A control circuit for a semiconductor memory device and semiconductor memory system

Abstract: A control circuit for a dynamic memory device comprises first timer means (2, 4) for delaying the RAS signal by a first delay time (τ1) and supplying the delayed RAS signal ( RINT ) to a row control circuit (91), and a second timer means (3, 5) for delaying the RAS signal by a second delay time (τ2) and supplying this delayed RAS signal ( RINT ) to a column control circuit (91).

Description: The present invention relates to a control circuit for a semiconductor memory device and, more particularly to a dynamic memory and also a dynamic memory system, capable of performing either RAS time-out function or a function equivalent thereto, in accordance with a RAS signal supplied from an external device. To use a conventional dynamic random-access memory (hereinafter referred to as "DRAM"), the user must input a RAS (Row-Address Strobe) to the memory at the timing shown in Fig. 11. A new type of a DRAM has a RAS time-out function and can automatically maintain an internal row-address strobe ( RINT ) at an active level (i.e., a low level) for a prescribed period as is illustrated in Fig. 13, in response to the row-address strobe ( RAS ) supplied from an external device. Due to the RAS time-out function, the user need not care about the timing of precharging the RAS to a high level, and it is easy for him or her to use the DRAM. Also, due to the RAS time-out function, the data stored in the memory cells of the DRAM remains undestroyed even if noise enters the RAS , inevitably precharging the RAS . The RAS time-out function is indeed helpful in this respect, but it raises the following problems when it is applied to a high-integration, high-speed DRAM. As is shown in Fig. 14, there are two terms during which signals can be input to a DRAM. The first is a RAS -active term tRAS, and the second is a RAS -precharge term tRP. The cycle time tRC of the DRAM is given as follows: tRC = tRAS + tRP + 2tT where tT is the transition time tT required for switching the RAS from the low level to the high level, or vice versa. The lower limits for tRC, tRAS, tRP, and tT are predetermined, as is shown in the following table. This means that the DRAM must be operate normally even if tRC, tRAS, tRP, and tT are reduced to these minimum values. One of the advantages of the RAS time-out function is that no lower limit is set to tRAS practically. (If the lower limit is set, the RAS must be held at the low level for the period of 80 ns, as can be understood from the above table.) Hence, the user can set the RAS at the high level much earlier than the case where a lower limit is set to tRAS. Since the number of critical timings of operation the DRAM is thus reduced by one, it becomes easy for the user to use the DRAM. However, the DRAM must have a timer for maintaining the internal RAS (i.e., the RINT ) for 80 ns. In the conventional DRAM, not able to perform the RAS time-out function, the potential of a word line increases to supply data stored in a memory cell to a bit line, upon lapse of some time after the RAS has switched to the low level, as can be understood from Fig. 12. At the time a potential difference is made between the pair of bit lines, the sense amplifier connected these bit lines is activated, thus amplifying the potential difference. When the RAS is switched to the high level, the potential of the word line decreases, whereby the data is stored into the memory cell. The moment the potential of the word line decreases to 0 V, these bit lines are equalized to potential VCC/2, where VCC is the power-source potential. The higher the integration density and speed of the DRAM, the longer the time the sense amplifier requires to amplify the potential difference between the pair of bit lines, and at the same time, the shorter the time allowed for the restoring of the bit lines. For instance, if the RAS access time is 80 ns (as in a 4M-bit DRAM of the first generation), tRASmin is 80 ns, and the bit lines must be restored within 80 ns. Further, in the case of a 4M-bit DRAM manufactured by the conventional method and having three polysilicon layers and one aluminum layer, the bit line must be restored within only 20 ns after the P-channel sense amplifier has been activated, when VCC = 4 V, Vtn = 1.0 V, Vtp = -1.0 V, and Tc = 85°C -- all being the worst conditions possible. (Vth is the threshold voltage of the N-channel transistors used in the DRAM, and Vtp is that of the P-channel transistors used therein.) It is practically impossible to restore the bit lines to a sufficiently high level within so short a time as 20 ns. The time required for sensing and amplifying the potential difference between the pair of bit lines can be reduced by using a multi-level metal wiring technique. The use of such technique requires a more complex method to manufacture the DRAM, and results in a higher cost of the DRAM. Hence, in the DRAM unable to perform RAS time-out function, if the RAS is precharged within the tRASmin, the potential of each word line inevitably will decrease to 0 V before the potential difference between the bit lines is sufficiently amplified. Consequently, sufficient charges cannot be stored into the memory cell, inevitably causing soft errors and impairing the data holding characteristic of the DRAM. To avoid soft errors and the deterioration of data holding characteristic, the DRAM must be modified to perform RAS time-out function and be equipped with a timer to hold the internal RAS (i.e., RINT ) at the low level for a period longer than tRASmin of 80 ns. If the DRAM is so modified and has a timer, the potential difference between the bit lines will be sufficiently amplified within the tRAMmin of 80 ns, provided that the potential of each word line reduces to 0 V and the pair of bit lines are equalized to potential VCC/2, fully within the tRPmin of 60 ns. However, when the timer holds the internal RAS at the low level for a period longer than tRASmin of 80 ns, a problem will arise. If data-reading operation is prohibited during the preceding operation cycle of the DRAM (for example RAS only refresh cycle), no data should be read from the DRAM during a period tRPC starting at the precharging of the RAS and ending at the activation of CAS (Column-Address Strobe) as shown in Fig. 4. (The period tRPC must have its minimum value of 0 ns even if tRASmin is 80 ns.) This condition cannot be satisfied when the internal RAS is set at the low level longer than tRASmin of 80 ns. As has been described, the RAS time-out function makes it unnecessary for the user to care about the timing of precharging the RAS to a high level, and also prevents the data stored in the memory cells of the DRAM from being destroyed even if noise enters the RAS . Nonetheless, this RAS time-out function cannot apply to a DRAM which has a higher integration density and operates at a higher speed, and in which a longer time is required to amplify the potential difference between each pair of bit lines, and the tRASmin is shorter. In order to lengthening the term tRAS, without changing the term tRC, the term tRP can be reduced internally. If this method is used, however, the term tRPC will fail to have its minimum value. This problem results from the fact that the DRAM capable of performing the RAS time-out function has only one timer, and the output of this timer, i.e., the internal tRAS, controls all other circuits of the DRAM when the external tRAS is too short. JP-A-62 134 894 discloses a control circuit for a DRAM generating from an external RAS signal a single internal RAS signal having an expanded low level time due to delaying the trailing edge of the external RAS signal. The expanded low level time of the single internal RAS signal is dependent from the delay time of a delay circuit and the low level time of the external RAS signal as well. An object of the present invention is to provide a dynamic memory and also a dynamic memory system, either capable of operating fast and correctly, merely by activating an externally supplied timing signal RAS for a short time of 80 ns. This object is achieved by a control circuit for a semiconductor memory device comprising: a RAS input terminal for receiving an external row-address strobe signal; a row control circuit including a row-address control circuit for controlling row addresses for the semiconductor memory device and a word line driver circuit for driving word lines; a column control circuit including a column-address control circuit for controlling column addresses for the semiconductor memory device and an output control circuit for controlling data output; an internal RAS signal generator for generating at least a first internal RAS signal being supplied to said row control circuit and a second internal RAS signal being supplied to said column control circuit such that the low level time of the first internal RAS signal is expanded by at least a first delay time and is longer than the low level time of the external RAS signal and also longer than the low level time of the second internal RAS signal. Preferably the low level time of the second internal RAS signal is expanded by at least a second delay time and is longer than the low level time of the external RAS signal. According to a further development of the invention the internal RAS signal generator generates a third internal RAS signal being supplied to a column decoding control in said column control circuit for controlling column address signals to be decoded such that the low level time of the third internal RAS signal is expanded by at least a third delay time and is shorter than the low level time of the first internal RAS signal and longer than the low level time of the second internal RAS signal. Further preferred developments and embodiments of the invention appear from claims 4 to 15. In particular the invention provides a semiconductor memory device comprising an array of memory cells, a plurality of sense amplifiers, a row control circuit, a column control circuit, a RAS input terminal for receiving a row-address strobe signal, and at least two timer means for expanding the RAS signal and supplying the expanded RAS signals to the row control circuit and the column control circuit, respectively. Since the two expanded RAS signals output by the timer means are supplied to the row control circuit and the column control circuit, sufficient periods of time are provided for precharging the RAS signal and operating the sense amplifiers completely. Therefore, the memory device according to the present invention can operate at high speed. The timer means can either be incorporated in the memory-cell chip or be arranged outside the memory-cell chip. In either case, the memory device according to the invention can operate at high speed even if it has a high integration density. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1A is a diagram showing the internal RAS signal generating section incorporated in a first memory device according to the invention; Fig. 1B is a block diagram illustrating two groups of control circuits which are incorporated in the first memory device and operate under the control of the internal RAS signals generated by the section shown in Fig. 1A; Fig. 1C is a timing chart explaining how the internal RAS signals output by the section shown in Fig. 1A control the circuits shown in Fig. 1B; Fig. 2A is a circuit diagram showing the internal RAS signal generating section incorporated in a second memory device according to the invention; Fig. 2B is a block diagram illustrating the control circuits which are used in the second memory device and operate under the control of the internal RAS signals generated by the section shown in Fig. 2A; Fig. 2C is a timing chart explaining how the internal RAS signals output by the section shown in Fig. 2A control the circuits shown in Fig. 2B; Fig. 3A is a circuit diagram showing the internal RAS signal generating section incorporated in a third memory device according to the invention; Fig. 3B is a block diagram illustrating two groups of control circuits which are used in the third memory device and operate under the control of the internal signals generated by the section shown in Fig. 3A; Fig. 3C is a timing chart explaining how the internal RAS signals output by the section shown in Fig. 3A control the circuits shown in Fig. 3B; Fig. 4 is a timing chart explaining the operation of DRAM; Figs. 5, 6, and 7 are graphs representing the operation characteristics which the memory devices of the present invention can have; Figs. 8A, 8B, and 8C are block diagrams schematically illustrating those units of the first, second, and third memory devices, each of which comprises at least one internal RAS signal generating section and two control circuits; Figs. 9A, 9B, and 9C are block diagrams schematically illustrating those units of the fourth, fifth, and sixth memory devices according to the invention, each of which comprises at least one RAS signal generating section and two control circuits; Figs. 10A to 10F are circuit diagrams, each showing circuit which can be used in the present invention; and Figs. 11, 12, 13, and 14 are timing charts explaining the operation of a conventional dynamic memory devices. A DRAM, which is a first embodiment of the present invention, will now be described with reference to Figs. 1A, 1B, and 1C. Fig. 1A illustrates the internal RAS signal generating section incorporated in this DRAM, and Fig. 1B shows the two control circuits which are also incorporated in the DRAM and operate under the control of the internal RAS signals generated by the RAS signal generating section, as will be explained later in detail. As is illustrated in Fig. 1A, the internal RAS signal generating section comprises an external RAS input buffer 1, a first internal RAS signal generating circuit 2, a second internal RAS signal generating circuit 3, a first timer circuit 4, and a second timer circuit 5. The buffer 1 is connected to receive a RAS signal supplied from an external device. It comprises two inverters 6 and 7 coupled in series. The circuits 2 and 3 are designed to generate a first internal RAS signal ( RINT ) and a second internal RAS signal ( KRIN T), respectively, from the external RAS signal. The signals RINT and KRINT will control a row-address control circuit and a column-address control circuit (both later described), respectively. The first internal RAS signal generating circuit 2 comprises a set-reset type flip-flop FF1 and an inverter 8 connected to the set output of the flip-flop FF1. Similarly, the second internal RAS signal generating circuit 3 comprises a set-reset type flip-flop FF2 and an inverter 9 connected to the set output of the flip-flop FF2. The flip-flop used in either internal signal generating circuit is composed of two 2-input NAND gates which are cross-connected to each other. The signals output by the inverters 8 and 9 are the internal RAS signals RINT and KRINT , respectively. The set terminals of the flip-flops FF1 and FF2 are connected to the output of the RAS input buffer 1. The reset terminals of the flip-flops FF1 and FF2 are connected to the outputs of the timer circuits 4 and 5 to receive the output signals RTM and KRTM of these timer circuits, respectively. The first timer circuit 4 comprises a 2-input NAND gate 10, a first delay circuit 11, and two inverters 12 and 13. The first input of the NAND gate 10 is connected to receive a word line-driving signal WDRV. The inverter 12 receives the word-driving signal WDRV and inverts the signal. The first delay circuit 11 receives the output signal of the inverter 12 and delays the signal by time τ1 and supplies the signal to the second input of the NAND gate 10. The inverter 13 inverts the output signal of the NAND gate 10. The signal output by this inverter 13 is the signal RTM. The second timer circuit 5 comprises a 2-input NAND gate 14, a second delay circuit 15, and two inverters 16 and 17. The first input of the NAND gate 14 is connected to receive a word-driving signal WDRV. The inverter 16 receives the word-driving signal WDRV and inverts the signal. The second delay circuit 15 receives the output signal of the inverter 16 and delay the signal by time τ2 and supplies the signal to the second input of the NAND gate 14. The inverter 17 inverts the output signal of the NAND gate 10. The signal output by this inverter 17 is the signal KRTM. The control circuits 91 and 92, both incorporated in the DRAM, will now be described, with reference to Fig. 1B. The circuit 91, which is a row control circuit, comprises a row-address buffer controller 21, a word line driver 22, a row-address buffer 23, a row decoder 24, and two delay circuits 25 and 26. The row-address buffer controller 21 controls the row-address buffer 23 in accordance with the signal RINT output from the first internal RAS signal generating circuit 2. The output signal SEN of the delay circuit 25 is used to activate the N-channel sense amplifiers (not shown) incorporated in the DRAM. The circuit 92, which is a column control circuit, comprises a column-address buffer controller 27, a column-address buffer 28, a column controller 29, a column decoder 30, an address-transition detector 31, an output controller 32, and a write controller 33. The column-address buffer controller 27 controls the column-address buffer 28 in accordance with the signal KRINT output from the second internal RAS signal generating circuit 3. The operation of the circuits shown in Figs. 1A and 1B will now be explained, with reference the timing chart shown in Fig. 1C. First, the word-line driving signal WDRV is at a low level. When the word-line driving signal WDRV rises from the low level to a high level, a word line WL is driven. The output RTM of the first timer circuit 4 then rises from the low level to the high level, thereby setting the flip-flop FF1 incorporated in the first internal RAS signal generating circuit 2. The inverter 8 inverts the output signal of the flip-flop FF1 into the signal RINT . The signal RINT is not influenced by the external RAS signal and is thus clamped at the low level. In other words, the RAS time-out function is performed. Some time thereafter, the sense amplifiers (not shown) are activated. Upon lapse of the delay time τ1 of the first delay circuit 11 after the signal WDRV has risen to the high level, the output RTM of the first timer circuit 4 falls to the low level. At this time, the RAS time-out function is stopped, and the signal RINT is influenced by the external RAS signal and starts performing its function. The delay time τ1 of the first delay circuit 11 is equal to the longer one of either the time required for increasing the potential of the higher-potential bit line of each pair to the VCC potential after the sense amplifiers (not shown) have been activated, or the time required for decreasing the potential of the lower-potential bit line of each pair to the VSS potential after the sense amplifiers have been activated. Hence, the delay time τ1 can be determined solely by the speed of the charging the bit lines, not influenced by the shortest possible term during which the RAS signal remains at the low level. On the other hand, when the word-line driving signal WDRV rises from the low level to the high level and thus starts driving the word line WL, the output KRTM of the second timer circuit 5 rises from the low level to the high level. As a result, the flip-flop FF2 incorporated in the second internal RAS signal generating circuit 3 is set, and the output signal KRINT of this circuit 3 is clamped at the low level, not influenced by the external RAS signal. In other words, the RAS time-out function is started. Upon lapse of the delay time τ2 of the second delay circuit 15 after the signal WDRV has risen to the high level, the output signal KRTM of the second timer circuit 5 falls to the low level. At this time, the RAS time-out function is stopped, and the signal KRINT is influenced by the external RAS signal and starts performing its function. The delay time τ2 of the second delay circuit 15 is equal to either tRASmin or a little shorter than tRASmin, not influenced by the speed of the charging the bit lines. Hence, the term during which the internal RAS signal remains at the low level is equal to tRASmin even if the term, during which the external RAS signal supplied to the DRAM remains at the low level, is shorter than tRASmin. (Needless to say, the external signal must remain at the low level longer than the time τ0 between the trailing edge of the external RAS signal and the leading edge of the word-line driving signal WDRV.) In other words, the delay time τ1, the delay time τ2, and the term tRAMmin have the following relationship: τ0 + τ2 ≦ tRASmin < τ0 + τ1 Even when tRASmin is set at 80 ns, the time required for sufficiently increasing the potential difference between the pair of bit lines is lengthened up to about 40 ns by changing the term tRAS toward the term tRP (i.e., the RAS precharging term). Therefore, the potential difference between the pair of bit lines is sufficiently increased. As a result, the DRAM makes no soft errors, nor is its data holding characteristic impaired at all. Hence, the DRAM can operate at high speed. As has been described, the column control circuit 92 is controlled by the internal RAS signal KRINT which has been generated from the output signal of the second timer circuit 5, and the delay time τ2 of the delay circuit 15 used in the second timer circuit 15 is shorter than the delay time τ1 of the delay circuit 11 of the first timer circuit 4. Therefore, the column control circuit 92 can be stopped earlier than the row control circuit 91 in response to the second internal RAS signal KRINT. Data can be correctly written into, and read from, the DRAM. Hence, the number of critical timings is reduced by one, by virtue of the RAS time-out function performed by the DRAM. Fig. 2A illustrates the internal RAS signal generating section incorporated in a DRAM which is a second embodiment of the invention. This internal RAS signal generating section is designed to output three internal RAS signals RINT1 , RINT2 , and RINT3 . The signal RINT1 will control a row control circuit, and the signals RINT2 and RINT3 will control a column control circuit, as will be described later in detail. As is shown in Fig. 2A, the section comprises an external RAS input buffer 1, three internal RAS signal generating circuits 2, 3, and 3-2, three timer circuits 4, 5, and 5-2. The buffer 1 is of the same structure as that shown in Fig. 1A. The internal RAS signal generating circuits 2, 3, and 3-2 are identical in structure to those shown in Fig. 1A. Also, the timer circuits 4, 5, and 5-2 have the same structure as those shown in Fig. 1A. The first timer circuit 4 starts operating at the leading edge of a word-line driving signal WDRV, and stops operating upon lapse of time τ1 when the potential difference between the pair of bit lines increases sufficiently. During this period of time, τ1, the signal RINT1 remains at a low level, regardless of the voltage value of the external RAS signal supplied to the external RAS input buffer 1. The signal RINT1 is supplied to the row control circuit 91. More precisely, as is shown in Fig. 2B, this signal RINT1 is supplied to the low-address buffer controller 21 and the word line driver 22, both incorporated in the control circuit 91. The term tRAS (i.e., the term during which the RAS signal remains at the low level) is short but longer than the time required for increasing the potential of the word line WL to the high level. Therefore, the potential of the word line WL remains at the high level up until the potential difference between the bit lines increases sufficiently. When this potential difference becomes great enough, the word line is disabled. Thereafter, the output signal of the row-address buffer 23, also used in the row control circuit 91, equalizes the bit lines; that is, it sets the bit lines at the same potential. On the other hand, the signal RINT2 , which has been generated from the output signal of the second timer circuit 5, is supplied to the column control circuit 92. More precisely, as is shown in Fig. 2B, the signal RINT2 is input to the column-address buffer controller 27 and the column controller 29, both incorporated in the control circuit 92. Like the signal RINT1 , this signal RINT2 falls to the low level at the trailing edge of the external RAS signal. The signal RINT2 remains at the low level regardless of the voltage value of the external RAS signal. This is because the second timer circuit 5 starts operating at the leading edge of the word-line driving signal WDRV. The second timer circuit 5 stops operating upon lapse of time τ2 which is shorter than the delay time τ1 of the delay circuit 11 incorporated in the first timer circuit 4. Hence, the signal RINT2 is synchronous with the external RAS signal. The period during which this timer circuit 5 functions is either equal to the term tRASmin of the DRAM or slightly shorter than the term tRASmin. Therefore, the second timer circuit 5 enables the DRAM to write and read data correctly even if the term tRAS is relatively short. In other words, the second timer circuit 5 also serves to reduce the number of critical timings by one, by virtue of the RAS time-out function performed in the DRAM. The third timer circuit 5-2, which characterizes the second embodiment of the invention, has time constant τ3 which is greater than τ2 and less than τ1; that is, τ2 < τ3 < τ1. The signal RINT3 , which has been generated from the output signal of the third timer circuit 5-2, is input to the column control circuit 92. More precisely, as is illustrated in Fig. 2B, the signal RINT3 is supplied to the column decoder 30 used in the control circuit 92. The term tRAS of the DRAM is relatively short. Therefore, the signal RINT2 input to the circuits 27 and 29 is reset at the high level earlier than the other internal RAS signals RINT1 and RINT3 . For some time after the signal RINT2 has been thus reset, the signal RINT3 input to the column decoder 30 remains at the low level. Hence, the column-select line CSL remains selected. However, the signal RINT3 will be reset to the high level earlier than the signal RINT1 input to the row-address buffer controller 21 and the word line driver 22. This is because, τ3 < τ1. The reason for τ3 < τ1 will be discussed briefly. If the column-select line CSL is reset simultaneously with the signal RINT2 , data cannot be completely written into the DRAM in some cases, when the external tRAS is smaller than the tRASmin. This is because the line CSL is inevitably reset before the completion of the data-writing operation. On the other hand, if the column-select line CSL is reset simultaneously with the signal RINT1, no time can be spared for the word line WL to close after the lower-potential one of the bit lines has been set at the VSS potential. Consequently, the magnitude of the data signal to be rewritten into a memory cell is insufficient in some cases, making it impossible to write correct data into the DRAM. The lower-potential bit line is at a potential about 1 V higher than the VSS potential when the column-select line CSL is selected. This is because a P-channel load transistor is connected to a DQ line to protect the data stored in the memory cell. (The DQ line is coupled to the bit line as long as the column-select line CSL remains at the high level, and the DQ line is also at a potential about 1 V higher than the VSS potential when the column-select line CSL is selected.) Nonetheless, since the term tRAS is relatively long, and the RAS signal is precharged during this term tRAS, the potential difference between the bit lines is sufficiently great. The magnitude of the data signal is therefore sufficient even if the word line WL is closed when the lower-potential bit line is still at a potential about 1 V higher than the VSS potential. The data can be correctly written into the memory cell. The DRAM according to the second embodiment is advantageous in two respects. First, the DRAM can operate at high speed even if its integration density is comparatively high. Second, the data can be correctly written into the DRAM. The DRAM can operate at high speed since the term tRP (i.e., the term for precharging the RAS signal) is relatively long. Part of the long tRP is added to the term tRAS, thus lengthening this term tRAS during which the signal RINT1 remains at the low level, while maintaining the term tRC (i.e., the RAS-cycle time uncharged). Thus, the term tRAS is longer than tRASmin, the potential difference between the pair of bit lines increases sufficiently in a short cycle time. The data can be correctly written into the DRAM since the signal RINT3 , generated from the output signal of the timer circuit 5-2 which stops operating before the term tRASmin runs out, is reset earlier than the signal RINT1 and later than the signal RINT2 , thereby to amplify the data signal to be rewritten into a memory cell before the word line is set at the low level. This is because if the signal RINT3 is reset at the expiration of the term tRASmin, the data-writing operation cannot be correctly performed, and if the signal RINT3 is reset at the leading edge of the signal RINT1 , the lower-potential bit line may fail to set at the VSS potential. Fig. 3A shows the internal RAS signal generating section incorporated in a DRAM which is a third embodiment of the invention. Fig. 3B illustrates a row control circuit 91 and a column control circuit 92, both incorporated in this DRAM. Fig. 3C is a timing chart explaining the operations of the control circuits 91 and 92 shown in Fig. 3B. In Figs. 3A and 3B, the same reference numerals as those used in Figs. 1A and 1B designate the same components as those shown in Figs. 1A and 1B. As may be understood from Figs. 3A, 3B, and 3C, the third embodiment is characterized in that the timer circuit 4 has a time constant τ1 greater than the term tRASmin, so that the potential difference between a pair of bit lines can be increased sufficiently within the term tRASmin. Since the term tRP (i.e., the period for precharging the bit lines) is relatively long, the the RAS -cycle time tRC need not be lengthened even if part of the term tRP is used to lengthen the internal tRAS. In order to keep the term defined from the time of precharging the RAS signal at tRASmin, the external RAS signal, not the internal RAS signal generated from the output signal of the timer circuit 4, is used to control the column-address buffer controller 27 and the column-address controller 29, both incorporated in the column control circuit 92. The operation of the third embodiment will now be described, with reference to Figs. 3A, 3B, and 3C. First, the external RAS input buffer 1 receives an external RAS signal. The section shown in Fig. 3A outputs two internal RAS signals RINT -1 and KRINT -1. The signal RINT -1 is not influenced by the external RAS signal and remains at the low level, once the external signal has fallen to the low level, and the word-line driving signal WDRV has risen to the high level. The signal RINT -1 comes to be influenced again upon lapse of time τ during which the potential difference between the bit lines increases sufficiently. This signal RINT -1 is input to the row-address buffer controller 21 and the word-line driver 22 only, as is illustrated in Fig. 3B. Thus. the word line is disabled upon lapse of a period during which the potential difference between the bit lines increases sufficiently even if the term tRASmin is 80 ns. Thereafter, the bit lines are equalized by the output signal of the row-address buffer 23. On the other hand, the signal KRINT-1 is totally synchronous with the external RAS signal. This signal is input to the column-address buffer controller 27 and the column-address controller 29, as is illustrated in Fig. 3B. Hence, the RAS signal can be precharged within the term tRASmin. In other words, the signal KRINT-1 can control the column control circuit 92, regardless of the value of τ. In Fig. 4, two period of time, tRPC ( RAS -to- CAS Precharge Time) and tRRH (Read Command Hold Time Reference-to- RAS ) are defined. The period tRPC starts when the RAS is precharged and ends when the CAS signal is activated. However, there is the rule that the DRAM must not read data Dout through an output buffer (not shown) even if tRPCmin = 0 at the end of any operation cycle (e.g., RAS -only refresh cycle) other than a read cycle. Needless to say, this rule should be observed when tRASmin is 80 ns. To observe this rule, the DRAM must not read the data Dout in the region 41 of the graph shown in Fig. 5, wherein the term tRAS and the period tRPC are plotted on the X axis and the Y axis, respectively. If the column-address buffer 27 and the column-address controller 29 are controlled by the signal RINT -1 generated from the output signal of the timer circuit 4 whose delay time is longer than the term tRASmin, the DRAM will output the data Dout in the fail region 43 shown in Fig. 6. This region includes a sub-region 42, in which the DRAM should not output the data Dout. This is because the fail region 43 is too large since the rising of the signal RINT -1 is delayed. In the third embodiment of the invention, the signal KRINT -1 does not interfere with the signal RINT -1, and the fail region 43-2 is small and narrow as is illustrated in Fig. 7. As is evident from Fig. 7, the DRAM does not output the data Dout in the region 41, thus observing the above-mentioned rule. The period tRRH defined in Fig. 4 starts when the RAS signal is precharged and ends when the write-enable signal WE is activated. There is the rule that the DRAM must not write data Din through an input buffer (not shown) even if tRRHmin = 0 at the end of any operation cycle (e.g., RAS -only refresh cycle) other than a write cycle. Since the column-address buffer 28 and the column-address controller 29 are controlled by the signal KRINT -1, not the signal RINT -1, the DRAM observes this rule, too, even when the term tRASmin is 80 ns. In the third embodiment of the present invention, the signal KRINT -1, which is synchronous with the external signal, is input to the column-address control circuit 92. This solves the following problems inherent in the conventional DRAM. In the conventional DRAM, the time required for sufficiently increasing the potential difference between any pair of bit lines falls within the term tRASmin (i.e., the minimum pulse width of the RAS signal). Therefore, the RAS time-out function is performed for two purposes: (i) to reduce the number of critical timings, thus making it easy for the user to operate the DRAM; (ii) to prevent destruction of the data stored in the DRAM. It is increasing demanded that DRAMs operate at higher speed. To meet this demand, tRAS should also be as short as 80 ns or 60 ns. It is also demanded that DRAM having a larger storage capacity be provided. Generally, the greater the storage capacity of a DRAM, the longer the time required for sufficiently increasing the potential difference between each pair of bit lines. Under these circumstances, the RAS time-out function be used for another purpose, that is, to provide a time longer enough to increase said potential difference sufficiently even if the user has set tRASmin of 80 ns or 60 ns to the DRAM. To provide such a long time, a timer having a large time constant must be incorporated in the DRAM chip. Even if this timer is used, the term tRC can be maintained unchanged since the tRP set in the DRAM is far shorter than tRPmin. In other words, it is possible to divide tRC into tRAS and tRP freely in a chip. However, when the column control section of this DRAM, as well as the bit lines and word lines thereof, are controlled by the output signal of the timer, it becomes no longer possible for the DRAM to operate if the period tRPC or the period tRRH are set at their respective minimum values. This problem is solved in the third embodiment of the present invention, by supplying the signal KRINT -1, which is synchronous with the external RAS signal, to the column control circuit 92. Hence, the DRAM according to the third embodiment can be a high-speed, large-capacity one which is free of soft errors and has good data holding characteristic. Figs. 8A, 8B, and 8C are block diagrams schematically showing those units of the first, second, and third embodiments, each of which comprises at least one internal RAS signal generating section and two control circuits. More specifically, the unit shown in Fig. 8A has a chip 51 in which the timer circuits 4 and 5 and the control circuits 91 and 92 -- all illustrated in Fig. 1B, are incorporated. As can be understood from Fig. 8A, the external RAS signal is input to the chip 51. In the chip 51, the timer circuit 4 generates an internal RAS signal RINT for controlling the row control circuit 91, whereas the timer circuit 5 generates an internal RAS signal KRINT for controlling the column control circuit 92. The unit illustrated in Fig. 8B has a chip 52 in which the timer circuits 4, 5, and 5-2 and the control circuits 91 and 92 -- all illustrated in Fig. 2B, are incorporated. As is evident from Fig. 8B, the external signal is input to the chip 52. In the chip 52, the timer circuit 4 generates an internal RAS signal RINT1 for controlling the row control circuit 91, the timer circuit 5 generates an internal RAS signal RINT2 for controlling the column control circuit 92, and the timer circuit 5-2 generates an internal RAS signal RINT3 for controlling also the column control circuit 92. The unit shown in Fig. 8C has a chip 53 in which the timer circuit 4 and the control circuits 91 and 92 -- all illustrated in Fig. 3B are incorporated. As is evident from Fig. 8C, the external RAS signal is input to the chip 53. In the chip 53, the timer circuit 4 generates two internal RAS signals RINT-1 and KRIN-1 for controlling the row control circuit 91 and the column control circuit 92, respectively. Figs. 9A, 9B, and 9C are block diagrams schematically showing those units of a fourth, fifth, and sixth embodiments, each of which comprises at least one internal RAS signal generating section and two control circuits. More precisely, the unit shown in Fig. 9A is formed of two chips 61 and 51-2. Timer circuits 4 and 5 are formed in the chip 61, and control circuits 91 and 92 are formed in the chip 51-2. As is shown in Fig. 9A, the external RAS signal is input to the chip 61 via an input terminal 96. In the chip 61, the timer circuit 4 generates an internal RAS signal RINT for controlling the row control circuit 91, whereas the timer circuit 5 generates an internal RAS signal KRINT for controlling the column control circuit 92. The signal RINT is supplied to the input terminal 64 of the chip 51-2, and the signal KRINT is supplied to the input terminal 65 of the chip 51-2. The external RAS signal can be supplied from a circuit located outside the chip 61. The unit illustrated in Fig. 9B has chips 62 and 52-2. Three timer circuits 4, 5, and 5-2 are formed in the chip 62, and two control circuits 91 and 92 are formed in the chip 52-2. As is shown in Fig. 9B, the external RAS signal is input to the chip 62 via an input terminal 97. In the chip 62, the timer circuit 4 generates an internal signal RINT1 for controlling the row control circuit 91 the timer circuit 5 generates an internal RAS signal RINT2 for controlling the column control circuit 92, and the timer circuit 5-2 generates an internal RAS signal RINT3 for controlling also the column control circuit 92. The signal RINT1 is supplied to the input terminal 66 of circuit 91, and the signals RINT2 and RINT3 are supplied to the input terminal 65 of the circuit 92. The terminals 67 and 68 of the circuit 92, respectively, external RAS signal can be supplied from a circuit located outside the chip 62. The unit shown in Fig. 9C has chips 63 and 53-2. One timer circuit 4 is formed in the chip 63, and two control circuits 91 and 92 are formed in the chip 53-1. The external RAS signal is input to the chip 63 through an input terminal 98. In the chip 63, the timer circuit 4 generates an internal RAS signals RINT- 1 for controlling the row control circuit 91. The external RAS signal is directly or synchronously supplied to the chip 53-2 as KRINT-1 . The signal RINT-1 is supplied to the input terminal 69 of the circuit 91, and the signal KRINT-1 to the input terminal 70 of the circuit 92. The external RAS signal can be supplied from a circuit outside the chip 63. Figs. 10A to 10F are circuit diagrams, each showing a circuit for generating an internal RAS which can be used in the present invention. The circuit illustrated in Fig. 10A can be used in the section shown in Fig. 1A. The circuit shown in Fig. 10B is a modification of the circuit illustrated in Fig. 10A; the external RAS input buffer 1, the timer circuits 4 and the internal RAS generating circuit 2 are different from their counterparts of the timer circuit shown in Fig. 10A. The circuit shown in Fig. 10C is different from the circuit of Fig. 10A, in that a NAND gate 71 is used in place of the flip-flop (Fig. 10A). The circuit shown in Fig. 10D is different from the circuit of Fig. 10B, in that a NAND gate 72 is used in place of the flip-flop (Fig. 10B). The circuit illustrated in Fig. 10E is a combination of the circuit shown in Fig. 10A and a circuit 81 for preventing the row control circuit 91 from being reset. The signal WDUR input to the circuit 81 is at the low level during the write duration of the DRAM. The circuit shown in Fig. 10F is a modification of the circuit shown in Fig. 10E; it differs in the structure of the timer circuit 4 and the circuit 81. The circuits shown in Figs. 10A to 10F can be used as circuits for generating an internal RAS signal for controlling both the row control circuit 91 and the column control circuit 92. The present invention is not limited to the embodiments described above. Various changes and modifications can be made without departing the scope of the present invention. As has been described, the present invention provide a dynamic memory and a dynamic memory system which reliably restore data on the bit lines, and operate at high speed at correct various RAS timings.

Claims: A control circuit for a semiconductor memory device comprising: a RAS input terminal (93-98) for receiving an external row-address strobe ( RAS ) signal; a row control circuit (91) including a row-address control circuit for controlling row addresses for the semiconductor memory device and a word line driver circuit (22) for driving word lines; a column control circuit (92) including a column-address control circuit (27) for controlling column addresses for the semiconductor memory device and an output control circuit (32) for controlling data output; an internal RAS signal generator (1-5) for generating at least a first internal RAS signal ( RINT , RINT1 ) being supplied to said row control circuit (91) and a second internal RAS signal ( KRINT , RINT2 ) being supplied to said column control circuit (92) such that the low level time of the first internal RAS signal ( RINT ) is expanded by at least a first delay time (τ1) and is longer than the low level time of the external RAS signal and also longer than the low level time of the second internal RAS signal ( KRINT ). A control circuit for a semiconductor memory device according to claim 1, wherein the low level time of the second internal RAS signal ( KRINT , RINT2 ) is expanded by at least a second delay time (τ2) and is longer than the low level time of the external RAS signal. A control circuit for a semiconductor memory device according to claim 1, wherein the internal RAS signal generator (1-5) generates a third internal RAS signal ( RINT3 ) being supplied to a column decoding control (30) in said column control circuit (92) for controlling column address signals to be decoded such that the low level time of the third internal RAS signal ( RINT3 ) is expanded by at least a third delay time (τ3) and is shorter than the low level time of the first internal RAS signal ( RINT1 ) and longer than the low level time of the second internal RAS signal ( RINT2 ). A control circuit for a semiconductor memory device according to claim 1, further comprising a semiconductor chip (61 to 63) including said RAS input terminal (96-98) for receiving said external row-address strobe, RAS , signal and a first timer means (4) for expanding the low level time of the external RAS signal by at least a first delay time (τ, τ1) to provide the first internal RAS signal ( RINT-1 , RINT ) ; and a semiconductor memory device (53-2) including a first input terminal (64, 66, 69) for receiving said first internal RAS signal ( RINT-1 , RINT ) expanded by said first timer means (4) and supplying said first internal RAS signal ( RINT-1 , RINT ) to said row control circuit (91), and a second input terminal (65, 67, 70) for receiving the second internal RAS signal and supplying the second internal RAS signal to said column control circuit (92). A control circuit for a semiconductor memory device according to claim 4, wherein said semiconductor chip (61, 62) further includes a second timer means (5) for expanding the low level time of the external RAS signal, for at least a second delay time ( 2) to provide the second internal RAS signal ( KRINT , RINT2 ); and said semiconductor memory device (51-2) has its second input terminal (65, 67) connected for receiving said second internal RAS signal ( KRINT , RINT2 ) expanded by said second timer means (5) and supplying said second internal RAS signal ( KRINT , RINT2 ) to said column control circuit (92). A control circuit for a semiconductor memory device according to claim 5, wherein said semiconductor chip (62) further includes a third timer means (5-2) for expanding the low level time of the external RAS signal for at least a third delay time (τ3) to provide a third internal RAS signal ( RINT3 ); and said semiconductor memory device (51-2) includes a third input terminal (68) for receiving said third internal RAS signal ( RINT3 ) expanded by said third timer means (5-2) and supplying said third internal RAS signal ( RINT3 ) to said column control circuit (92). A control circuit for a semiconductor memory device according to claim 2, 3, 5 or 6, wherein the first delay time (τ1) is longer than the second delay time (τ2). A control circuit for a semiconductor memory device according to claim 3 or 6, wherein the first delay time (τ1) is longer than the third delay time (τ3), and the third delay time (τ3) is longer than the second delay time (τ2). A control circuit for a semiconductor memory device according to any preceding claim, wherein said RAS signal is a pulse signal having a width of less than 80 nsec, and said first timer means (4) is activated for said first delay time (τ1) of time after said first timer means (4) responds to a word-line driving signal (WDRV). A control circuit for a semiconductor memory device according to claim 2, 3, 5 or 6, wherein said RAS signal is a pulse signal having a width of less than 80 nsec, and said second timer means (5) is activated for said second delay time (τ2) of time after said second timer means (5) responds to a word-line driving signal (WDRV). A control circuit for a semiconductor memory device according to claim 3 or 6, whrein said RAS signal is a pulse signal having a width of less than 80 nsec, and said third timer means (5-2) is activated for said third delay time (τ3) of time after said third timer means (5-2) responds to a word-line driving signal (WDRV). A control circuit for a semiconductor memory device according to claim 1 or 4, characterized in that said first timer means (4) comprises a first inverter circuit (12) for receiving a word-line driving signal (WDRV), a first delay circuit (11) connected to the output of said first inverter circuit (12), for delaying the word-line driving signal by a first delay time (τ1), and a 2-input NAND gate circuit (10) having a first input for receiving the word-line driving signal, and a second input connected to the output of said first delay circuit (11). A control circuit for a semiconducctor memory device according to claim 2 or 5, characterized in that said second timer means (5) comprises a second inverter circuit (16) for receiving a word-line driving signal (WDRV), a second delay circuit (15) connected to the output of said second inverter circuit (16), for delaying the word-line driving signal by the second delay time (τ2), and a second 2-input NAND gate circuit (14) having a first input for receiving the word-line driving signal, and a second input connected to the output of said second delay circuit (15). A control circuit for a semiconductor memory device according to claim 3 or 6, characterized in that said third timer means (5-2) comprises a third inverter circuit (16-2) for receiving a word-line driving signal (WDRV), a third delay circuit (15-2) connected to the output of said third inverter circuit (16-2), for delaying the word-line driving signal by the third delay time (τ3), and a third 2-input NAND gate circuit (14-2) having a first input for receiving the word-line driving signal, and a second input connected to the output of said third delay circuit (15-2). A semiconductor memory device system comprising a control circuit according to any preceding claim and a semiconductor memory arranged to be controlled by the control circuit.

Inventors: OHSAWA TAKASHI C O INTELLECTUA, OHSAWA, TAKASHI C/O INTELLECTUAL PROPERTY DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: G11C11
IPC codes: G11C8, G11C11