Abstract:
In a memory comprising an upper word line and a lower word line for selecting memory cells connected therebetween, a delay circuit connected to the upper word line provides a first signal having a predetermined level when a voltage applied to the upper word line is between a selection voltage and a predetermined voltage, and a second signal, which is a delayed signal of the upper word line voltage signal, when the upper word line voltage changes from the predetermined voltage toward the non-selection voltage. The output of the delay circuit is used to control a switch circuit for discharging the lower word line therethrough.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a memory, and more particularly, to a bipolar memory. 
     In prior-art bipolar memories, each word line is constructed of a pair of word lines which consist of an upper word line and a lower word line, and memory cells are arranged at the points of intersection between each pair of word lines and pairs of data lines. The selection of the memory cell is executed by changing the voltage of the upper word line from a non-selection voltage to a selection voltage. In such a memory, in order to obtain a high speed for the memory selecting operation, it is necessary that after the word line is switched from select to non-select condition the word line voltage is changed at high speed from the selection voltage to the non-selection voltage. 
     FIG. 1 is a schematic circuit diagram of a memory which has previously been proposed to this end by the assignee of the present application in Japanese Published Unexamined patent application No. 53-41968 and in U.S. Pat. No. 4,156,941, which is herein incorporated by reference. 
     Shown in FIG. 1 are upper word lines L X0  and L X1 , lower word lines L ST0  and L ST1 , data lines D 00 , D 01 , D 10  and D 11 , memory cells C 0  -C 3  which are arranged at the points of intersection between the word and data lines, word line voltage-detecting circuits 20a and 20b which are respectively connected to the upper word lines L X0  and L X1 , delay circuits 21a and 21b for delaying output signals of these detector circuits, switch circuits 22a and 22b which are disposed in correspondence with the respective lower word lines L ST0  and L ST1  in order to supply currents thereto in response to the levels of output signals of these delay circuits, and constant-current sources 10a and 10b for supplying constant currents to the lower word lines L ST0  and L ST1 . 
     By way of example, when a pulse for selecting the upper word line L X0  is applied to a terminal X 0 , an emitter-follower transistor Q 201  detects the selection pulse. An emitter output of this transistor rises fast but its falling edge is delayed by the delay circuit 21a which is composed of transistors Q 202  and Q 203 , resistances R 201  -R 203 , voltage sources V EE , and capacitances C 201  -C 203 . This signal with the delayed falling edge is applied to the switch circuit 22a which is composed of a transistor Q 204  and a voltage source V EE  as is disposed for the corresponding lower word line. Thus, even after the selection pulse has been removed from the upper word line L X0 , current is caused to flow from the switch circuit 22a to the lower word line L ST0  for a predetermined period. It is characteristic of the proposed memory that the switch circuits are disposed in correspondence with the respective lower word lines and have the current sources respectively. 
     Voltage and current waveforms produced in the memory of FIG. 1 are illustrated in FIG. 2. FIG. 2(a) shows the voltage waveform of the selected upper word line, while FIG. 2(b) shows the waveform of the current (ΔI st ) flowing through the transistor Q 204 . As is apparent from the figure, the current ΔI st  starts flowing at a point t 1  at which the word line voltage V X  begins to rise, and it reaches its maximum current value at a time t 2  at which the voltage V X  arrives at a high level. Conversely, when the voltage V X  falls, the current ΔI st  starts falling at a time t 3  at which the fall of the voltage V X  is initiated. The current ΔI st  has its fall delayed by the delay circuit 21a or 21b and becomes zero at a time t 5 . That is, the large current ΔI st  continues to flow at a time t 4  at which the voltage V X  falls perfectly. As a result, the discharge of charges stored in stray capacitances C S1  and C S2  attendant upon the upper word line L X0  and the lower word line L ST0  respectively is effected at high speed, and the fall rates of the voltages of the word lines L X0  and L ST0  become high. In consequence, the access time and cycle time of the memory operation can be shortened as compared with prior memory circuits which lack the circuits 20a, 21a and 22a. 
     In addition to achieving a rapid fall, the circuit of FIG. 1 also decreases the influence of double selection occurring during the transition of the change-over of address signals as discussed below. 
     A large number of memory cells are usually arranged in the form of a matrix on a memory LSI chip, and in order to select desired cells from among them, a plurality of address signals are applied. The change-over of the address signals is effected by switching the levels of some of the plurality of address signals. Ideally, the switching of the levels should be simultaneous for all the address signals. Actually, however, some deviations are ordinarily involved in the timings at which the levels of the respective address signals applied to an address signal input pin are switched. This can be caused, for example, by unequal lengths of printed interconnections from gates for driving the address signals to the address signal input pin, and other circuit pecularities. Hereinbelow, this type of deviation shall be termed &#34;address skew&#34;. 
     FIG. 3(a) shows an example of the switching of the address signals applied to the address signal input pin of FIG. 1. In the absence of any address skew, the respective address signals change as indicated by the solid lines. That is, at the time when the address signal a 1  switches from the high level to the low level, the other address signals, e.g., signals a 2  and a 3 , switch from the low level to the high level. Accordingly, the change-over of the levels of the address signals occurs simultaneously. At this time, the voltage of the upper word line which shifts from the selected state into the non-selected state switches from the high level to the low level as illustrated by a waveform b 1  in FIG. 3(b), whereas the voltage of the upper word line which shifts from the non-selected state into the selected state switches as illustrated by a waveform b 2 . The levels of both the waveforms b 1  and b 2  switch without a time lag relative to each other. All the voltages of the other upper word lines remain at the non-selection level. 
     In the presence of an address skew, however, the situation becomes different. By way of example, as illustrated by a broken line in FIG. 3(a), it is presumed that the address skew is involved in the address signal a 3 , so the timing of the level switching of the signal a 3  lags over the timings of the level switchings of the other address signals a 1  and a 2 . In this case, during the period after the levels of the signals a 1  and a 2  have switched and before the level of the signal a 3  switches, the upper word line which is determined by the condition that the signals a 1 , a 2  and a 3  are at the low, high and low levels respectively is transiently selected. When the level of the signal a 3  has thereafter switched, the desired upper word line is selected. Accordingly, the voltage waveforms of the upper word lines in this case become as shown in FIG. 3(c). 
     Referring to these voltage waveforms shown in FIG. 3(c), in correspondence with the switchings of the levels of the signals a 1  and a 2 , the voltage b 1  of the upper word line having been previously selected begins to fall, and the voltage b 3  of the different upper word line begins to rise transiently. However, this upper word line is selected only transiently, and its voltage b 3  rises only slightly and thereafter begins to fall. At this time after a 3  begins to rise, the actually desired upper word line begins to be selected, and its voltage b 2  begins to rise. 
     In the circuit of FIG. 1, in response to the fact that the voltage b 3  of the upper word line transiently selected has become greater than the non-selection voltage, the switch circuit connected to that upper word line causes the current ΔI st  to flow slightly to the corresponding lower word line. This raises the speed at which the voltage b 3  of the upper word line which was incorrectly transiently selected falls to the non-selection level. As a result, the voltage of the upper word line transiently selected can be returned to the non-selection level more quicky than in the absence of the switch circuits 22a and 22b. Thus, the circuit of FIG. 1 lessens the likelihood of the destruction of information. 
     However, for rendering the speed of the operation of the memory still higher, it is desirable that the voltage b 3  falls more rapidly as indicated by a voltage b 4  in FIG. 3(c). As illustrated in FIGS. 2(a) and 2(b), unless the voltage of the upper word line rises sufficiently, the current ΔI st  owing to the switch circuit 22a or 22b flows only slightly, and hence, the voltage b 3  cannot be lowered as rapidly as the voltage b 4 . In a memory of very high operating speed, accordingly, information destruction sometimes takes place because two upper word lines have been simultaneously selected transiently. In FIG. 3, the case where an address skew is involved in only one address signal has been referred to. When two or more address signals involve respectively different skews, double or further multiple selection occurs and the situation worsens. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a memory which relieves the influence of the double selection due to address skews in the high-speed operating state. 
     To accomplish this object, the present invention consists in improving the delay circuit of the prior art by providing a delay circuit which generates a signal for controlling a switch circuit so that it may cause a sufficiently great current to flow to a lower word line even when an upper word line voltage has not reached a selection voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a prior-art bipolar memory; 
     FIGS. 2(a) and 2(b) are time charts of signals for explaining the operation of the circuit in FIG. 1; 
     FIGS. 3(a), 3(b) and 3(c) are diagrams illustrative of the changes of word line voltages for explaining a problem of the circuit in FIG. 1; 
     FIG. 4 is a circuit diagram of a memory according to the present invention; and 
     FIGS. 5(a) through 5(d) are time charts of signals for explaining the operation of the circuit in FIG. 4. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of this invention shown in FIG. 4, wherein like numerals designate like elements to FIG. 1, differs from the circuit of FIG. 1 in the point that a diode D 0  which has a voltage source V CL  connected to its cathode is connected to the base of the transistor Q 203 , and the point that the value of a resistance R 204  &#39; is selected to be smaller than the value of the resistance R 204  in the circuit of FIG. 1. The diode D 0  serves to clamp the base voltage of the transistor Q 203  to the supply voltage V CL . 
     By way of example, as shown in FIG. 5(a), let it be supposed that in a case where the upper word line L X0  is selected, the voltage of the word line L X0  begins to rise from the non-selection voltage towards the selection voltage at a time t 1  until it reaches the selection voltage at a time t 2 . This voltage then falls over from a time t 3  to a time t 4 . The supply of the voltage to the word line L X0  is executed by a known driver (not shown). The driver delivers the voltage pulse switching from the non-selection voltage to the selection voltage as stated above, to the upper word line to-be-selected in response to an address signal. 
     The emitter-follower transistor Q 201  detects this change of the voltage of the word line, and its emitter voltage changes with the same waveform as the waveform of FIG. 5(a). This voltage change produces a voltage change as shown in FIG. 5(b) in the base of the transistor Q 203  under the action of the resistance R 201   as well as the constant-current circuit which is composed of the transistor Q 202 , the resistance R 202  and the voltage source V EE . When the voltage of the word line L X0  rises from the non-selection voltage towards the selection voltage, the base voltage of the transistor Q 203  also rises. 
     In this case, in order to prevent the base voltage of the transistor Q 203  from rising in a rise time longer than that of the voltage of the word line L X0  under the action of the stray capacitance C 201  connected to the collector of the transistor Q 202  and to permit it to rise at substantially the same rise time as that of the voltage of the word line L X0 , the speeding-up capacitance C 202  is disposed in parallel with the resistance R 201 . 
     However, after the voltage of the word line L X0  has reached a predetermined voltage intermediate between the selection voltage and the non-selection voltage at a time t 2  &#39; and the base voltage of the transistor Q 203  has become greater than a supply voltage (V CL  +V F ) (where V F  denotes the forward voltage drop of the diode D 0 ), this base voltage of the transistor Q 203  is clamped to the voltage (V CF  +V F ) under the action of the diode D 0  even when the voltage of the word line L X0  rises more. 
     The emitter voltage of the emitter-follower transistor Q 203  varies as shown in FIG. 5(c) in response to the change of the base voltage thereof. That is, the emitter voltage holds a fixed value after the time t 2  &#39; at which the base voltage having risen in response to the rise of the voltage of the word line L X0  reaches the supply voltage (V CL  +V F ). 
     Depending upon this emitter voltage of the transistor Q 203 , the switch circuit 22a causes a current as shown in FIG. 5(d) to the lower word line L ST0 . The switch circuit 22a has the value of the resistance R 204&#39;  selected so that the allowable maximum current of the circuit may flow when the emitter voltage of the transistor Q 203  has reached the fixed value at the time t 2  &#39;. Thereafter, even when the voltage of the word line L X0  rises more, the switch circuit 22a continues to cause the maximum current to flow because the base voltage of the transistor Q 203  is clamped. 
     Subsequently, as the voltage of the word line L X0  begins to fall at the time t 3 , the emitter voltage of the transistor Q 201  also begins to fall. However, the base voltage of the transistor Q 203  is kept clamped to the supply voltage (V CL  +V F ) by the diode D 0 , and the aforecited maximum current continues to flow from the switch circuit 22a. 
     When a time t 3  &#39; is reached, the base voltage of the transistor Q 203  determined from the emitter voltage of the transistor Q 201  becomes smaller than the supply voltage (V CL  +V F ), and the diode D 0  releases the clamping action. As a result, the base voltage of the transistor Q 203  begins to fall at the time t 3  &#39;. At this time, the emitter voltage of the transistor Q 203  has its fall rate retarded under the action of the stray capacitance C 203  connected to the emitter thereof. In consequence, even after the voltages of the upper word line L X0  and the base of the transistor Q 203  have perfectly fallen to the non-selection levels at the time t 4 , the emitter voltage of the transistor Q 203  continues to fall, and this voltage completes its fall at a time t 5  &#39;. In response to the fall of the emitter voltage of the transistor Q 203 , the current of the switch circuit 22a similarly begins to fall at the time t 3  &#39; and completes the fall at the time t 5  &#39;. 
     The circuits 20b, 21b and 22b connected to the other pair of word lines L X1  and L ST1  have the same arrangements and perform the same operations as the circuits 20a, 21a and 22a connected to the pair of word lines L X0  and L ST0 , respectively. 
     The delay circuits 21a and 21b and the switch circuits 22a and 22b are constructed as described above, whereby the double selection of the word lines ascribable to an address skew is prevented. As illustrated by the waveform b 3  in FIG. 3(c), in a case where a certain upper word line, e.g., L X0  has been transiently selected due to the address skew, the voltage of this word line L X0  rises from the non-selection voltage and thereafter falls. According to this invention, the switch circuit 21a causes the maximum allowable current to flow before the voltage of the word line L X0  reaches the selection voltage. Therefore, even when the voltage of the word line L X0  transiently selected is comparatively small, a current greater than that in the prior art flows from the switch circuit 22a in the delayed fashion. It is accordingly possible to significantly increase the speed at which the voltage of the transiently selected word line falls to the non-selection voltage. 
     In order to permit the allowable maximum current to flow from the switch circuit before the word line voltage reaches the selection level as described above, the curent ΔI st  is caused to flow to the memory cell connected to the selected word line immediately after the initiation of the selection of the word line. Accordingly, this invention also has the beneficial result that the information of the memory cell connected to the word line which shifts from the non-selected state into the selected state is more difficult to destroy. 
     This invention is not restricted to the foregoing embodiment, but also covers other circuit arrangements such as those which are disclosed in the specification of Japanese Published Unexamined patent application No. 53-41968 and U.S. Pat. No. 4,156,941. For example, although the word line voltage-detecting circuit 20a is shown as being connected to the upper word line and is constructed so as to detecct the change of voltage of the particular word line, it can be replaced with a word line voltage-detecting circuit which is connected to the driver (not shown) for applying the voltage to the upper word line. Such an arrangement could be constructed so as to detect a voltage change within the driver and to substitute this detection for the detection of the change of the word line voltage. Also, the voltage-detecting circuit 20a may be omitted in FIG. 4. In this case, the resistance R 201  and the capacitors C 202  would be connected directly to the word line L X0 . 
     It is to be understood that the above-described arrangements are simply illustrative of the application of the principles of this invention. Numerous other arrangements may be readily devised by those skilled in the art which embody the principles of the invention and fall within its spirit and scope.