SEMICONDUCTOR MEMORY DEVICE

Abstract: A semiconductor memory device having an operational mode such as a nibble mode or page mode, wherein, with a first address strobe signal ( RAS ) kept in an active state, a second address strobe signal ( CAS ) is successively switched between an active state and standby state, thereby enabling successive data output (Dout). Previous data output is once reset in ac­ cordance with the switchover of the second address strobe signal to the active state while the first address strobe signal is in the active state, before data output is performed, and the reset operation of the data output is also performed when both the first and second address strobe signals are switched to the standby state, so that the period of the output data can be expanded.

Description: The present invention relates to a semiconductor memory device, more particularly to a semiconductor memory device such as a dynamic random access memory or other device wherein an operation is performed in a nibble mode or page mode. In a dynamic random access memory, operation under a nibble mode (4-bit block of word mode), page mode, etc. when reading out data enables a plurality group of bits of data to be read out at a high speed. However, when reading out data under the nibble mode, page mode, etc., the time interval of each bit of the read out signal becomes extremely short and the period of the group of output data becomes considerably short. When the period of the output data is too short, the circuits connected to the memory device cannot respond sufficiently and accurate data readout becomes impossible. Therefore, even in data readout under the nibble mode, page mode, etc., it is desirable that the period of each bit of the output data be made as long as possible. In a conventional memory device, each successive data readout in the nibble mode or page mode is output after the fall of an inverted column address strobe signal and is reset at the rise of the inverted column address strobe signal. Therefore, the time period of each bit of output data is considerably short. To eliminate this disadvantage, there is known another memory device, for example, the Model 2716 16K RAM from Intel Co., USA. In this memory device, output data is reset at the fall of the inverted column strobe signal, thereby considerably expanding the time period of the output data. However, this memory device has the disadvantage of competition occurring between outputs of a plurality of memory chips when a plurality of memory chips are combined to constitute a large capacity memory system, as explained in detail later. An embodiment of the present invention, in consideration of the above-mentioned problems in the conventional art, can provide a semiconductor memory device in which the period of a data output during operation under a nibble mode, page mode, etc. is expanded and the output data of the memory chips do not compete even in the case of a memory system constituted by a plurality of memory chips having common output lines. According to the present invention, there is provided a semiconductor memory device arranged to receive a first address strobe signal and a second address strobe signal for controlling its data output, and having an operational mode wherein, with said first address strobe signal kept in an active state, said second address strobe signal is successively switched between a standby state and an active state, thereby enabling successive data output; the device comprising reset means arranged to perform a reset operation of previous data output in response to switching of said second address strobe signal to the active state whilst said first address strobe signal is in the active state; characterised in that: said reset means is also arranged to perform an additional reset operation of data output in response to switching of said second address strobe signal to the standby state whilst said first address strobe signal is in the standby state, said reset means comprising an exclusive OR circuit, to which said first and second address strobe signals are input, and which outputs an output signal initiating said reset operation and said additional reset operation. Reference is made, by way of example, to the accompanying drawings in which:- Figure 1 is a block diagram of the schematic constitution of a conventional semiconductor memory device; Figure 2 is a waveform diagram explaining the operation of the memory device of Fig. 1; Figure 3 is a block diagram of the constitution of a memory system using a plurality of memory chips, to which the present invention may be applied; Figure 4 is a waveform diagram of the conventional way of operation of the memory system of Fig. 3; Figure 5 is a waveform diagram of the operation of a semiconductor memory device according to an embodiment of the present invention; Figure 6 is a waveform diagram of the operation of a memory system constituted by using a plurality of memory chips of the semiconductor memory device according to an embodiment of the present invention; and Figure 7 is an electrical circuit diagram of the detailed circuit constitution of an output buffer used in a semiconductor memory device according to an embodiment of the present invention. Before describing the preferred embodiments, an explanation will be given of a conventional semiconductor memory device having the nibble mode or page mode. Figure 1 shows the schematic constitution of a dynamic random access memory used as a conventional semiconductor memory device. The memory device in the figure is constituted by, for example, four cell blocks 1, 2, 3, and 4, each including a plurality of memory cells; data buffers 5-1, 5-2, 5-3, and 5-4 connected to cell blocks 1, 2, 3, and 4, respectively; and an output buffer 6. Each data buffer 5-1, 5-2, ..., and 5-4 is constituted by a basically flip-flop type circuit. In the memory device of Fig. 1, data read out from the memory cells in the cell blocks 1, 2, 3, and 4 selected by a row decoder and column decoder (not shown) is temporarily stored in the data buffers 5-1, 5-2, 5-3, and 5-4, respectively. Next, based on part of the column address signals, i.e., 2 bits in the memory device of Fig. 1, one of the data buffers 5-1, 5-2, 5-3, and 5-4 is selected and 1 bit of data is taken out through the output buffer 6 as the output data D out . However, in a memory device having the above-mentioned constitution, the dynamic random access memories (data buffers) are connected in cascade to constitute a shift register and it is possible to take out data read out from the cell blocks 1, 2, 3, 4 at a high speed. This operational mode is called the nibble mode. In other words, as shown in Fig. 2, during operation under the nibble mode, an inverted row address strobe signal RAS becomes low level, the address signal at that time is input and fixed as the row address, and the inverted column address strobe signal CAS is then repeatedly turned ON and OFF for a successive output of data from the cell blocks 1, 2, 3, 4. Therefore, in the operation under the nibble mode, an extremely high speed readout is possible compared with the usual readout operation wherein address selection is performed for both rows and columns for every 1 bit. However, in the above-mentioned conventional memory device, the data output D out during a nibble mode operation is reset at the rise of the inverted column address strobe signal CAS , and thus, the data period T of each bit of the output data becomes considerably short and input of the output data D out by the circuits connected to the memory device becomes very difficult. Therefore, this device has the disadvantage of insufficient utilization of the functions of the nibble mode in actual operation. In particular, in recent random access memories, the cycle time for ON-OFF switching of the inverted column address strobe signal CAS is short, i.e., 4⊘ to 5⊘ ns. Consequently, in the nibble mode, the effective period T of the D out becomes extremely short, i.e., 15 to 2⊘ ns. As a method of eliminating this disadvantage and of increasing the effective period of the output data, it is known to use a memory device in which the output data reset is performed upon the fall of the inverted column address strobe signal CAS (for example, as in Model 2716 16K RAM of Intel Co., USA). In such a memory device, the period of the output data can be considerably expanded, however, there is the disadvantage of competition between outputs of a plurality of memory chips where a plurality of memory chips 7, 8, 9, and 1⊘ are combined to constitute a large capacity memory system, as shown in Fig. 3. In other words, in the memory system of Fig. 3, the memory chips 7 and 9 make common use of the output line D1 and a common inverted row address strobe signal RAS1 is applied. Also, memory chips 8 and 1⊘ make common use of the output line D2 and a common inverted row address strobe signal RAS2 is applied. Further, the memory chips 7 and 8 receive as input a common inverted column address strobe signal CAS1 while memory chips 9 and 1⊘ receive a common inverted column address strobe signal CAS2 . In the memory system shown in Fig. 3, when, for example, successively reading out data from the memory chips 7 and 9, the inverted row address strobe signal RAS1 becomes low level then the inverted column address strobe signal CAS1 repeatedly turns ON and OFF, as shown in Fig. 4. Conventionally, after readout of data from the memory chip 7, the inverted row address strobe signal RAS1 remains at―――――――low level and the inverted column address strobe signal CAS2 repeatedly turns ON and OFF. By this operation, the output LSI1 from the memory chip 7 and the output LSI3 from the memory chip 9 are successively output on the common data line D1. However, in this case, as is clearly shown in Fig. 4, the output of the memory chip is only reset upon the fall of the inverted column address strobe signal, therefore the output LSI1 of the memory chip 7 remains as output even after the completion of the ON-OFF operation of the inverted column address strobe signal CAS1 and is held until the inverted column address strobe signal CAS1 next falls. In the same way, the output LSI3 of the memory chip 9 is held until the inverted column address strobe signal CAS2 falls, therefore the memory chips 7 and 9, which commonly use the output line D1, compete in output. Consequently, a memory device which resets output data at the fall of the inverted column address strobe signal has a disadvantage in that it cannot be used for a memory system constituted by a plurality of memory chips having common output lines. The same problem exists with the so-called page mode, wherein, at the state where the row address is fixed, the column address is input at each fall of the inverted column address strobe signal for successive output of data. The operation of a semiconductor device according to the present invention will be explained with reference to the drawings. The semiconductor memory device of a first embodiment of the present invention has, for example, the constitution of the memory device of Fig. 1 with the output buffer 6 replaced with a below-mentioned (Fig. 7) circuit. In a preferred embodiment of the semiconductor memory device of the present invention, during operation under the nibble made or page mode, the output data is reset based on a signal obtained by an exclusive OR operation of the row address strobe signal and column address strobe signal. In other words, as shown in Fig. 5, after the inverted row address strobe signal RAS becomes low level, and when the inverted column address strobe signal CAS once becomes low level, the output data D out is output after a predetermined time delay from the point of the fall thereof. The output data D out is reset by the fall of a signal EOR obtained by an exclusive OR operation of the row address strobe signal and the column address strobe signal. Consequently, when the inverted column address strobe signal CAS next becomes low level, the previous data output is once reset and the output data D out is then output after the above-mentioned predetermined time delay. Subsequently, if the inverted row address strobe signal RAS is already at high level when the inverted column address strobe signal CAS once again becomes high level, the output data D out is reset in response to the rise of the former inverted column address strobe signal CAS . Therefore, by using this reset method, when the inverted row address strobe signal RAS is active, i.e., at low level, the previous output data D out is once reset at the fall of the inverted column address strobe signal (transition to active state) and then the effective data (memory data) is output. When the inverted row address strobe signal RAS is on standby, i.e., at high level, the output data is reset at the time of the rise of the inverted column address strobe signal. Therefore, the period of the output data can be expanded while maintaining interchangeability with conventional dynamic random access memories. In Fig. 5, waveform A represents the output waveform in a conventional memory device wherein the output signal is reset by the rise of the inverted column address strobe signal CAS and waveform B represents the waveform of the output data in a conventional memory device wherein the output data is reset at the fall of the inverted column address strobe signal CAS . An explanation will be given about the signals CL, CL', and R in the description of later embodiments. Figure 6 shows the operation of a memory system using a plurality of semiconductor memory devices according to the above-mentioned embodiment of the present invention and constituted by the above-mentioned circuit shown in Fig. 3. In Fig. 6, after the inverted row address strobe signal RAS1 has become low level, the inverted column address strobe signal CAS1 repeatedly turns ON and OFF. The output LSI1 of the memory chip 7 at this time, as shown in Fig. 6, is not output (i.e. is reset) after the signal RAS1 is temporarily made high (compare Figs. 4 and 6) and ――――― the inverted column address strobe signal CAS1 becomes high level. Further, even when the inverted column address strobe signal CAS2 repeatedly turns ON and OFF, the output LSI3 of the memory chip 9 is not output before the time that the inverted column address strobe signal CAS2 initially becomes low level. Therefore, the outputs of the two memory chips 7 and 9 making common use of the output line D1 do not compete. Figure 7 shows the detailed constitution of an output buffer used in the semiconductor memory device according to an embodiment of the present invention. The output buffer of the figure is provided with a buffer portion 11 constituted by transistors Q1 to Q1⊘, etc. and a reset signal generating circuit portion 12 constituted by transistors Q11 to Q26, etc. In the output buffer shown in Fig. 7, the data buses DB and DB of the buffer portion 11 are connected to the outputs of the shift registers constituted by the data buffers of each cell block (not shown) for a semiconductor memory device with a nibble mode function. When a clock pulse CL' is at high level, the outputs of the data buffers DB and DB are applied to the gates of the transistors Q2 and Q3 through the transistors Q1 and Q6, respectively. The clock signal CL' rises and becomes high level following a predetermined time delay after which data output is possible, this being after the inverted column address strobe signal CAS falls when the inverted row address strobe signal RAS is in the active state (low level), and, after the inverted column address strobe signal CAS moves to the high level, maintains this high level in a floating state. Referring to Fig. 5, a clock signal CL becomes high level when the two strobe signals RAS and CAS enter the active state and data output is possible in the readout state. This is an internal clock signal which is used also for the conventional device (output type of Fig. 5 waveform A) and which falls in accordance with the rise of the latter ( CAS ). The above clock signal CL' is similar to the clock signal CL, however, after the rise of the inverted column address strobe signal CAS , the high level is held at the floating state and then becomes the low state only after the reset operation of the buffer circuit 11 explained below. Then, for example, if the data bus DB is a high level and the data bus DB is a low level, the clock signal CL' becomes high level and, thereby, the transistor Q5 becomes on and the transistor Q4 becomes off. Thus, the gate of the transistor Q9 of the output circuit becomes low level and the transistor Q9 is cut off, the gate of the transistor Q1⊘ becomes high level, and the transistor Q1⊘ becomes ON. Therefore, for example, a low level signal is output as the output data D out · Even if the clock signal CL' is in the floating state, this output state is maintained so long as the clock signal CL' is held at a high level. However, in the reset signal generating circuit 12, when the inverted row address strobe signal RAS and the inverted column address strobe signal CAS are both at high level, the transistors Q21 and Q26 both become ON and the gate voltages of the transistor Q23 and Q24 both become low level. Consequently, these transistors Q23 and Q24 both are cut off, and, therefore, the gate of the transistor Q16 becomes high level. By this, the transistor Q16 becomes ON, the exclusive OR output EOR of the two strobe signals becomes low level, and the transistor Q18 becomes cut off. If, in this state, the clock pulse CL is low level, the reset signal output R becomes high level. Further, when the inverted row address strobe signal RAS and the inverted column address strobe signal CAS both are low level, the transistors Q12 and Q13 are both cut off and the gate voltage of the transistor Q15 becomes high voltage. Consequently, the transistor Q15 becomes ON and the EOR signal output terminal, i.e., the gate voltage of the transistor Q18, is pulled down to a low level and the transistor Q18 is cut off. Therefore, in this case too, if the clock pulse CL is low level, the reset signal output R is high level. On the other hand, if one of the inverted row address strobe signal RAS and the inverted column address strobe signal CAS is high level and the other is low level, the gate voltages of the transistors Q15 and Q16 both become low level and the transistors Q15 and Q16 are both cut off. Consequently, the inverted exclusive OR output EOR is high level, the transistor Q18 turns ON, and the output R becomes low level. As is clear from the above, the reset signal generating circuit 12 of Fig. 7 includes a portion for operating as an exclusive NOR circuit of the signals RAS and CAS and outputs a reset signal R at times other than times prescribed by the clock signal CL, which is high level after the output data becomes effective while the two strobe signals RAS and CAS are in the active state and times while the EOR output is high level. When the output R of the above-mentioned reset signal generating circuit 12 is high level, the transistors Q7 and Q8 of the buffer portion 11 both become ON. Therefore, the gates of the totempole connected output transistors Q9 and Q1⊘ both become low level, these transistors Q9 and Q1⊘ both enter the cutoff state, and the output terminal enters the high impedance state. In other words, by employing the output buffer circuit of Fig. 7 instead of the output buffer 6 of the memory device of Fig. 1, the information incorporated (sent) from the data buffer at the time when the clock pulse CL temporarily becomes high level to the flip-flop circuit constituted by the transistors Q4 and Q5 is reset at the time when the reset signal R rises. At the same time, the clock signal CL' drops from the floating high level state to low level and the output terminal then is in the high impedance state. In the circuit of Fig. 7, the transistor Q19 provided at the output stage of the reset signal generating circuit 12 is provided for preventing the output data of the buffer portion 11 from being reset when the clock pulse CL becomes high level. In this way, according to the present invention, during operation under the nibble mode or page mode, resetting of the output data is effected based on the signals obtained by exclusive OR operations of the row address strobe signal and the column address strobe signal, therefore, the period of the output data can be made sufficiently long and also no competition occurs between output data in a memory system constituted by a plurality of memory chips having common output lines.

Claims: A semiconductor memory device arranged to receive a first address strobe signal ( RAS ) and a second address strobe signal ( CAS ) for controlling its data output, and having an operational mode wherein, with said first address strobe signal ( RAS ) kept in an active state, said second address strobe signal ( CAS ) is successively switched between a standby state and an active state, thereby enabling successive data output; the device comprising reset means (11,12) arranged to perform a reset operation of previous data output in response to switching of said second address strobe signal ( CAS ) to the active state whilst said first address strobe signal ( RAS ) is in the active state; characterised in that: said reset means (11,12) is also arranged to perform an additional reset operation of data output in response to switching of said second address strobe signal ( CAS ) to the standby state whilst said first address strobe signal ( RAS ) is in the standby state, said reset means comprising an exclusive OR circuit (12), to which said first and second address strobe signals ( RAS , CAS ) are input, and which outputs an output signal (R) initiating said reset operation and said additional reset operation. A semiconductor memory device according to claim 1, wherein said first address strobe signal is a row address strobe signal ( RAS ) and said second address strobe signal is a column address strobe signal ( CAS ). A semiconductor memory device according to claim 1 or 2, wherein said semiconductor memory device comprises a plurality of memory cell blocks (1,...4), a shift register (5) receiving readout data from said memory cell blocks in parallel to corresponding stages (5-1,...5-4) of said shift register, and an output buffer circuit (6,11) which receives the output signal from said shift register (5) after resetting the previously received output signal by using the output signal (R) of said exclusive OR circuit (12). A semiconductor memory device according to claim 3, wherein said output buffer circuit (11) comprises a flip-flop circuit (Q₂...Q₅) to which output data of said shift register is sent, an output amplifier circuit (Q₉,Q₁₀) receiving the output signal of said flip-flop circuit, and a reset circuit (Q₇,Q₈) for resetting the output signal of said flip-flop circuit. A semiconductor memory device according to claim 4, wherein said output amplifier circuit comprises a pair of totempole connected transistors (Q₉,Q₁₀) each receiving complementary output signal from said flip-flop circuit. A semiconductor memory device according to claim 5, wherein said reset circuit comprises a pair of transistors (Q₇,Q₈) which are connected in parallel to transistors constituting said flip-flop circuit (Q₂,...Q₅) and which are turned on to reset the output signals of said flip-flop circuit on the basis of the output signal (R) of said exclusive OR circuit.

Inventors: NAKANO, MASAO, NAKANO, TOMIO SHIRAHATADAI-JUTAKU 12-404, SATO, KIMIAKI, TAKEMAE, YOSHIHIRO
Applicants: FUJITSU LIMITED
Application year: 1984
Main classification: G11C8
IPC codes: G11C8, G11C7, G11C11