Strobe signals in semiconductor memory devices

Abstract: A dynamic random access memory accomplishes a high speed data access operation by supplying a plurality of row address strobe signals RASi to a chip. A plurality of row address strobe signals RASi are supplied to a plurality of pins, and each row address strobe signal RASi is sequentially supplied with an active signal during a data access operation. Therefore, data in a plurality of memory cell sub-arrays is accessed during one access cycle time. Thus, since a large number of random data are provided, the data access time becomes faster and the performance of a system can be greatly improved.

Description: The present invention relates to semiconductor memory devices, and more particularly to dynamic random access memories. The progress of semiconductor memory devices has brought a high degree of the integration, and the demand for high speed access of a signal has become strong. Generally, high speed operation of a chip means that the accessing of cell data stored in a memory array from the exterior of the chip by the control of a central processing unit (CPU) of a system is performed at high speed. Generally, in a DRAM, a data access operation is performed by synchronizing a system clock generated in the system. As the speed of the system clock is increased, the data access operation of the chip should be performed with a speed corresponding to the speed of the system clock. It is known in the art that the speed of the system clock has been proposed up to 66MHz (which means that a pulse period of the system clock is 15 nano-seconds (ns)), and this will become faster in future. The data access operation of the chip must be performed with a speed corresponding to the system clock speed. However, the data access operation of the chip does not tend to keep up with the processing speed of the system with the construction of current semiconductor memory devices. Figure 1 of the accompanying diagrammatic drawings shows the construction of a general DRAM chip. An array construction of Figure 1 is divided into four memory arrays each having a number of sub-arrays. Moreover, the array construction may be changed according to the degree of integration of the chip. Data in each sub-array is accessed by the control of the system, and the control of the system is executed through pins at the exterior of the array. Figure 2 of the accompanying diagrammatic drawings is a diagram showing the connection of pins of Figure 1. A row address strobe signal RAS and a column address strobe signal CAS are applied with a signal of a TTL (transistor-transistor logic circuit) level controlled by the CPU of the system. At this time, it should be noted that the row address strobe signal RAS is connected to only one pin. Figure 3 of the accompanying diagrammatic drawings is a block diagram illustrating a conventional data access operation. In the construction of the DRAM shown in Figure 1, a number of sub-arrays having a number of accessible memory cells with multiple rows and columns are provided depending on the integration degree of the chip. In Figure 3, a memory array 100 includes one sub-array or more, and in the case of a 4 Mega DRAM, 16 Mega DRAM or more highly integrated semiconductor memory device, usually a plurality of sub-arrays are arranged. In addition, a sense amplifier and a data line, etc. are separately provided and simultaneously a buffer for generating data to the exterior of the chip is provided. An operational characteristic of Figure 3 is described by way of an example of a nibble mode in an operating mode of the DRAM. The nibble mode means an operating cycle achieved by a desire to perform a page mode cycle at high speed in the operating mode of the DRAM. In the page mode, a column address is not applied from the exterior, but incremented in the interior of the DRAM device in the page mode. An example of an operation during the nibble mode is shown in Figure 4 of the accompanying diagrammatic drawings. The nibble mode is identical to the page mode except that a column address is not strobed, and the parts which are not strobed can be repeated at high speed. In the case of 1M DRAM, an accessible memory cell with the nibble mode cycle comprises data of 4 bits. In Figure 3, one row address strobe signal RAS , one column address strobe signal CAS , and a number of addresses A0 to A9 control the access of data. If the row address strobe signal RAS is activated to logic "low" level to enable the chip, the number of sub-arrays are activated, and then, data stored in the memory cell is generated to the exterior of the chip by a series of active cycles. On the other hand, in order to generate new data by a new row address strobe signal, the row address strobe signal RAS should be set to logic "high" level. Therefore, a data output chain is reset and an interval t1 exists as shown in Figure 4. Thereafter, the above-mentioned process is repeated by the new row address strobe signal RAS of logic "low" level. Thus, data of 4 bits is produced in response to the column address strobe signal RAS of logic "low" level. While the row address strobe signal RAS is reset, that is, the signal RAS is precharged, output data is in a "don't care" state at the interval t1, and after the signal RAS is activated, data of 4 bits is produced. In order to improve the performance of the system, a method for increasing an access speed of the DRAM, or another method for providing more random data during a given time should be proposed. However, the method for increasing the access speed has limitations in a given process and design techniques, and in view of the speed of the CPU, the access speed of data is too slow. Further, in the nibble mode, if a cycle time of the column address strobe signal CAS is 15ns and that of the row address strobe signal RAS is 120ns (in this case, a precharge time of the row address strobe signal RAS becomes 60ns), data of 4 bits is produced during 60ns and during the remaining 60ns of the interval t1, no data can be generated. Consequently, since data can not be generated during a precharge cycle, the increase of the access speed of data has limitations. DATABUS, No. 2, Feb. 1985 describes a Dynamic Random Access Memory consisting of two memory array groups. Input address lines are multiplexed in an input multiplexer and applied to the two memory array groups. The data outputs from the memory array groups are recombined in an output multiplexer. The memory array groups are read alternately and refreshed during the period when their respective RAS signals are inactive. Thus, the memory includes two memory array groups which are alternately accessed and each of which is refreshed during the active cycle of the other. The memory works adequately even if the timings of the RAS signals are mismatched and this is achieved by using two column address signals, CAS1 and CAS2 . Each is associated with a respective memory array group and a respective RAS signal. In addition, the column address strobe signals are cycled after an initial enable interval to generate in series four bits of data which are shifted into a shift register in series (FIG. 4). EDN Electrical Design News, vol. 34, no.8 describes a DRAM refresh process in which the refresh takes place during the inactive cycle of RAS . It is an aim of preferred embodiments of the present invention to provide a DRAM operated in response to a high speed system clock supplied from the exterior. It is another aim to provide a DRAM capable of successively generating data produced from different memory cell arrays from each other in response to a high speed system clock supplied from the exterior during one data access cycle. It is a further aim to provide a DRAM capable of successively generating data produced from one memory cell array, and data produced from another memory cell array precharged when the memory cell array is in an active state and activated when the memory cell array is in a precharge state in response to a high speed system clock supplied from the exterior during one data access cycle. According to one aspect of the present invention, there is provided a dynamic random access memory comprising: a number of memory array groups each having a plurality of memory cells in a row and column matrix form; an equal number of row address strobe signal input terminals for receiving respective row address strobe signals, each row address strobe signal input terminal being associated with a respective memory array group; a clock signal input terminal for receiving an external clock signal; means for sequentially accessing the memory array groups in response to sequential activation of the respective row address strobe signals and activation of a column address strobe signal so that each access of each memory array group provides a plurality of bits of data in parallel; means for latching the said plurality of bits of data in parallel; and means for precharging bit lines of the memory array groups corresponding to inactive row address strobe signals. characterised in that: the memory includes a single column address strobe signal input terminal for receiving a single column address strobe signal and associated with all the memory array groups; and the means for sequentially accessing the memory array groups is adapted to do so for each memory array group in response to activation of the single column address strobe signal. Preferably, the means for latching the said plurality of bits further comprises means for reproducing them in series in response to the external clock signal. Preferably, the means for latching and reproducing the said plurality of bits of data comprises a shift register. For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to Figures 5 to 10 of the accompanying diagrammatic drawings, in which: Figure 5 is a block diagram illustrating one example of a data access operation according to the present invention; Figure 6 is a diagram illustrating one example of the connection of pins of a DRAM according to the present invention; Figures 7A to 7D are block diagrams illustrating control signal of Figure 5; Figures 8A and 8B are diagrams illustrating the arrangement of sub-arrays; Figure 9 is a circuit diagram illustrating a data read route of Figure 5; and Figure 10 is a timing chart of a data access operation of Figure 5. There is a tendency in highly integrated semiconductor memory devices for the number of pins included in one chip to be increased as the degree of integration of a semiconductor memory device is improved. It should be noted that, as the integration degree is improved, embodiments of DRAMs in accordance with the present invention can be effectively operated. Furthermore, the number of external row address strobe signals and pins as input terminals corresponding thereto is determined by the arrangement of a memory array group included in one chip. The construction of Figure 5 is different from that of Figure 3 in that a plurality of row address strobe signals RAS1 to RASi and a system clock SYSC supplied from the system are applied to a control & clock part 40. In Figure 6, the connection of pins in Figure 5 is shown. The number of pins included in a semiconductor memory device is more and more increased according to the increased integration degree, and therefore, the number of addresses A0 to Aj may be increased. Hence, the pins for receiving the row address strobe signals RAS1 to RASi respectively and another pin for receiving the system clock SYSC should be provided. Control signals of the control & clock part 40 of Figure 5 are indicated in Figure 7A. The row address strobe signals RAS1 to RASi supplied from the exterior of the chip are converted into signals RAS1 to RASi respectively as internal signals, and the system clock SYSC as an external signal is converted to a system clock SYSC as an internal signal. An address buffer 20 of Figure 5 includes row and column address buffers 21 and 22 as shown in Figure 7B. The addresses A0 to Aj are supplied to the row and column address buffers 21 and 22 respectively. The row address buffer 21 is enabled when any one of the row address strobe signals RAS1 to RASi is activated, and an output signal thereof is a signal BLSi for selecting a desired sub-array. In the preferred embodiment, a sub-array drive clock generating device for driving a desired sub-array is required and simultaneously a control device for driving the sub-array drive clock generating device is necessitated as shown in Figure 7C. The internal row address strobe signals RAS1 to RASi generated from the control & clock part 40 of Figure 7A are applied to a sense amplifier & input/output (I/O) line enable clock generating circuit 91. The number of the sense amplifier & input/output (I/O) line enable generating circuits is determined by the number of the internal row address strobe signals RAS1 to RASi, that is, by the number of sub-array groups. Output signals CSI1 to CSIi of each sense amplifier & input/output (I/O) line enable clock generating circuit are supplied to sub-array select clock parts to generate a given sub-array select clock SC as indicated in Figure 7D. In more detail, the sub-array select clock part 92 receives the block select signal BLSi generated from the row address buffer 21 of Figure 7B and the output signals CSI1 to CSIi generated from the sense amplifier & input/output (I/O) line enable clock generating circuit 91, to produce the sub-array select clock SC. It will be readily appreciated that the number of sub-array select clock parts is determined by the number of sub-array groups. The sub-array select clock SC is a signal for driving a sense amplifier included in a selected sub-array, which will be described later on. Further, one sub-array select clock part should have a plurality of sub-array select clock circuits and the number thereof is determined by the number of sub-arrays in one sub-array group. In the case that the sub-array in the memory array is arranged with two groups, an example of a data access operating characteristic will be described hereinafter. In this case, since two row address strobe signals should be applied, two pins for receiving the two row address strobe signals are required. The arrangement of two sub-array groups is shown in Figure 8A. One sub-array group has a plurality of sub-arrays; in some cases, one sub-array group may have one sub-array. In the preferred embodiment, two row address strobe signals RAS1 and RAS2 activate or precharge two sub-array groups. The configuration of sub-arrays adjacent to each other is shown in Figure 8B. Figure 9 is a circuit diagram showing a data read routine in each sub-array of Figure 8A. An example of a reading operation of data in a given sub-array will now be described. If the row address strobe signal RAS1 synchronized with the system clock SYSC is activated to logic "low" level, a given sub-array in a sub-array group 1 is activated through the routine shown in Figures 7B to 7D to generate data. In this case, the sub-array select clock SC of Figure 7D enables sense amplifiers 101 and 102 in each sub-array of the sub-array group 1, and the data in the selected sub-array is connected to each input/output (I/O) line. Moreover, the data is supplied to a shift register part 110 for sequentially producing the data through input/output & sense amplifiers (I/O-S/A) 71 to 74 enabled by the signal CSI1 generated from the sense amplifier & input/output (I/O) line enable clock generating circuit of Figure 7C. The data stored in the shift register part 110 is synchronized with the system clock SYSC and successively produced. Next, the row address strobe signal RAS1 is set to logic "high" level, then the row address strobe signal RAS2 synchronized with the system clock SYSC is activated to logic "low" level while the sub-array activated in the sub-array group 1 is precharged, and a given sub-array in a sub-array group 2 is activated to generate given data through the routine shown in Figures 7B to 7D. At this time, the sub-array select clock SC of Figure 7D enables a sense amplifier in each sub-array of the sub-array group 2, and then, data in the selected sub-array is connected to each input/output (I/O) line. Furthermore, each data is supplied to the shift register part 110 through the input/output & sense amplifiers (I/O-S/A) 71' to 74' enabled by the signal CSI2 generated from the sense amplifier & input/output (I/O) line enable clock generating circuit of Figure 7C. Therefore, data stored in the shift register part 110 is synchronized with the system clock SYSC and is successively generated. Thus, data of different sub-array groups are successively produced during one data access cycle as shown in Figure 10. At this time, a data output time of one bit corresponds to one pulse period of the system clock. For example, if the cycle time of the row address strobe signal RAS1 is 120ns (In this case, a precharge time of the row address strobe signal RAS1 becomes 60ns), data of 4 bits is generated during the data access time of 60ns. Thereafter, further data of 4 bits is sequentially generated, since the row address strobe signal RAS2 is activated while the row address strobe signal RAS1 is precharged during the precharge time of 60ns thereof. Thus, the row address strobe signals RAS1 and RAS2 have an alternative data access time with a time interval of a given level. Therefore, the rate of the data access during a unit time is increased by two times in comparison with that of Figure 3. The data access operation is continuously accomplished while the system clock SYSC is applied. In the timing chart of Figure 10, the row address strobe signal RAS2 falls at a rising point of the row address strobe signal RAS1 . However, since the system clock SYSC is continuously applied, even if the row address strobe signal RAS2 falls at a delayed point by a given level in comparison with the rising point of the row address strobe signal RAS1 , the same result can be obtained. As described above, even if the data access time is predetermined, a large number of random data are achieved by supplying a plurality of row address strobe signals, and thus the data access time becomes faster and system performance can be greatly improved.

Claims: A dynamic random access memory comprising: a number of memory array groups each having a plurality of memory cells in a row and column matrix form; an equal number of row address strobe signal input terminals for receiving respective row address strobe signals, each row address strobe signal input terminal being associated with a respective memory array group; a clock signal input terminal for receiving an external clock signal; means for sequentially accessing the memory array groups in response to sequential activation of the respective row address strobe signals and activation of a column address strobe signal so that each access of each memory array group provides a plurality of bits of data in parallel; means for latching said plurality of bits of data in parallel; and means for precharging bit lines of the memory array groups corresponding to inactive row address strobe signals. characterised in that: the memory includes a single column address strobe signal input terminal for receiving said column address strobe signal as a single column address strobe signal and associated with all the memory array groups; and the means for sequentially accessing the memory array groups is adapted to do so for each memory array group in response to activation of the single column address strobe signal. A memory according to claim 1 in which the means for latching the said plurality of bits further comprises means for reproducing them in series in response to the external clock signal. A dynamic random access memory according to claim 2 in which the means for latching and reproducing the said plurality of bits of data comprises a shift register.

Inventors: CHIN DAE-JE, CHO SOO-IN, CHOI YUN-HO, HAQ EJAZ UL, CHIN, DAE-JE, CHO, SOO-IN, CHOI,YUN-HO, HAQ, EJAZ UL
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO. LTD.
Application year: 1992
Main classification: G11C11
IPC codes: G11C11, G11C8, G06F12