one of the individual strings from one memory circuits (52). The module (50) comprises an additional memory circuit (10) which includes a plurality of additional signal lines and a plurality of additional data lines. A first of the additional signal lines is wired in common with the signal line of a first one of the data memory circuits and a second of the additional signal lines is wired in common with the signal line of a second one of the data memory circuits. The additional circuit is responsive to transfer a bit of data along one of the additional data lines when an individual data string is transferred from one of the memory circuits.

Description: FIELD OF THE INVENTION This invention relates to semiconductor memory devices and, more particularly, it relates to a memory circuit having multiple, independently controllable data paths as well as a memory module formed with such a circuit. BACKGROUND OF THE INVENTION Progressively larger semiconductor memories of all types are being fabricated as higher bit densities and smaller cell designs are realized. In 1972 efforts were being made to produce 4K bit Dynamic Random Access Memories (DRAMs) and by 1983 256K devices became available. One megabit DRAM devices were introduced by 1987, 4 megabit devices will be sold widely by 1990 and 16 megabit devices are currently in design stages. Memory circuits with 64 Megabit, 256 Megabit or even higher density may be manufactured during the 1990's. Despite rapid increases in memory density for DRAMs, Static Random Access Memories (SRAMs) and nonvolatile memory devices, there continues to be a need to form microprocessor based systems with memory capacity larger than that available from a single device. Memory modules provide a standard solution to the problem of meeting ever-increasing memory requirements. Generally, a memory module comprises plural discrete memory devices mounted on a common substrate. For example, a 256K memory module can be configured with four 64K DRAMs to form a 64K X 4 memory, i.e., a composite memory with four I/O paths for storing 64K four bit words. Similarly, X 8 modules handle eight bit words while X 9 modules accomodate a parity bit as well as eight data bits. Memory modules of much higher density can be formed with 256K and 1 Megabit devices. By way of example, DRAM module TMO24EAD9, manufactured by Texas Instruments Incorporated, incorporates nine one-Megabit DRAMs to provide a 1,048,576 X 9 organization in a 30-pin Single-In-Line Package (SIP). In this module the column address strobe ( CAS ) control lines on eight of the devices are connected in common to the same control pin to provide eight parallel data lines for X 8 operation. A separate CAS input is provided for the ninth device, which stores the parity bit. In many memory applications it is important to assure data accuracy by performing a parity check for each word of data. Thus it is common for module designs to include additional memory circuitry to store parity information. Module architectures can be formed to accomodate parity data for larger word sizes, e.g., 16, 32 or 64 bit data I/O. An example is DRAM module TM256KBC36, also manufactured by Texas Instruments Incorporated, which comprises eight-one megabit DRAMs and four-256K DRAMs to provide a X 36 organization. The storage depth of this module, i.e., the number of words which can be stored on the module, is 256K. In addition to providing a 36 bit word length with a 256K depth, this architecture provides an even greater depth for shorter words. That is, the TM256KBC36 is a X 36 module capable of storing either 262,144 36-bit words, 524,288 18-bit words or 1,048,576 9-bit words. The TM256KBC36, schematically illustrated in Figure 1, is configured as a single-in-line package with four groups of memory devices. Each group includes two 256K X 4 DRAMs and one 256K X 1 DRAM. Each group provides 256K of storage depth for eight bits of data and a parity bit. The CAS lines of all devices in the same group are wired to a common module control pin. Thus each of four module control pins is associated with a different 9-bit data group stored in three of the 12 module devices. This enables read/write operations in integer multiples of 9-bit words. Although modules such as the TM256KBC36 provide a convenient and flexible means for expanding memory density, it is well known that the advantages of such are accompanied by an increased cost per bit of memory over the cost per bit of a discrete device. A portion of these increased costs is inherent to the formation of a complex circuit with multiple integrated circuit devices. The cost of packaging and testing a module also increases in proportion to the number of discrete components on the board. In addition, significant costs are associated with development and fabrication of memory boards which accomodate a large number of intergrated circuits. In particular, designs which minimize thermal stress and mechanical vibration problems become more costly as the weight, physical size and power requirements of a module increase. Surface mount technology, which reduces module size and fabrication costs, has provided only a partial solution to some of these problems. In view of these factors and a growing demand for progressively larger memory systems, there is a desire in the art to further reduce the physical size and cost per bit of memory modules. SUMMARY OF THE INVENTION Accordingly, the invention provides an architecture by which a given number of discrete components can provide a memory module of increased capacity. It is an object of the invention to reduce the number of discrete memory circuit components which are required to store a given size word. It is another object of the invention to reduce the board level costs of packaging and testing a memory module. According to the invention, the memory module as defined in the precharacterizing part of claim 1 is characterized by the features of the characterizing part of claim 1. In a preferred embodiment of the invention the additional memory circuit provides four or more bits of parity data and each bit of parity data is associated with an eight bit string of data provided by one or more of the data memory circuits. BRIEF DESCRIPTION OF THE DRAWING The invention may best be understood by reference to the following detailed description when read in conjunction with the accompanying drawing, wherein: Figure 1 illustrates a prior art memory module; Figure 2 illustrates a semiconductor memory device which, according to the invention, includes multiple CAS inputs; Figure 3 illustrates details of the Figure 2 device; Figure 4 further illustrates details of the Figure 2 device; Figures 5 - 9 are timing diagrams which illustrate relationships between control signals and data I/O during various modes of operation for the device of Figure 2. Figure 10 is a schematic of a memory module which can be formed with the device of Figure 2; and Figure 11 is a timing diagram illustrating static column mode operation. Identical reference numerals and nomenclature are employed in the various figures unless otherwise noted. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Dynamic random access memory arrays are generally described in U.S. Patent No. 4,081,701, issued to White, et al. and assigned to Texas instruments Incorporated. High density monolithic semiconductor memory devices constructed with multiple arrays formed thereon are well known in the art. See, for example, U.S. Patent No. 4,636,986 to Pinkham, also assigned to Texas Instruments Incorporated and incorporated herein by reference, which illustrates a dual port, i.e., both random and serial access, memory device such as provided for video RAM applications. Normally, each of the multiple arrays comprises an equal number of active memory elements arranged in rows and columns. Strings of address data are provided to common row and column decoders to simultaneously access a memory cell in each array. Separate input/output buffers are associated with each array for parallel transfer of data to and from an external source. It has been proposed that the input to individual arrays on such a device can be controlled to selectively write data to less than all the memory cells having a common address. To this extent, the U.S. Patent 4,636,986 to Pinkham discloses a write mask circuit for inhibiting data transfer to addressed memory locations and also teaches formation of a bit mapped video display memory device wherein writing of data into each of several arrays on the device is controllable with a separate Column Address Strobe signal. In the past the concept of restricting data input to less than all memory cells having a common address has been largely, if not solely, applied to dual port memory design because unique advantages are manifest when writing graphics data or providing enhanced graphics capabilities. Moreover, while devices which include the write mask feature have become commercially popular, it is not known that video RAM designs which incorporate multiple CAS control lines have generated any significant commercial interest. Perhaps this is because an additional CAS signal pin would be required to control each separate array. According to the invention it is recognized that the provision of a separate Column Address Strobe signal for each of several arrays on a memory device provides advantages beyond that of restricting data input to less than all memory cells having a common address. Referring to Figure 2, there is illustrated one example of a semiconductor memory device 10 comprising four memory arrays blocks. The device 10 is of the so-called 1-Megabit size, having 2²⁰ or 1,048,576 memory cells distributed into the four arrays 10a, 10b, 10c and 10d with each array containing 262,144 cells. For each array there are 512 row lines. All of the row lines are connected to one of two row decoders 11a or 11b. Each row decoder receives a nine-bit row address from address input pins 12 by way of row address latches 13 and lines 14. A nine-bit column address is also applied to the input pins 12, in time multiplexed manner, for input to buffers 15. Eight data Input/Output (I/O) lines 16 are positioned in the center of the array. Four of these eight lines 16 are simultaneously selected for data input or output by a 1-of-2 multiplexer 17. Multiplexer output is connected via four I/O lines through buffers 18a, 18b, 18c and 18d to data I/O pins DQ1, DQ2, DQ3 and DQ4. The multiplexer 17 receives one bit of column address data transmitted along line 20 from one of the column address buffers 15. Two of the eight I/O lines 16 are connected to each of the memory array blocks by I/O lines 21. A 2-of-16 column selection is made in each of sixteen sets of sixteen intermediate output buffers 24 in each array block, using four bits of the column address on lines 25 from the buffers 15. For each of the arrays 10a, 10b, 10c and 10d there are 512 sense amplifiers 26 each connected to an array. Each column comprises a pair of bit lines each of which is selectively coupled to a number of bit line segments as more fully described in U.S. Patent No. 4,658,377 assigned to the assignee of the present invention and incorporated herein by reference. Each of the buffers 24 is coupled for selection between one of two columns based on one bit of column address information received from the buffers 15 on line 27. The arrays 10a, 10b, 10c and 10d could, of course, be further partitioned for power reduction and noise reduction purposes, e.g., into four sub-arrays with additional decoding. The number of data I/O lines 16 could also be increased and it is contemplated that the lines 16 will be increased to at least 16 data lines with a 1-of-4 multiplexer (in lieu of the multiplexer 17) receiving two address bits and providing four bit data I/O. The memory device 10 receives a Row Address Strobe ( RAS ) signal on input pin 28 and multiple Column Address Strobe signals CAS 1, CAS 2, CAS 3, and CAS 4 on pins 29a, 29b, 29c and 29d, respectively. Each of the output buffers 18a, 18b, 18c and 18d is enabled, respectively, by a CAS 1, CAS 2, CAS 3 or CAS 4 input to transfer data to or from an associated DQ pin. Selection of a read or write operation is controlled by a read/write (R/ W ) control signal which is input to pin 30. A clock generator and control circuit 31 produces all of the internal clocks and control signals as needed for the various random access operations. Figure 3 illustrates in greater detail the I/O lines 16, the first and second level intermediate output buffers 22 and 24, and the sense amplifiers 26 for a block containing one of the arrays 10a, 10b, 10c and 10d. Sixteen intermediate output buffers 22 are arranged in groups of eight (22-1 ... 22-8 and 22-9 ... 22-16). Each group of eight buffers 22 is connected by a line 21 to one of two lines 16. Associated with each of the buffers 21-1 through 21-16 there is a set of sixteen buffers 24. Each set 24-1 through 24-16 of sixteen buffers 24 is coupled to a group of 32 sense amplifiers 26 that are arranged in pairs. The array includes 512 row lines 34 which intersect the bit lines 33 and two rows of dummy cells 34a which are each coupled to the bit lines in the usual manner and selectable with one of the row decoders 11a or 11b according to one bit of the nine-bit row address. Each sense amplifier 26 is connected to a pair of bit lines 33 which, in turn, are selectively coupled to bit line segments as described in U.S. Patent No. 4,658,377. One bit of the column address from the buffers 15 is transmitted along line 27 to selectively connect one of two sense amplifiers 26 in each of the sixteen pairs in a set of sense amplifiers to one of the sixteen buffers 24 via lines 37. Figure 4 presents a portion of the Figure 3 circuitry to illustrate in greater detail the relationships between the set of sense amplifiers 26 associated with the set of sixteen buffers 24-1. Each sense amplifier 26 has two bit lines 33 extending therefrom in the well known folded bit line configuration. The row lines 34 intersect the bit lines 33 and memory cells are located at intersections of row and bit line segments. A multiplexer 42 for each pair of sense amplifiers 26 selects one sense amplifier 26, based on the value of an address bit transmitted along line 27, for connection to a buffer 24. Based on four column address bits transmitted over the lines 25, only one of the sixteen buffers 24-1 is selected at any one time for coupling to the lines 38 and 39. Selection of the buffer 22-1, illustrated in Figure 4, is controlled by a 2-of-16 select provided by three bits of data on lines 23. Selection of the buffer 22-1 couples the dual-rail I/O lines 38 and 39 to one of the two single rail I/O lines 16 associated with the array. With reference again to Figure 2 and as described above, each of the memory arrays 10a, 10b, 10c and 10d shares a common row decoder and a common column decoder. A row address and an associated Row Address Strobe ( RAS ) signal activate the addressed row in each of the arrays. Each of the CAS signals CAS 1, CAS 2, CAS 3 and CAS 4 activates an addressed column in a corresponding array 10a, 10b, 10c or 10d. Absent this feature the sharing of a common column decoder among all four arrays would require reading or writing of data to all of the arrays 10a, 10b, 10c and 10d simultaneously. By providing a separate CAS control signal for each of the arrays 10a, 10b, 10c and 10d it becomes possible to selectively transfer data between any one of the DQ pins and a corresponding one of the arrays 10a, 10b, 10c or 10d. This separate I/O operation allows the device 10 to operate in a parity mode as more fully described below. Figures 5 - 9 illustrate timing relationships, respectively, for the following operations: read; write; early write; read-write/read-modify-write; and page mode. These operations are now summarily discussed in conjunction with transfer of parity data although the device 10 may be applied to provide other functions wherein the four independent CAS input pins 29a - 29d provide independent control of the four data I/O pins DQ1 - DQ4. Associated pin nomenclature is defined in the legend of Figure 5. Further detail relating to this exemplary embodiment may be acquired from Advance Information Data Sheets for Part No. TMS44C260 available from Texas Instruments Incorporated, Post Office Box 1443 Houston, Texas 77001, incorporated herein by reference. For read or write cycles the column address is latched on the falling edge of the first CAS signal on any of the pins 29 to transition to a logic low state. In addition, each time a CAS signal goes low the corresponding DQ pin is enabled to transmit data corresponding to the latched address to or from one of the memory arrays 10a - 10d. All address set up and hold parameters are referenced to the first falling edge of the CAS signals. The delay time, i.e., from the falling edge of a CAS signal to the time when "valid data" becomes available, is measured from the falling edge of a particular CAS signal to the time when data appears at the corresponding DQ pin. In order to latch a new column address all of the CAS pins 30 must be brought to a logic high level. The column precharge time, parameter t CP , is measured from the last CAS signal which undergoes a transition high to the falling edge of the first CAS signal in the next cycle. For early write cycles data is latched on the first falling edge of a CAS signal. However, only the DQ pins which correspond to a logic low CAS signal will transmit data to memory. Page mode operation allows for faster access times by retaining one row address while selecting a series of random access column addresses. This eliminates the time required for row-address setup and hold times which would otherwise occur in a time-multiplexed fashion. Unlike conventional page-mode operations, the column address buffers 15 in the device 10 are activated on the falling edge of RAS . The buffers 15 act as "transparent", or flow-through latches while all of the CAS pins 29 are in a logic high state. This feature permits the device 10 to operate at a higher data bandwidth than conventional page-mode parts. That is, data retrieval begins as soon as the column address becomes valid rather than when a corresponding CAS signal transitions low. The valid column address can be presented immediately after the row address hold time, t RAH , has been satisfied. Eighteen address bits of information are required to decode each of the 262,144 storage cell locations in each of the arrays 10a - 10d. Nine row address bits are input on pins A0 through A8 and latched with the RAS signal. Then nine column address bits of information are set up on the pins A0 through A8 and latched onto the device 10 as the first CAS signal falls. The RAS signal is similar to a chip enable signal in that it activates the sense amplifiers as well as the row decoders lla and llb. When a particular CAS signal goes to logic low it acts as a chip select because this transition activates the corresponding output buffer 18. The output enable signal, G , controls the impedance of the output buffers. When G is high the buffers 18 will remain in the high impedance state. Bringing G low during a normal cycle activates the output buffers 18, putting them in a low-impedance state. It is necessary for both the RAS signal and a CAS signal to be brought low in order for an associated output buffer to go into a low-impedance state. A buffer will remain in the low-impedance state until either G or the associated CAS is brought to a logic high level. Read and Write operations are selected through the write-enable input, W , to pin 30. A logic high selects the read mode and a logic low signal selects the write mode. Data input is disabled when the read mode is selected. If W is in the logic low state prior to any of the CAS input signals transitioning to logic low, (early write), the output pins will remain in the high impedance state, permitting a write operation with G grounded. Data is written during a write or read-modify-write cycle. To initiate an early write cycle W is brought low prior to a transition low by one or more of the CAS signals. Data propagates to and from an addressed memory cell when the corresponding CAS signal transitions low. In a delayed write, or a read-modify-write cycle, CAS signals will already have transitioned to logic low states. Thus the data will be strobed in by the transition of W to logic low. In a delayed write or a read-modify-write cycle G must be in a logic high state in order to bring the output sections of the buffers 18 to a high impedance state prior to impressing data on the I/O lines. Figure 10 illustrates a X 36 memory module 50 incorporating the device 10 and eight 256K X4 (one Megabit) DRAM devices 52. The 256K X4 device 10 serves as a parity DRAM in lieu of four 256K X1 DRAM devices such as illustrated in the prior art module of Figure 1. A feature of the module 50 is that all of the devices 10 and 52 can be formed with a common technology, e.g., they are the same density and can be manufactured with the same technology. That is, because the devices 10 and 52 are of the same density it is likely that they can be manufactured with the same generation technology. For example, the device 10 may be part TMS44C260 and the devices 52 may each be part TMS44C256, all available from Texas Instrumnents Incorporated P.O. Box 1443 Houston Texas 77001. In contrast, prior art X 36 memory modules of the same capacity were formed with four 256K X 1 DRAM devices as illustrated in Figure 1. From the viewpoints of cost, performance and device availability it is advantageous to form the module 50 with same generation parts. Each of the devices 52 has one RAS input pin, one CAS input pin and four data pins, DQ1, DQ2, DQ3 and DQ4. To effect transfer of eight-bit words the devices 52 are grouped in pairs 62, 64, 66 and 68 with the CAS input pins of each pair wired in common. In addition, each of the four CAS pins 29a - 29d of the device 10 is wired in common with the commonly wired CAS pins of one of the pairs of devices 52. In this exemplary module 50 the RAS pins for two pairs of devices 52 are wired in common to receive signal RAS 0 and the RAS pins of the remaining two pairs of devices 52 are wired in common to receive the signal RAS 2. In addition, RAS 0 and RAS 2 are input to a NAND gate 70 the output of which is provided to the RAS pin 28 of the device 10. This architecture enables data I/O from the device 10 when RAS 0 device pairs 62 and 64 are accessed and/or enables data I/O from the device 10 when RAS 2 device pairs 66 and 68 are accessed. Thus the device 10 is accessed with an appropriate RAS signal in order to transfer a ninth data bit for each eight-bit word being transfered to or from the devices 52. Another advantage of forming all of the module components with devices that are manufactured with the same technology is that the features of the most advanced design can be made common to all of the module devices. For example, the 256K X1 devices of Figure 1 may be NMOS while the associated 256K X4 devices is CMOS. In addition to including four CAS inputs the device 10 can include special features common to devices 52 of the same generation which have single CAS inputs. These may include nibble mode, static column mode or serial mode. With this improvement the module can be used to provide one or more special features which in the past could not be implemented because not all of the devices included that feature. For example, all of the devices 10 and 52 could function in the static column mode wherein the CAS signal is held in a logic low state and the address signals are statically strobed as shown in the timing diagram of Figure 11. Static column mode operation is also discussed in the Data Sheet for part TMS44C257 (256K X4 DRAM) published by Texas Instruments and incorporated herein by reference. Although a one-Megabit DRAM having four CAS inputs has been illustrated, other embodiments of the invention will be apparent to those skilled in the art. Volatile and nonvolatile memory devices may be constructed with multiple CAS input pins to individually control data paths. While it is contemplated that integer multiples of four CAS inputs will be most useful to the specific applications discussed herein, the invention is not so limited. Nor is it limited to applications where the data stored in a module device having multiple CAS inputs is parity data. Of course, when parity data is stored in a module according to the invention, the parity data need not be stored in the particular device that has multiple CAS input pins. Rather, a feature of the invention is the provision of a device architecture, which when applied to a memory module, results in a convenient and cost effective means for storing strings of data having lengths which are integer multiples of nine bits. Larger devices, e.g., 256K X8, 1M X4, 1M X8, 16M X8, etc., having multiple CAS input pins can be used to form memory modules according to the invention. A module can be formed with a 256K X4 device like that of the device 10 in conjunction with eight 1 Megabit X4 devices like the devices 52. Memory modules formed with multiple CAS input devices can be configured to provide 32 bit, 64 bit or wider data I/O. Such modules can also be configured to also provide smaller word sizes. To reduce access times the devices may be configured into data blocks which can be individually accessed by interleaving the RAS 0 and RAS 2 signals. Although certain preferred embodiments of the invention have been described, it should be appreciated that numerous other modifications can be made without departing from the scope invention which is limited only by the claims which follow.

Claims: A memory module (50) of the type formed with a plurality of discrete memory circuits, said module comprising: a plurality of discrete data memory circuits (52) each organised to provide an individual string having length equal to an integer multiple of four bits, said circuits arranged in groups to provide a combined data string having a length equal to the sum of the individual string lengths, each circuit including a signal line connected to control transfer of individual data strings; a plurality of data pins (DQ1, DQ2, DQ3 and DQ4) each associated with a different bit of the combined data string and connected to transfer a datum, associated with one of the individual strings, from one of the memory circuits for output from said module; and a plurality of control pins with each signal line connected to a control pin to receive an external signal ( RAS , CAS , RAS0 , RAS2 , CAS0 , CAS 1 , CAS2 , CAS3 , CAS4 ) for initiating transfer of one of the individual data strings from one of the memory circuits; characterised in that the module further comprises an additional single chip memory circuit (10) having a plurality of additional signal lines ( CAS1 , CAS2 , CAS3 , CAS4 ) and a plurality of additional data lines, with a first of the additional signal lines wired in common with the signal line of a first group of the data memory circuits to render said additional circuit responsive to transfer a bit of data along one of the additional data lines when a combined data string is transferred from the first group of memory circuits; and a second of the additional signal lines wired in common with the signal line of a second group of the data memory circuits to render said additional circuit responsive to transfer a bit of data along one of the additional data lines when a combined data string is transferred from the second group of memory circuits. The module of claim 1, wherein all of the memory circuits are dynamic random access memory circuits and said additional circuit includes four signal lines which control the transfer of parity data. The module of claim 1 or claim 2, wherein the discrete data memory circuits are electrically arranged in pairs (62, 64, 66, 68) to provide a data string length that is an integer multiple of eight bits, said plurality of data memory circuits further arranged to provide a combined data string having a length equal to the sum of the individual data string lengths. The module of any preceding claim, wherein the first of the additional signal lines is wired in common with the signal line of a third one of the data memory circuits to provide a data string length that is a multiple of nine bits and wherein the second of the individual data lines is wired in common with the signal line of a fourth one of the data memory circuits to also provide a data string length that is a multiple of nine bits. The module of any preceding claim, wherein there are an equal number of additional signal lines and additional data lines in the additional memory circuit. The module of any preceding claim, wherein the number of data lines in the additional memory circuit is four. The module of any preceding claim, wherein the number of signal lines in the additional memory circuit is four. The module of any preceding claim, wherein each additional data line is wired to associate a ninth data bit with each of eight bits in the combined data string. The module of any preceding claim, wherein the additional data lines are wired to associate one bit of parity data with every eight bits of the combined data word.

Inventors: NEAL JOSEPH H, POTEET KENNETH A, NEAL, JOSEPH H., POTEET, KENNETH A.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1990
Main classification: G11C7
IPC codes: G11C5, G11C7, G11C29, G06F11, G11C11