Memory Circuit Having a Plurality of Cell Arrays

The present invention relates to a memory circuit, and more particularly to a dynamic memory circuit having a plurality of memory cell arrays.

The storage capacity of semiconductor memory devices has been becoming greater and greater, and efforts are being made to increase the integration density of integrated circuit chip.

However, if such increase in the storage capacity is achieved by simply increasing the number of memory cells coupled to each word line, the storage capacitance of each word line would become large, resulting in low speed operation.

Furthermore, in the case where the memories are of dynamic type, the written information disappears when a predetermined time has elapsed. Therefore, it is necessary to execute a rewriting cycle called "refresh" within the above time. The refresh is done on the respective word lines one by one and hence, the number of the word lines in the memory corresponds to the cycle times of refresh within the predetermined period of time.

Under the above circumstance, it has been practiced that the memory cells are divided into two or more memory cell arrays each having word lines to which reduced number of memory cells are coupled. In such construction, refresh operations are executed in the respective memory cell arrays in parallel, reducing the time required for refresh for the whole memory cells.

For example, memory cells of a 64 K-bit dynamic RAM are arrayed in two memory cell arrays each having 128 word lines and 256 bit lines, and the whole memory cells are refreshed through 128 refresh cycles. In this kind of memories, one word line in each memory cell array is driven and two word lines are simultaneously driven as a whole in the memory in response to the same driving pulse. Accordingly, the drive circuit for generating the driving pulse is required to have a large driving capacity in order to attain high speed operation. Moreover, since two word lines are driven at the same time to be charged to a power voltage, a large value of peak current necessarily flows to the two word lines through the drive circuit. This peak current causes noise and affects the memory operation, resulting in malfunction.

It is an object of the present invention to provide a memory circuit which can operate stably without malfunction.

It is another object of the present invention to provide a memory circuit which can operate with a reduced peak value of drive current.

The memory circuit according to the present invention is of the type that a plurality of memory cells are divided into a plurality of arrays each provided with a peripheral circuit, and is featured that the peripheral circuits associated to different arrays are driven at different timings.

According to this feature, simultaneous drive of a plurality of peripheral circuits can be avoided so that a peak value of the driving current caused through the memory operation can be remarkably reduced.

In the peripheral circuit, a word line drive circuit consumes the major part of the peak current, and hence it is very effective to drive at least the word line drive circuits associated to different arrays at different timings.

FIG. 1 is a schematic block diagram of a conventional memory circuit;

FIG. 2 is a circuit diagram of an example of a conventional block generating circuit;

FIG. 3 is a waveform diagram showing operation of the memory in Fig. 1;

FIG. 4 is a schematic block diagram of a major part of a memory according to a first embodiment of the invention;

FIG. 5 is a time chart explanatory of the operation of the embodiment shown in FIG. 4;

FIG. 6 is a schematic block diagram of a major part of a memory according to a second embodiment of the invention;

FIG. 7 is a schematic circuit diagram showing the timing signal switching circuit 16 of FIG. 6; and

FIG. 8 is a block diagram showing a third embodiment of the invention.

FIG. 1 is a schematic block diagram of a dynamic RAM which is known as a random access memory (referred to as "RAM", hereinunder) having a large capacity of 64 Kilo bits.

This RAM has two memory cell arrays 1 and 2 each composed of 128 word lines WL.sub.1, WL.sub.2 . . . arranged in the row direction, 256 bit line pairs (BL.sub.1, BL.sub.1) . . . arranged in the column direction, memory cells C.sub.s coupled between the word lines and bit lines, and 256 sense amplifier circuits 3. Each of the memory cell array 1 and 2 is provided with a word selecting circuit 4.

In this known RAM, refreshing of all memory cells C.sub.s is completed in 128 refreshing cycles. To this end, one word line is selected in each array per one access cycle such as to operate 512 sensing amplifier circuits. The word selecting circuits 4 provided for both arrays 1 and 2 are operated simultaneously in response to an activating signal .phi..sub.W which is generated by a timing generator 5 in response to an external clock RAS.

In such a case, the circuit 5 has to have a large capacity in order to attain high speed operation. A sense enable signal .phi..sub.S is the signal to enable the sense amplifiers 3 each composed of cross-coupled transistors Q.sub.A and Q.sub.B when the .phi..sub.S is at low in level. The signal .phi..sub.S is generated by a generator 6.

FIG. 2 shows a circuit diagram of an example of the clock generating circuit 5 for use in a memory which operates with a 5 V single power supply. This circuit 5 is well known and is composed of N-channel MIS transistors (referred to as "transistors", hereinunder) Q.sub.1 to Q.sub.10 and a boot capacitor C.sub.B. In this circuit, the transistors Q.sub.1 and Q.sub.2 have large driving capacity in order to attain a high speed generation of the signal .phi..sub.W. This in turn increases not only the current for charging the signal .phi..sub.W but also the current flowing through the transistors Q.sub.1 and Q.sub.2 within a period of 5 nS to 10 nS. The waveforms of RAS, .phi..sub.W and a current I.sub.M of .phi..sub.W are shown in FIG. 3. In case of a memory having a large capacity such as 64 kilo bit or 256 kilo bits, the load capacitance of the signal .phi..sub.W is on the order of several tens of pF and the sum of the charging current and the current through the transistors Q.sub.1 and Q.sub.2 becomes as large as 50 mA or greater. Thus, the changing rate of the peak current and the current during the operation of the activating signal .phi..sub.W takes a considerably large level of 10 mA/nS or greater.

When the apparatus is composed of a plurality of memories, a specific consideration has to be given in the design of the power source line and the ground line, in order to avoid any unfavourable effect from noise produced by the peak current. The design of the memory itself requires due consideration of way of avoiding the influence of the noise, particularly in the impedance design of wirings.

For instance, when a wiring in which the current is changed at a rate of 10 mA/nS has an impedance of 20 nH, a noise on the order of about 200 mV is inevitably generated. When the information transmitted from the memory cell to the bit line after the selection of the word is sensed and amplified by the bit line sensing amplifier circuit, the potential transmitted to the bit line is on the order of several hundreds of mV so that there is a risk of malfunctioning of the memory even by some fractions of the noise mentioned above. In addition, in order to avoid the influence of this noise, it is necessary to preserve a sufficiently long period of time between the selection of the word line and the sensing and amplification of the bit line.

Thus, the conventional dynamic memory suffers from a problem in that a large charging current and a large throughcurrent flow during the operation of the word line to cause a generation of noise due to a large peak current having a large changing rate, resulting in malfunctioning of the memory.

Referring to FIGS. 4 and 5, the memory circuit according to one embodiment is described.

As shown in FIG. 4, the memory circuit is basically composed of two memory cell arrays 1 and 2. Each of the arrays 1 and 2 includes memory cells C.sub.S coupled between word lines WL.sub.1, WL.sub.2 . . . and bit line pairs (BL.sub.1, BL.sub.1) . . . and sense amplifiers 3 provided for the respective bit line pairs. The word line driver circuit 4-1 is provided for the array 1 while the word line driver circuit 4-2 is provided for the array 2. The driver circuit 4-1 includes a plurality of driver units (NOR.sub.1 and Q.sub.41) each coupled to each one of word lines. Each of driver units is composed of a NOR gate NOR.sub.1 receiving address signals A.sub.D and a driving transistor Q.sub.41 having a drain receiving an activation signal .phi..sub.W, a gate coupled to the output of the NOR gate NOR.sub.1 and a source connected to the word line WL.sub.1. Other driver units connected to other word lines in the array 1 included in the driver circuit 4-1 have the same circuit structure as that for the word line WL.sub.1. The driver circuit 4-2 for the array 2 also includes a plurality of driver units each composed of a NOR gate NOR.sub.2 and a driver transistor Q.sub.42 operating in a source-follower circuit. A drain of the transistor Q.sub.42 receives another activation signal .phi.'.sub.W which is applied to the word line WL.sub.1 when the NOR gate NOR.sub.2 applies an ON signal to the gate of the transistor Q.sub.42. The clock generator 5 generates a word line activation signal .phi..sub.W which is directly applied to the driver circuit 4-1. While the signal .phi..sub.W is inputted to a delay circuit 15 from which a delayed activation signal .phi.'.sub.W is generated. The signal .phi.'.sub.W is used as an activation signal for the driver circuit 4-2. In other words, the activations of the driver circuits 4-1 and 4-2 are executed separately in response to .phi..sub.W and .phi.'.sub.W, respectively.

In operation, the driver circuit 4-1 corresponding to the memory cell array 1 is enabled first and, with a certain delay of time of the delay circuit 15, the driver circuit 4-2 corresponding to the array 2 is enabled.

The operation of this embodiment will be described hereinunder with reference to FIG. 5.

When the memory is activated by a change of the external signal RAS from "H" level to the "L" level, the activating signal .phi..sub.W for activating the driver circuit 4-1 is energized and, with a certain delay of time of the delay circuit 15, the activating signal .phi.'.sub.W for activating the driver circuit 4-2 is energized. Subsequently, the external signal RAS is changed from "L" level to "H" level so that the state of the memory is changed into precharge state. As a result, the driver circuit 4-1 is precharged and after a delay time provided by the delay circuit 15, the driver circuit 4-2 is precharged.

In consequence, the current I.sub.M flowing in the memory has two small peaks a and b during the activation and small peaks c and d during the precharge.

In the conventional memory shown in FIG. 1, both activating signals .phi..sub.W and .phi.'.sub.W charge and discharge simultaneously so that the peaks a and b, as well as the peaks c and d, are overlapped so that peaks of levels about two times as high as that exhibited by the embodiment are thus formed in the current waveform.

For the same reason, the changing rate of the current in the conventional memory is about two times as large as that in the embodiment.

Thus, the described embodiment of the dynamic memory in accordance with the invention allows a reduction of the level of peak current, as well as the changing rate of the current, during the operation of the word line selecting circuit down to about a half of that in the conventional memory. According to the invention, therefore, it is possible to obtain a dynamic memory which can operate stably without suffering from noise caused by the peak current.

FIG. 6 shows a second embodiment of the invention. In this embodiment, a switch circuit 16 for switching .phi..sub.W and .phi.'.sub.W is provided, as illustrated.

The switch circuit 16 receives the signals .phi..sub.W and .phi.'.sub.W and applies one of the signals .phi..sub.W and .phi.'.sub.W to the driver circuit 4-1 and the other of the .phi..sub.W and .phi.'.sub.W to the driver circuit 4-2 in response to an address signal .phi..sub.A which is used to designate which one of the arrays 1 and 2 is to be selected. Namely, when the array 1 is to be selected, the circuit 16 transmits the non-delayed signal .phi..sub.W to the driver circuit 4-1 and the delayed signal .phi.'.sub.W to the driver circuit 4-2. On the other hand, in the case where the array 2 is to be selected, the circuit 16 applys .phi..sub.W to the driver circuit 4-2 and .phi.'.sub.W to the driver circuit 4-1.

One example of the switch circuit 16 is shown in FIG. 7.

Thus, this second embodiment, therefore, can avoid any increase in the access time which may be experienced in the first embodiment due to the delayed signal .phi.'.sub.W . More specifically, an address information, particularly the row address information (.phi..sub.A) is used to determine which one of two memory cell arrays contains the memory cell to be selected in the preceding access cycle for the outputting or inputting to and from the external device. In this embodiment, making use of this information (.phi..sub.A), the driver circuit corresponding to the memory cell array having the memory cell to be accessed is operated first and then the driver circuit belonging to the other memory cell array is operated, so that any undesirable increase in the access time can be avoided advantageously. Needless to say, it is possible to make a design such as to determine the operation sequence appropriately even when the cycle is the refreshing cycles performed in response to an internal address signal.

In the second embodiment, after the completion of the access cycle, the precharge of the driver circuit which was operated first is precharged first, followed by the precharging of the other word line driver circuit. According to this arrangement, all the driver circuits can share an equal activating time, so that any increase in the required activated time due to the time division can be suppressed advantageously.

Namely, since the activating time for activating the memory is controlled by the external signal, the activating time for the driver circuit which was operated second is shortened as compared to that for the driver circuit which was operated first. In the second embodiment, this problem is avoided by the control of the timing of commencement of the precharging.

Referring to FIG. 7, a third embodiment of the present invention is explained. The memory includes two memory cell arrays 1 and 2, a first group of peripheral circuits including a word driver circuit 2-1, sense amplifiers 3-1, and a column selection circuit 7-1 provided for the array 1, a second group of peripheral circuits including a driver circuit 3-2, sense amplifiers 3-2 and a column selection circuit 7-2 provided for the array 2, and an input/output selection circuit 8. The driver circuits 4-1 and 4-2 receive row address signals ADR and are enabled to select one of the word lines in the arrays in response to activation signals .phi..sub.11 and .phi..sub.12, respectively. The column selection circuits 7-1 and 7-2 receive column address signals ADC and are enabled to select bit line pairs in each array in response to activation signals .phi..sub.C1 and .phi..sub.C2, respectively. The selection circuit 8 selectively couple an input/output terminal I/0 to one of the column selection circuits 7-1 and 7-2 in response to one bit address signal.

The signal generator 5, the delay circuit 15 and the selection circuit 16 are the same as those designated by the same reference numerals in FIGS. 6 and 7.

The outputs A and B of the switching circuits 16 are applied to the driver circuits 4-1 and 4-2 as the signals .phi..sub.11 and .phi..sub.12, respectively.

Timing signals generators 17-1 and 17-2 generate timing signals .phi..sub.S1 and .phi..sub.S2 for enabling the sense amplifiers 3-1 and 3-2 in response to the signals .phi..sub.11 and .phi..sub.12, respectively. Timing signal generators 18-1 and 18-2 generate timing signals .phi..sub.C1 and .phi..sub.C2 for enabling the column selection circuits 7-1 and 7-2 in response to the signals .phi..sub.S1 and .phi..sub.S2, respectively.

Namely, the timing signal generators 17-1 and 18-1 are provided for the array 1 while the generators 17-2 and 18-2 are provided for the array 2. Through the switch circuit 16, one of the signals .phi..sub.W and .phi.'.sub.W is applied to the generators 17-1 and 18-1 connected in cascade while the other of the signals .phi..sub.W and .phi.'.sub.W is applied to the generator 17-2 and 18-2 in response to the address signal .phi..sub.A .

In this embodiment, not only the activation signals for the driver circuits 4-1 and 4-2, but also the timing signals for other peripheral circuits such as sense amplifiers, column selection circuits are controlled dependent on which one of the arrays is to be selected.