Constant Operation Speed Logic Circuit

1. Field of the Invention

The present invention relates to a logic circuit such as a NAND circuit or an NOR circuit. Such a logic circuit is often used in address decoders, a bit line control clock generator circuit, etc of a memory device.

2. Description of the Prior Art

A prior art logic circuit such as a NAND circuit or a NOR circuit comprises a load, a single driver circuit driven by the potentials at two or more input terminals, and an output terminal connected between the load and the driver circuit. The driver circuit has a plurality of gates which are driven by the potentials at the input terminals.

In the above-mentioned prior art, however, the presence or absence of charges at an intermediate node of the gates generates a fluctuation in operation speed. In this case, the operation speed is dependent upon the condition of input signals. As a result, the overall operation speed is substantially reduced.

It is an object of the present invention to provide a logic circuit in which the operation speed is constant under any condition of input signals.

According to the present invention, at least two driver circuits are provided. Preferably, the same number of driver circuits is provided as input terminals. The driver circuits are connected in parallel with each other. The gates of the driver circuits connected directly to the output terminal are driven by different potentials at the output terminals. Therefore, at least one of the driver circuits operates rapidly in response to changes of input signals so as to change the potential at the output terminal. As a result, no fluctuation in operation speed is generated under any condition of the input signals, and, in addition, the operation speed is increased.

The present invention will be clearly understood from the description as set forth below contrasting the prior art with the present invention and referring to the accompanying drawings.

FIG. 1 is a circuit diagram of a prior art logic circuit;

FIGS. 2A through 2D and FIGS. 3A through 3D are timing diagrams of the signals appearing in the circuit of FIG. 1;

FIG. 4 is a circuit diagram of another prior art logic circuit;

FIGS. 5A through 5D and FIGS. 6A through 6D are timing diagrams of the signals appearing in the circuit of FIG. 4;

FIGS. 7A through 7D, FIGS. 8A through 8D, FIGS. 9A and 9B, and FIGS. 10A and 10B are circuit diagrams of embodiments of the logic circuit according to the present invention;

FIG. 11 is a circuit diagram of a semiconductor device to which the present invention is applied;

FIG. 12 is a circuit diagram of a memory cell of FIG. 11;

FIG. 13 is a circuit diagram of the bit line precharging circuit of FIG. 11;

FIG. 14 is a circuit diagram of the bit line sensing circuit of FIG. 11;

FIG. 15 is a circuit diagram of the pre-decoders of FIG. 11;

FIG. 16 is a circuit diagram of the bit line control clock generator circuit of FIG. 11;

FIG. 17 is a circuit diagram of the address buffer of FIG. 11;

FIG. 18 is a circuit diagram of the circuit for generating the pre-clock signals PA.sub.i (i=0 to 7) of FIG. 16;

FIGS. 19A through 19E are timing diagrams for explaining the operation of the circuit of FIG. 16;

FIGS. 20A and 20B are circuit diagrams of the other embodiments bit line control clock generator circuit of FIG. 11; and

FIGS. 21A through 21E are timing diagrams for explaining the operation of the circuits of FIGS. 20A and 20B.

In FIG. 1, which is a prior art two-input complementary metal-insulator-semiconductor (CMIS) NAND circuit, two P-channel transistors Q.sub.1 ' and Q.sub.2 ' are connected in parallel between a power supply V.sub.cc and an output terminal OUT to form a load L. The transistors Q.sub.1 ' and Q.sub.2 ' are driven by the potentials at the input terminals IN.sub.1 and IN.sub.2, respectively. In addition, two N-channel transistors Q.sub.11 and Q.sub.12 are connected in series between a power supply V.sub.ss, having a lower potential than the power supply V.sub.cc, and the output terminal OUT to form a driver circuit D.sub.1. The transistors Q.sub.11 and Q.sub.12 are also driven by the potentials at the input terminals IN.sub.1 and IN.sub.2, respectively. Note that the "apostrophe" on the references designates "P-channel type".

The operation of the circuit of FIG. 1 will now be explained with reference to FIGS. 2A through 2D and FIGS. 3A through 3D. As illustrated in FIGS. 2A and 2B, when the potential at the input terminal IN.sub.1 remains at V.sub.cc and, in addition, the potential at the input terminal IN.sub.2 is changed from V.sub.ss to V.sub.cc, the potential at the output terminal OUT is changed from V.sub.cc to V.sub.ss, as illustrated in FIG. 2D. In this case, however, before the change of the potential at the input terminal IN.sub.2, the transistors Q.sub.2 ' and Q.sub.11 are both turned on and the transistors Q.sub.1 ' and Q.sub.12 are turned off, so that a node N.sub.1 connecting the transistor Q.sub.11 to the transistor Q.sub.12 is charged at V.sub.cc -V.sub.th3, where V.sub.th3 is the threshold voltage of the transistor Q.sub.11, as illustrated in FIG. 2C. As a result, even when the potential at the input terminal IN.sub.2 is changed from V.sub.ss to V.sub.cc so as to turn off the transistor Q.sub.2 ' and turn on the transistor Q.sub.12, the potential at the output terminal OUT does not become V.sub.ss until the node N.sub.1 is discharged, resulting in a relatively large delay time t.sub.1.

On the other hand, as illustrated in FIGS. 3A and 3B, when the potential at the input terminal IN.sub.2 remains at V.sub.cc and, in addition, the potential at the input terminal IN.sub.1 is changed from V.sub.ss to V.sub.cc, the potential at the output terminal OUT is also changed from V.sub.cc to V.sub.ss, as illustrated in FIG. 3D. This operation is logically the same as the above-mentioned operation. In this case, before the change of the potential at the input terminal IN.sub.1, the transistors Q.sub.1 ' and Q.sub.12 are turned on and the transistors Q.sub.2 ' and Q.sub.11 are turned off, so that the potential at the node N.sub.1 is at V.sub.ss, as illustrated in FIG. 3C. Therefore, when the potential at the input terminal IN.sub.1 is changed from V.sub.ss to V.sub.cc, so as to turn off the transistor Q.sub.1 ', and turn on the transistor Q.sub.11, the potential at the node N.sub.1 is changed with a relatively small delay time t.sub.2, since it is unnecessary to discharge the node N.sub.1.

Thus, a difference in speed between the operation as shown in FIGS. 2A through 2D and the operation as shown in FIGS. 3A through 3D, is generated. This difference is inherent in the construction of the series connection of the transistors Q.sub.11 and Q.sub.12.

The above-mentioned difference in operation speed similarly occurs in the case of a NOR circuit as illustrated in FIG. 4, which is a prior art two-input CMIS NOR circuit. In FIG. 4, two N-channel transistors Q.sub.1 and Q.sub.2 are connected in parallel between the power supply V.sub.ss and the output terminal OUT to form a load L, while two P-channel transistors Q.sub.11 ' and Q.sub.12 ' are connected in series between the power supply V.sub.cc and the output terminal OUT to form a driver circuit D.sub.1. As illustrated in FIGS. 5A through 5D, when the potential at the input terminal IN.sub.1 remains at V.sub.ss and, in addition, the potential at the input terminal IN.sub.2 changes from V.sub.cc to V.sub.ss, it is necessary to charge the node N.sub.1, having the potential V.sub.th2, where V.sub.th2 is the threshold voltage of the transistor Q.sub.11 ', whereby a relatively large delay time t.sub.1 is generated. However, as illustrated in FIGS. 6A through 6D, when the potential at the input terminal IN.sub.2 remains at V.sub.ss and, in addition, the potential at the input terminal IN.sub.1 is changed from V.sub.cc to V.sub.ss, it is unnecessary to charge the node N.sub.1. As a result, a relatively small delay time t.sub.2 is generated. Therefore, a difference in operation speed is generated between the operation as illustrated in FIGS. 5A through 5D and the operation as illustrated in FIGS. 6A through 6D.

In the present invention, the relatively large delay time t.sub.1 is shortened so as to eliminate the above-mentioned difference in operation speed.

FIGS. 7A, 7B, and 7C are first, second, and third embodiments of the present invention, i.e., a two-input CMIS NAND circuit, a three-input CMIS NAND circuit, and a four-input CMIS NAND circuit, respectively. FIG. 7D is a fourth embodiment of the present invention, also a four-input CMIS NAND circuit.

In the embodiment of FIG. 7A, two driver circuits D.sub.1 and D.sub.2, connected in parallel, are provided. In the driver circuit D.sub.1, a first N-channel transistor Q.sub.11, connected directly to the output terminal OUT, is driven by the potential at the input terminal IN.sub.1, while a second N-channel transistor Q.sub.12 is driven by the potential at the input terminal IN.sub.2. In the driver circuit D.sub.2, a first N-channel transistor Q.sub.21 connected directly to the output terminal OUT is driven by the input terminal IN.sub.2 and a second N-channel transistor Q.sub.22 is driven by the input terminal IN.sub.1.

When the potentials at the input terminals IN.sub.1 and IN.sub.2 are V.sub.cc and V.sub.ss, respectively, the transistors Q.sub.2 ', Q.sub.11 and Q.sub.22 are turned on and the transistors Q.sub.1 ', Q.sub.12 and Q.sub.21 are turned off. As a result, the potential at the node N.sub.1 is V.sub.cc -V.sub.th, and the potential at the node N.sub.2 is V.sub.ss. Therefore, when the potential at the input IN.sub.2 is changed from V.sub.ss to V.sub.cc, the transistor Q.sub.2 ' is turned off and the transistor Q.sub.21 is turned on, so that the potential at the output terminal OUT is changed quickly from V.sub.cc to V.sub.ss under the influence of the discharged node N.sub.2. However, when the potential at the input terminal IN.sub.2 remains at V.sub.cc and, in addition, the potential at the input terminal IN.sub.1 is changed from V.sub.ss to V.sub.cc, the potential at the output terminal OUT is also changed quickly from V.sub.cc to V.sub.ss under the influence of the discharged node N.sub.1, in the same way as in FIG. 1. That is, in all cases, the operation time is t.sub.2 as shown in FIG. 3D.

In the embodiment of FIG. 7B, three driver circuits D.sub.1, D.sub.2, and D.sub.3, connected in parallel, are provided. The structure of the circuit of FIG. 7B is similar to that of the circuit of FIG. 7A. That is, first N-channel transistors Q.sub.11, Q.sub.21 and Q.sub.31 of the driver circuits, connected directly to the output terminal OUT, are driven by the potentials at the input terminals IN.sub.1, IN.sub.2 and IN.sub.3, respectively. Therefore, when two of the potentials at the input terminals IN.sub.1, IN.sub.2 and IN.sub.3 remain at V.sub.cc and the other potential remains at V.sub.ss, the potential at only one of the nodes N.sub.1, N.sub.2 and N.sub.3 remains at V.sub.ss. For example, when the potentials at the input terminals IN.sub.1 and IN.sub.2 remain at V.sub.cc, and the potential at the input terminal IN.sub.3 remains at V.sub.ss, the potential at only the node N.sub.3 is V.sub.ss. As a result, when the potential at the input terminal IN.sub.3 is changed from V.sub.ss to V.sub.cc, the potential at the output terminal OUT is changed quickly from V.sub.cc to V.sub.ss under the influence of the discharged node N.sub.3.

In the embodiment of FIG. 7C, four driver circuits, D.sub.1, D.sub.2, D.sub.3 and D.sub.4, connected in parallel, are provided. The structure of the circuit of FIG. 7C is also similar to that of the circuit of FIGS. 7A or 7B. That is, first N-channel transistors Q.sub.11, Q.sub.21, Q.sub.31 and Q.sub.41 of the driver circuits, connected directly to the output terminal OUT, are driven by the potentials at the input terminals IN.sub.1, IN.sub.2, IN.sub.3 and IN.sub.4, respectively. Therefore, when three of the potentials at the input terminals IN.sub.1, IN.sub.2, IN.sub.3 and IN.sub.4 remain at V.sub.cc and the other potential remains at V.sub.ss, the potential at only one of the nodes N.sub.1, N.sub.2, N.sub.3 and N.sub.4 remains at V.sub.ss. For example, when the potentials at the input terminals IN.sub.1, IN.sub.2 and IN.sub.3 remain at V.sub.cc, and the potential at the input terminal IN.sub.4 remains at V.sub.ss, the potential at only the node N.sub.4 is V.sub.ss. As a result, when the potential at the input terminal IN.sub.4 is changed from V.sub.ss to V.sub.cc, the potential at the output terminal OUT is changed quickly from V.sub.cc to V.sub.ss under the influence of the discharged node N.sub.4.

In the embodiment of FIG. 7D, a four-input CMIS NAND circuit is illustrated, but only two driver circuits D.sub.1 and D.sub.2 are provided. In this case, the potentials at the input terminals IN.sub.1 and IN.sub.2 are more important than the potentials at the input terminals IN.sub.3 and IN.sub.4. That is, first N-channel transistors Q.sub.11 and Q.sub.21 of the driver circuits D.sub.1 and D.sub.2, connected directly to the output terminal OUT, are driven by the potentials at the input terminals IN.sub.1 and IN.sub.2, respectively. Therefore, when the potential at input terminal IN.sub.1 or IN.sub.2 remains at V.sub.ss and the potentials at the other input terminals are V.sub.cc, the potential at the node N.sub.1 or N.sub.2 is V.sub.ss. Therefore, when the potential at the input terminal IN.sub.1 or IN.sub.2 is changed from V.sub.ss to V.sub.cc, the potential at the output terminal OUT is changed quickly from V.sub.cc to V.sub.ss. However, when the potential at the input terminal IN.sub.3 or IN.sub.4 is changed from V.sub.ss to V.sub.cc, the potential at the output terminal OUT cannot be changed quickly from V.sub.cc to V.sub.ss. Therefore, the circuit of FIG. 7D is useful only when to specific input conditions exist.

FIGS. 8A, 8B, 8C and 8D are fifth, sixth, seventh and eighth embodiments of the present invention, i.e., CMIS NOR circuits. Note that the embodiments of FIGS. 8A, 8B, 8C and 8D roughly correspond to the embodiments of FIGS. 7A, 7B, 7C and 7D, respectively. For example, the circuit of 8A is the same as the circuit of FIG. 7A, except the load L and the driver circuits D.sub.1 and D.sub.2 are reversed in respect with the output terminal OUT and the P-channel transistors of the load L are changed to an N-channel type and vice versa.

The operation of the circuits of FIGS. 8A, 8B, 8C and 8D are similar to the operation of the circuits of FIGS. 7A, 7B, 7C and 7D, respectively. For example, in the embodiment of FIG. 8B, when two of the potentials at the input terminals IN.sub.1, IN.sub.2 and IN.sub.3 remain at V.sub.ss and the other potential remains at V.sub.cc, the potential at only one of the nodes N.sub.1, N.sub.2 and N.sub.3 remains at V.sub.cc. For example, when the potentials at the input terminals IN.sub.1 and IN.sub.2 remain at V.sub.ss, and the potential at the input terminal IN.sub.3 remains at V.sub.cc, the potential at only the node N.sub.3 is V.sub.cc. As a result, when the potential at the input terminal IN.sub.3 is changed from V.sub.cc to V.sub.ss, the potential at the output terminal OUT is changed quickly from V.sub.ss to V.sub.cc under the influence of the charged node N.sub.3.

FIGS. 7A through 7D and FIGS. 8A through 8D are CMIS logic circuits. However, note that the present invention can be also applied to enhancement/depletion (E/D) type MIS logic circuits and conventional MIS logic circuit.

In an E/D MIS logic circuits such as an E/D MIS NAND circuit as illustrated in FIG. 9A or an E/D MIS NOR circuit as illustrated in FIG. 9B, the load L is constructed by a deletion-type transistor Q.sub.L1 (N-channel) or Q.sub.L2 ' (P-channel) in which the source and gate are connected to each other. On the other hand, in a conventional MIS logic circuit such as a NAND circuit as illustrated in FIG. 10A or an NOR circuit as illustrated in FIG. 10B, the load L is constructed by a enhancement-type transistor Q.sub.L3 (N-channel) or Q.sub.L4 ' (P-channel) in which the drain and gate are connected to each other.

The application of the circuits according to the present invention will now be explained with reference to FIG. 11.

In FIG. 11, a 64 Kbit static random access memory (RAM) is illustrated. References MCA1 and MCA2 designate memory cell arrays formed by a large number of memory cells MC as illustrated in FIG. 12; RAB row address buffers; CAB column address buffers; I/O input/output circuits; GEN a bit line control clock generator circuit; P-RD pre-decoders, as illustrated in FIG. 15; RD row decoders; WD word drivers; CD column decoders; BLL bit line precharging circuits as illustrated in FIG. 13; and BLC bit line sensing circuits as illustrated in FIG. 14.

In a highly integrated large-capacity memory device, each memory cell MC is very small in size. Accordingly, the pitch between adjacent rows is also extremely narrow. Therefore, it is difficult to place one decoder for each row. In order to overcome this, one row decoder is allocated to every two or four word lines. For example, as illustrated in FIG. 15, the pre-decoders P-RD determine the selection modes .PHI..sub.00, .PHI..sub.01, .PHI..sub.10 and .PHI..sub.11. In the pre-decoders P-RD, four NAND circuits NA.sub.1, NA.sub.2, NA.sub.3 and NA.sub.4 of FIG. 7A are used.

The logic circuits according to the present invention can also be applied to the bit line control clock generator circuit GEN of FIG. 11, which will now be explained in more detail. One bit line precharging circuit and one bit line sensing circuit are provided for each bit line pair. That is, during the read mode, the bit line precharging circuits BLL are controlled by a clock signal .phi..sub.BP from the bit line control clock generator circuit GEN to precharge the corresponding bit lines. After that, the bit lines become in a floating state. Next, one selected memory cell is connected to each bit line pair. As a result, the small difference in potential between the bit line pair is sensed and enlarged by the corresponding bit line sensing circuit BLC, which is controlled by a clock signal .phi..sub.BC from the bit line control clock generator circuit GEN.

FIG. 16 is one example of the bit line control clock generator circuit GEN of FIG. 11. In FIG. 16, the generator circuit GEN comprises a NOR circuit NO.sub.1 of FIG. 8D, a NOR circuit NO.sub.2 of FIG. 8C, a NAND circuit NA.sub.5 of FIG. 7A, and a plurality of inverters. Row address pre-clock signals PA.sub.0 through PA.sub.7 correspond to external row address signals ADRS.sub.i (i=0 to 7, FIG. 17), respectively. For example, when the row address signal ADRS.sub.0 changes, the row address pre-clock signal PA.sub.0 remains at a high level for a definite time. Otherwise, the signal PA.sub.0 always remains at a low level. Such pre-clock signals PA.sub.0 through PA.sub.7 are obtained by the circuits of FIGS. 17 and 18. Note that CE of FIG. 17 designates a memory control signal. The circuits NO.sub.1, NO.sub.2 and NA.sub.5 of FIG. 16 together comprise a circuit for detecting the change of row address signals.

Note that the NOR circuit NO.sub.1 comprises only two loads, since, in this case, the signals PA.sub.0 and PA.sub.1 corresponding to the row address signals A.sub.0 and A.sub.1 are more important than the signals PA.sub.2 and PA.sub.3 corresponding to the row address signals A.sub.2 and A.sub.3.

As illustrated in FIG. 19A, when at least one of the row address signals PA.sub.0 through PA.sub.3 changes, the output of the NOR circuit NO.sub.1 reaches a high level. Similarly, when at least one of the row address signals PA.sub.4 through PA.sub.7 changes, the output of the NOR circuit NO.sub.1 reaches a high level. In all cases, the output of the NAND circuit NA.sub.5 reaches a high level. As a result, the clock signal .phi..sub.BP changes as illustrated in FIG. 19C, and the clock signal .phi..sub.BC changes as illustrated in FIG. 19D. Therefore, the bit line sensing circuit BLC does not perform a sensing operation during the precharging operation mode and a predetermined time after the precharging operation is completed. As a result, the potentials at the bit lines change as illustrated in FIG. 19E.

Generally, the connections between the generator circuit GEN and the circuits BLL and BLC are formed by aluminum, which has a small delay time, while the word line WL is formed by polycrystalline silicon, which has a large delay time. As a result, the potential at the portion of a word line far from the word drivers WD changes slower than the portion of the word line near the word drivers WD. The change of the former is illustrated by a dot and dash line in FIG. 19B. In this case, the change of the potentials of the bit lines is illustrated by a dot and dash line in FIG. 19E. Therefore, it is necessary to delay the clock signals .phi..sub.BP and .phi..sub.BC in accordance with the delay of the change of the word line portion. For this purpose, the circuits of FIGS. 20A and 20B are added to the circuit of FIG. 16.

In the combination of the circuits of FIGS. 16, 20A and 20B, four kinds of bit line control clock signals .phi..sub.BP1, .phi..sub.BP2, .phi..sub.BP3 and .phi..sub.BP4 are generated for the four blocks B1, B2, B3 and B4 of the memory cell arrays MCA1 and MCA2, respectively. In addition, four kinds of bit line sensing clock signals .phi..sub.BC1, .phi..sub.BC2, and .phi..sub.BC3 and .phi..sub.BC4 are generated for the blocks B1, B2, B3 and B4, respectively.

For example, the circuit of FIG. 20A comprises four resistors R.sub.1, R.sub.2, R.sub.3 and R.sub.4 and four NAND circuits NA.sub.11, NA.sub.12, NA.sub.13 and NA.sub.14. However, these NAND circuits do not always require two driver circuits, since the sequence of the input conditions for the NAND circuits is definite. In FIG. 20A, when the potential of the signal .phi..sub.BP is changed from a low level to a high level and, accordingly, the potential of the signal .phi..sub.BP is changed from a high level to a low level, the potentials of the signals .phi..sub.BP1, .phi..sub.BP2, .phi..sub.BP3 and .phi..sub.BP4 rise simultaneously, as illustrated in FIG. 21C. However, when the potential of the signal .phi..sub.BP is changed from a high level to a low level and, accordingly, the potential of the signal .phi..sub.BP is changed from a low level to a high level, the signals .phi..sub.BP1, .phi..sub.BP2, .phi..sub.BP3 and .phi..sub.BP4 fall into the order illustrated by the dashed and dotted lines of FIG. 21C, due to the presence of the delay circuits.

Similarly, the potentials of the clock signals .phi..sub.BC1, .phi..sub.BC2, .phi..sub.BC3 and .phi..sub.BC4 change in the same way as those of the clock signals .phi..sub.BP1, .phi..sub.BP2, .phi..sub.BP3 and .phi..sub.BP4, except for the delay times of the fall of the potentials, as illustrated in FIG. 21D. Thus, the delay time of each block B1, B2 B3, and B4 of the memory cell arrays MCA1 and MCA2 can be compensated. In this case, note that it is not important that the potentials of the signals .phi..sub.BP1 through .phi..sub.BP4 rise simultaneously. In other words, the signals .phi..sub.BC,i +1 also be obtained by shifting the signal .phi..sub.BCi. Note that FIGS. 21A, 21B and 21E are the same as FIGS. 19A, 19B and 19E, respectively.

As explained hereinbefore, according to the present invention, no fluctuation in operation speed is generated regardless of the condition of the input signals. In addition, the operation speed is increased.