Abstract:
In a dynamic random access memory including means for inputting a row strobe signal and means for inputting a column strobe signal, and having the functions of incorporating a row address input in response to the row strobe signal and incorporating a column address input in response to the column strobe signal, an improvement comprising a first insulated gate field effect transistor having its drain coupled to a first junction point, its gate receiving the column strobe signal and its source supplied with a first power supply, means for charging the first junction point within a period of active level of the row strobe signal, a second insulated gate field effect transistor having its drain coupled to the first junction point, its gate coupled to a second junction point and its source supplied with the first power supply terminal, means for precharging the second junction point within a period of absence of the row strobe signal, and a third insulated gate field effect transistor having its drain coupled to the second junction point, its gate supplied with the column strobe signal and its source supplied with the first power supply. After completion of a desired operation with respect to a selected memory cell, the row strobe signal is placed at its inactive level to be reset while the column strobe signal is at its active level. Then, after a necessary reset period when the row strobe signal is made active, a change from inactive level to active level of the first junction point is suppressed to thereby inhibit active operation based on the column strobe signal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a memory circuit constituted of semiconductor elements, and more particularly to a memory circuit employing insulated gate field effect transistors (IGFET&#39;s) fabricated on a semiconductor chip. 
     In dynamic random access memories employing IGFETs (hereinafter abbreviated as dynamic RAM) a 2-clock multi-address system has been employed in a 16 K-bit memory as well as in a 64 K-bit memory both with a 16-pin package structure. The details of this multi-address system are disclosed in the specification and drawings of U.S. Pat. No. 3,969,706 issued to Robert J. Proebsting et al on July 13, 1976. Basically, two input clocks, that is, a row strobe signal defined as ROW ADDRESS STROBE (abbreviated as RAS) and a column strobe signal defined as COLUMN ADDRESS STROBE (abbreviated as CAS) are generated in a predetermined sequence. In the following description, an example is adopted where N-channel IGFET&#39;s are used and RAS and CAS are assumed to be active when they are low signals. At first, when RAS becomes active by turning it from a high level to a low level, input levels at address terminals at that time point are incorporated in the memory and held therein as row address inputs which are used to select a word line and refresh memory cells connected to the selected word line. After completion of latching of address inputs in response to RAS, CAS becomes active by being turned from a high level to a low level to thereby incorporate the input levels at the address terminals at this time in the memory as column address inputs. Depending upon a level of a write control signal defined as WRITE ENABLE (abbreviated as WE), if WE is not at its active level, i.e., it remains high, then data read-out from the selected memory cell is effected so that the data appear at a data output terminal. If WE is at its active level, on the other hand, then the data at a data input terminal at the time point is written in the selected memory cell. 
     Recently a refresh operation called &#34;HIDDEN REFRESH&#34; has been employed in the multi-address system. This refresh operation is as follows: 
     After read-out data from a selected memory cell has been obtained by sequentially making RAS and CAS active the refresh operations for the memory cells on the different word lines can be sequentially effected while maintaing the read-out data at the data output terminal by repeatedly making RAS active and maintaining CAS at the low level. In other words, during the period when the read-out data is effectively revealed, the refresh operation can be effected simultaneously, and this operation is thus called HIDDEN REFRESH. However, HIDDEN REFRESH must be performed under the condition that WE is maintained at the high level i.e. inactive level, and hence this mode of operation lacks flexibility in control. 
     SUMMARY OF THE INVENTION 
     It is one object of the present invention to provide a memory circuit having such flexibility that HIDDEN REFRESH may be controlled only by RAS and CAS irrespective of any other inputs such as WE. 
     Another object of the present invention is to provide a practical and useful circuit structure for realizing the aforementioned feature. 
     According to one aspect of the present invention, there is provided a memory circuit characterized by the following function: 
     After necessary operations for a selected memory cell have been completed by sequentially making RAS and CAS active, RAS is made inactive while CAS is kept active. Then, after a necessary inactive period of RAS, when RAS is again made active, all the the active operations based on active level of CAS are inhibited, whereby the state at the time point when the necessary operations were completed can be in itself maintained at the data output terminal. 
     According to another aspect of the present invention, there is provided a semiconductor integrated circuit for achieving the above-featured memory circuit, which comprises a load element having one end coupled to a first power supply terminal and the other end coupled to a first junction point, a first IGFET having its drain coupled to the first junction point, its gate receiving CAS and its source coupled to a second power supply terminal, a second IGFET having its drain coupled to the first junction point, its gate coupled to a second junction point and its source coupled to the second power supply terminal, a third IGFET having its drain coupled to the first power supply terminal, its gate receiving a first clock which takes its active level in response to RAS input going into an inactive period and then inactive within the same inactive period, and its source coupled to the second junction point, and a fourth IGFET having its drain coupled to the second junction point, gate receiving CAS and its source coupled to the second power supply terminal. 
     The above-mentioned and other features and objects of the present invention will become more apparent by reference to the following description of its preferred embodiments taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a timing diagram showing a read cycle and a write cycle of a RAM according to the 2-clock system; 
     FIG. 2 is a timing diagram representing a known page mode cycle; 
     FIG. 3 is a timing diagram showing known RAS ONLY REFRESH mode; 
     FIG. 4 is a timing diagram showing known HIDDEN REFRESH mode; 
     FIG. 5 is a block diagram of a RAM according to the 2-clock system in the prior art; 
     FIG. 6 is a schematic circuit diagram showing a general timing generator circuit of FIG. 5; 
     FIG. 7 is a timing waveform diagram showing timing signals appearing in FIG. 5; 
     FIG. 8 is a circuit diagram of an output control circuit in the circuit shown in FIG. 6; 
     FIG. 9 is a timing waveform diagram for the circuit according to the present invention; 
     FIG. 10 is a timing diagram showing an input-output timing relationship in HIDDEN REFRESH in the circuit system according to the present invention; 
     FIG. 11 is a circuit diagram of a basic circuit according to the present invention; 
     FIG. 12 is an operation waveform diagram for the circuit shown in FIG. 11; 
     FIG. 13 is a diagram for explaining the fact that the circuit according to the present invention is effective for any combination of timings of RAS and CAS; 
     FIG. 14 is a circuit diagram showing a practical circuit embodying the present invention; and 
     FIG. 15 is an operation waveform diagram for the circuit shown in FIG. 14. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following description, MOS transistors (hereinafter abbreviated as MOST) which are representative ones among IGFETs are employed, and it is further assumed that the MOSTs used herein are N-channel ones wherein a higher level is a logic-l level, and a lower level is a logic-O level. However, this invention is not limited to the use of MOSTs and N-channel IGFETs. 
     The operations of the known 2-clock multi-address system will be described in greater detail with reference to FIGS. 1 to 4. 
     Timing relations between RAS, CAS, WE, data input and address input in a read cycle and in a write cycle are illustrated in FIG. 1(A) and FIG. 1(B) together with a general waveform of the data output. In the read cycle of FIG. 1(A), after a necessary access time has elapsed from a change of CAS to a low level, the effective data appear at the data output and are maintained until CAS is changed to a high level to be reset. While in the write cycle of FIG. 1(B), according to a general output mode the data output terminal is kept at a high impedance state (Hi-Z) and there appear no data. 
     A so-called &#34;page mode&#34; will now be described with reference to FIG. 2. In this mode, RAS and CAS are changed to a low level to be enabled or activated in sequence, and the reset state and enabled state of CAS are repeated while maintaining RAS at a low level. At a result, with respect to the memory cells on the same word line selected by the operation based on RAS, write-in or read-out can be achieved in the memory cell at the column address designated by the address input in response to CAS only. In other words, if the operation is limited to memory cells on a single word line, it is possible to shorten the cycle time and the access time, and this operation is called &#34;page mode&#34;. 
     Next the operation of &#34;RAS only refresh (RAS ONLY REFRESH)&#34; will be explained with reference to FIG. 3. RAS are repeatedly made a low level and a high level while maintaining CAS at a high level, i.e., at its reset state. Then naturally only the circuit portion enabled by RAS operates, and the contents of the memory cells on the word line corresponding to the row address designated by the address input are refreshed. This operation is called &#34;RAS ONLY REFRESH.&#34; 
     In addition to the normal read cycle and write cycle, the page mode operation and the RAS ONLY REFRESH operation form the characteristic operation of the RAM of the 2-clock multi-address system, and still further, an operation as illustrated in FIG. 4 has been employed recently. 
     After read-out data from a selected memory cell has been obtained by sequentially making RAS and CAS low, if RAS are repeatedly made high and low while maintaining CAS at the low level, i.e., at its active or enabled state, then while the read-out data are maintained at the data output terminal, the contents of the memory cells on the word line designated by the address input can be refreshed. In other words, during the period when the read-out data is effectively revealed refresh can be effected simultaneously, and this operation is called &#34;HIDDEN REFRESH&#34;. Heretofore, a restriction was imposed in that as shown in FIG. 4, in the HIDDEN REFRESH cycle the input WE must be maintained at the high level i.e. its inactive level, and hence this mode of operation lacks versatility as compaed to the fact that in the RAS ONLY REFRESH shown in FIG. 3 no restriction is imposed upon the input WE. 
     Now, before making a more detailed explanation of the present invention in connection with its preferred embodiment, we will first explain a general circuit system of a RAM according to the 2-clock multi-address system in the prior art with reference to FIG. 5. The RAM shown in FIG. 5 comprises a row address inverter 50, row decoders 51 and 52, a column address inverter 53, memory cell arrays 54 and 57, column decoders 55 and 56, a sense amplifier 55&#39;, an output amplifier 58, a latch control circuit 59 and a control timing generator circuit 60 responsive to RAS and CAS. 
     The structure of the control timing generator circuit 60 is disclosed in FIG. 6. The respective timing signal waveforms generated via inverters I 1  to I 4  and buffers B 1  to B 9  in FIG. 6 are shown in FIG. 7. When RAS shits from the high level to the low level and thus goes into an active period, a level-inverted output RAS derived from the inverter I 1  rises at first. In response to the rise of RAS, precharge timing signals XP 0 , XP 1  and XP 2  are successively changed to the low level. A timing signal AERAS also rises, so that the output of the row address inverter 50 can respond to the address input. After a selection/non-selection operation in the row decoders 51 and 52 has been completed, a timing signal RA rises, and hence the selected word line rises in level. In response to the signal RA, a timing signal SE rises, so that the sense amplifier 55&#39; is enabled. Thus, the contents in the memory cells 54 and 57 on the selected word line are refreshed. The circuit operations in response to RAS have now been completed. 
     On the other hand, a gate of MOST Q 52  forming the load of the inverter I 3  to which CAS is applied, is charged in response to AERAS derived from RAS. If this charging has been completed and also CAS shifts from the high level to the low level to achieve its active state, then a level-inverted output CAS rises. In response to CAS, precharging timing signals YP 0  and YP 1  fall in level and a timing signal AECAS rises, so that the output of the column address inverter 53 responds to the address input. After completion of a selection/non-selection operation of the column decoder 56, a timing signal RE rises and hence the digit line of the selected column is connected to a data input/output line. In response to the timing signal RE, a timing signal DE rises in level, so that the output amplifier 58 is enabled, and hence the information in the selected memory cell that has appeared on the data input/output lines DB and DB is amplified. Thus, the necessary data are provided at a data output terminal DATA OUT. This completes the circuit operations in response to CAS. 
     Next, if only RAS is at the high level while maintaining CAS at low level, then at first RAS falls, and subsequently XP 0 , XP 1  and XP 2  rise in level to effect reset-precharge operations in the RAS system, while AERAS, RA and SE at the low level. At this moment, in response to the rise of XP 0 , CAS falls and subsequently YP 0  and YP 1  rise to effect reset-precharge operations in the CAS system while AECAS, RE and DE are at the low level. In other words, even if RAS is at the high level (reset state) while maintaining CAS are at the low level (active state), both the RAS system and the CAS system are subjected to reset-precharge. Then the latch control circuit 59 shown in FIG. 5 acts upon the output terminal DATA OUT, so that the read-out data are held at the output terminal DATA OUT. This latch function is provided, for example, by the output control circuit illustrated in FIG. 8. 
     Referring now to FIG. 8, when the CAS system is reset, MOST Q 81  becomes a non-conducting and MOST Q 82  becomes conducting, so that a junction point N 13  shifts to ground potential and hence MOSTs Q 83  and Q 84  become non-conducting. Junction points N 11  and N 12  remain at a read-out information level, which is dynamically maintained by their capacitance, and junction points N 14  and N 15  are reset to the ground potential on the side of the output amplifier 58. MOSTs Q 85  to Q 90  form a control circuit for maintaining the output terminal DATA OUT at a high impedance when CAS is at the high level. In this case, CAS is held at the low level and MOST Q 86  is non-conducting. Hence a junction point N 16  stores an electric charge passed through MOST Q 85  in response to the high level of RE, and thus the junction point N 16  is held at the high level. MOST Q 88  is chosen to have a sufficiently large current capacity with respect to MOST Q 87  so that a junction point N 17  may be held at the low level, and MOST Q 89  and MOST Q 90  are non-conducting, resulting in no influence upon the levels at the junction points N 11  and N 12 . By the above description, it has been explained that the read-out data can be maintained at the output terminal DATA OUT. Now, when RAS shifts to the low or active level while CAS is kept at the active level, then RAS, AERAS, RA and SE rise in succession. On the other hand when the charging of the load MOST Q 52  in the inverter I 3  to which CAS is applied in response to AERAS has been completed, CAS rises and subsequently AECAS and RE rise in succession, and hence the junction point N 13  in FIG. 8 rises, resulting in conducting of MOSTs Q 83  and Q 84 . Thus, the junction points N 11  and N 14  and the junction points N 12  and N 15  are respectively connected to each other and both shift to ground potential. Accordingly, MOST Q 61  and MOST Q 62  become non-conducting so that the output terminal DATA OUT is held at a high impedance. When DE rises subsequently to the rise of RE, the output amplifier circuit 58 is activated, and hence an information in a newly selected memory cell appears at the output terminal DATA OUT. However, since the address input upon this activation is meaningless, the revealed output data cannot be used effectively. Moreover, if the WE input is lowered to its active level at this moment, the meaningless input data will be written in the memory, and therefore, the activation of WE must be inhibited. Thus, the operation is such that while the read-out data appearing in response to the active level of both RAS and CAS is maintained at the output terminal, refresh is effected by repeating the active level of RAS as shown in FIG. 4, and this operation is called HIDDEN REFRESH. However, in the presently known HIDDEN REFRESH, it is required to maintain the WE at its inactive level, resulting in the lack of versatility of timing signals. 
     While the circuit system according to the present invention is similar to the prior art circuit system in that where RAS is in its reset state while maintaining CAS at its active state, both the RAS system and the CAS system are reset and precharged, the present invention is characterized in that where RAS is subsequently put at its active level while CAS is kept at its inactive level, the RAS system circuit can operate but the CAS system circuit is kept reset and does not operate. More particularly, although the timing signal CAS derived by inverting the level of CAS can be reset when RAS is reset while maintaining CAS active, upon subsequent activation of RAS the rise of the timing signal CAS is suppressed, so that the CAS system circuit does not change from the reset state at this time, and all the activation operations therein are inhibited. In this case, the digit line would not be connected to the data input/output, and moreover, since the WE series timing is normally driven by the CAS system circuit, the WE input is not involved in the circuit operation caused by the activation of RAS at all. The waveforms of the respective timing signals according to the present invention are illustrated in FIG. 9 in a similar fashion to FIG. 7. The timing signals RAS, AERAS, RA, SE, SP 0 , XP 1 , XP 2  generated in the RAS system timing generator circuit are the same as those in FIG. 7, and the RAS system timing generator circuit (see FIG. 11) is also identical to that shown in FIG. 6. However, the CAS system timing signals are modified according to the present invention, and they are represented by reference letters CAS&#39;, AECAS&#39;, RE&#39;, DE&#39;, YD 0  &#39; and YP 1  &#39;. The CAS system timing generator circuit is accordingly modified as shown in FIG. 11. Since the restriction imposed on the WE input in the prior art system shown in FIG. 4 is eliminated, HIDDEN REFRESH in the circuit system according to the present invention is effected irrespective of WE as shown in FIG. 10. Furthermore, the circuit system according to the present invention involves an additional advantage that since the activation operations in the CAS system circuit are entirely suppressed, the power consumption upon HIDDEN REFRESH can be made as small as that upon RAS ONLY REFRESH illustrated in FIG. 3. 
     A description will now be made of a practical circuit for realizing the above-described circuit system according to the present invention, with references to FIG. 11 which shows a basic circuit construction of the timing generator circuit and FIG. 12 which illustrates waveforms at principal junction points. As described previously, the activation timing signal CAS&#39; derived by level-inversion of the CAS input is normally generated by an inverter I 3  &#39; including MOSTs Q 71 , Q 72  and Q 73 . When RAS is turned to the active (low) level and AERAS rises, a junction point 71 is charged through MOST Q 71 . After a sufficient period of time, CAS is turned to the active (low) level to turn MOST Q 73  to non-conducting, and CAS&#39; begins to rise through MOST Q 72 , which is driven to an unsaturated region owing to the existence of a boot-strap capacitor C1 F, and hence CAS&#39; reaches the V DD  level. When the activation period of RAS terminates and it goes into a reset period, the clock XP 00  for MOST Q 74  rises immediately, and after a junction point 73 has been sufficiently charged through MOST Q 74  it shifts to the low level within the reset period of RAS. Due to the rise of the junction point 73, MOST Q 76  becomes conducting and CAS&#39; shifts to the ground potential. Assuming that CAS is maintained at the low level, MOST Q 75  is non-conducting, so that the junction point 73 is held at the high level, and after XP 00  becomes low, the junction point 73 is maintained at the high level by a capacitance at the junction point 73. Subsequently, when RAS is made at its active (low) level again while CAS is kept at the low level, the timing signal AERAS rises and thereby the junction point 71 is charged. However, if the current capacity of MOST Q 76  is chosen sufficiently large with respect to that of MOST Q 72 , then CAS&#39; cannot rise because the junction point 73 is kept at the high level. In other words, the requirement that the activation operations in the CAS system circuit should be all suppressed, can be fulfilled. In place of the timing XP 00 , XP 0  may similarly be used. Under the timing condition of RAS and CAS in the HIDDEN REFRESH mode, the circuit of MOSTs Q 74  to Q 76  can realize the timing condition in FIG. 9 and the operation mode illustrated in FIG. 10, and it can be proved by reference to FIG. 13 that this circuit does not affect any other timing condition of CAS during the period from the reset of RAS to the next activation of same. Other timing conditions of CAS are limited only to the case a where CAS is reset, and in this case normally rated limitation to the minimum value is imposed upon time periods t 1  and t 2  shown in FIG. 13. As will be seen from this, the period when CAS is held at the high level after the shift of XP 00  to the low level, can be assured at least for the time t 2 . In the circuit shown in FIG. 11, after XP 00  has shifted to the low level, MOST Q 74  is non-conducting, and the junction point 73 is maintained at the high level in a dynamic fashion. But when CAS shifts to the high level and MOST Q 75  becomes conducting, the junction point 73 is immediately discharged and shifts to ground potential. When MOST Q 76  becomes non-conducting as a result of the above, the circuit can respond to the next activation of the CAS series circuit without affecting CAS. The time t 2  necessary for discharging of the junction point 73 is normally assured. 
     While the operation of the basic timing circuit according to the present invention illustrated in FIG. 11 has been described above, now a practical circuit structure of one preferred embodiment of the present invention will be explained with reference to a circuit diagram shown in FIG. 14 in which major parts of the control timing generator circuits are shown, and to a waveform diagram showing operations waveforms at principal junction points in the circuit as illustrated in FIG. 15. In the circuit shown in FIG. 14, the circuit according to the present invention is applied to the inverter I 11  to which CAS is an input, and a necessary RAS system timing signal generator circuit is added to the circuit. The inverter I 3  &#34; corresponds to the inverter I 3  &#39; of FIG. 11, and other circuit blocks denoted as I 1 , I 5  and B 1  correspond to ones denoted by the references in FIG. 11. The output circuit has a construction as shown in FIG. 8 such that in the read cycle, after completion of an access operation, effective data can be maintained at the output terminal so long as CAS is not reset. Starting from the state where both RAS and CAS are reset, if RAS is first put at the active level, then RAS rises up to the V DD  level via MOST Q 2  owing to the boot-strap capacitor CIF. In response to RAS, a junction point 3 rises up to the level of (V DD  -threshold voltage), and so, by selecting a current capacity of MOST Q 8  sufficiently large with respect to that of MOST Q 7 , XP 0  is shifted from the V DD  level to the low level. On the other hand, in response to RAS, a timing signal AERAS rises up to the V DD  level, and thereafter activation operations in the RAS series circuit are carried out. In response to the rise of AERAS, a junction point 15 is raised via MOST Q 26  up to the level of (V DD  -threshold voltage), and after waiting for the later one of CAS being activated and a junction point 18, which is an output of a delay circuit responsive to AERAS, shifting to ground potential, that is after MOSTs Q 73  and Q 54  both have become non-conducting, CAS rises. Then CAS reaches the V DD  level via MOST Q 72  owing to a boot-strap capacitor C1 F. In response to CAS, activation operations in the CAS system circuit are effected, and assuming that the cycle under consideration is a read cycle, effective data appear at the output terminal. Under such a condition, when RAS is reset while maintaining CAS activated, RAS shifts to the low level and subsequently the junction point 3 shifts to ground potantial. MOST Q 8  becomes non-conducting, hence XP 0  rises via MOST Q 7  and reaches the V DD  level owing to a boot-strap capacitor C4F. In response to the rise of XP 0 , XP 00  rises via MOST Q 18 , showing a waveform synchronized with XP 0 , and also reaches the V DD  level owing to a boot-strap capacitor C10F. In response to XP 00 , a circuit consisting of MOSTs Q 20  to Q 25  operates, and as a result, when a junction point 14 rises, MOSTs Q 17  and Q 19  becomes conducting, resulting in a shift of XP 00  to ground potential. In other words XP.sub. 00 is held at the high level equal to V DD  during the operation period of the circuitry of MOSTs Q 20  to Q 25 . During this period, a junction point 19 is charged via MOST Q 35  up to the level of (V DD  -threshold voltage). The voltage at the junction point 19 is indicated as V 19  in FIG. 15. If only MOST Q 74  were to be used, after XP 00  has shifted to the ground potential the junction point 19 would take the high level theat is dynamically maintained, and if the period of this condition is long, there is a risk that the level may be attenuated. Therefore, as a back-up, MOST Q 37  having a small current capacity and having XP 0  applied to its gate, is added between the V DD  power source and the junction point 19. Since CAS is not reset, effective data are in themselves maintained at the output terminal. Thereafter, if RAS is made active level while also keeping CAS at the active level, then activation operations in the RAS system circuit are effected in a similar manner to the above, and thereby the contents in the memory cells on the word line corresponding to the designated row address at the address input can be refreshed. In responce to the rise of AERAS, a junction point 15 rises up to the level of (V DD  -threshold voltage). However, since the junction point 19 is also maintained at the level of (V DD  -threshold voltage) and since a current capacity of MOST Q 76  is sufficiently larger than that of MOST Q 72 , CAS&#39; cannot rise but is held at the low level. Accordingly, the activation operations in the CAS system circuit are all inhibited, and only refresh operations in response to the RAS input can be effected. So long as the CAS input is held at the low level, effective data can be maintained in themselves at the output terminal, and even if reset and activation of the RAS input are repeated, the circuit other than the RAS system circuit is held in a reset condition without operating at all, and thus refresh is effected in succession. As a result, HIDDEN REFRESH illustrated in FIG. 10 can be realized. 
     As described above, according to the present invention there is provided a system characterized by the function that after necessary operations for a selected memory cell have been completed by sequentially making RAS and CAS at their at active levels, RAS is reset while CAS is kept at its active level, and after a necessary reset period, when RAS is again made active, all the activation operations responsive to CAS are inhibited, whereby the state at the time point when said necessary operations were completed can be in itself maintained at the data output terminal. Furthermore, according to the present invention, the aforementioned function can be realized by adding to an inverter for generating CAS a circuit comprising a first MOST (Q 76  in FIG. 11) having its drain connected to a level-inversion output CAS for the CAS input, its gate connected to a first junction point (73 in FIG. 11) and its source connected to the ground potential, a second MOST (Q 74  in FIG. 11) having its drain connected to a power supply terminal (V DD  in FIG. 11), its gate connected to a one-shot precharge clock (XP 00  in FIG. 11) from the RAS system circuit and its source connected to the first junction point, and a third MOST (Q 75  in FIG. 11) having its drain connected to the first junction point, its gate connected to the CAS input and its source connected to ground potantial. 
     Since many changes could be made in the above circuit arrangement and many apparently widely different embodiments of this invention could be made without departing the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrated and not as a limitation to the scope of the invention.