BACKGROUND OF THE INVENTION

The present invention relates to an improvement in semiconductor full subtracters.

An array divider of carry save type requires a plurality of full subtracters. In such an array divider, each full subtracter receives a minuend signal A having a weight of +1, a subtrahend signal B having a weight of -1 and a borrow input signal Xi having a weight of -1, thereafter providing a difference output signal D having a weight of +1 and a borrow output signal Xo having a weight of -2. The difference signal D and the borrow output signal Xo for the full subtracter are expressed by the following logical expressions (1) and (2), respectively.

D=A.sym.B.sym.Xi (1)

Xo=(A.sym.B)Xi+AB=(A+B)Xi+AB (2)

With reference to FIG. 1, an example of the above-described prior art full subtracter is illustrated. This circuit has two NOR gates 100, 102, two AND/NOR composite gates 101, 103, an inverter 104, an OR/NAND composite gate 105 and two NAND gates 106, 107. All of these logic gates are commonly-used CMOS gates. Both a combination of the NOR gate 100 and the AND/NOR composite gate 101 and a combination of the NOR gate 102 and the AND/NOR composite gate 103 function as an EXOR gate.

FIG. 2 illustrates another example of the above-described prior art full subtracter. This full subtracter has a NOR gate 200, an AND/NOR composite gate 201, three NAND gates 202, 207, 208, two OR/NAND composite gates 203, 206, two inverters 204, 205 and two output inverters 209, 210. Like the first example, all of these logic gates are commonly-used CMOS gates. The output inverters 209, 210 are inversion buffers for high drive performance. A combination of the NOR gate 200 and the AND/NOR composite gate 201 functions as an EXNOR gate. A combination of the NAND gate 202 and the OR/NAND composite gate 203 functions as an EXNOR gate. These combinations are well-known ones in the art.

FIG. 3 is a truth table for the full subtracters shown in FIGS. 1 and 2. "0" represents the state of low logic level and "1" represents the state of high logic level. The fact that this truth table is in accordance with the logical expressions (1) and (2) can be proved easily.

The length of processing time, required in a case where input signals (SIGNALS A, B, Xi) are simultaneously supplied to the full subtracter circuit shown in FIG. 1, is explained. SIGNAL D appears in a propagation delay time required for input signal propagation through a total of four gate-stages, i.e., the logic gates 100-103. SIGNAL Xo appears in a propagation delay time required for input signal propagation through a total of three gate-stages, i.e., the gates 104, 105 (106) and 107.

For the case of the FIG. 2 full subtracter, SIGNAL D appears in a propagation delay time required for input signal propagation through a total of five gate-stages, i.e., the gates 200-203 and 209. On the other hand, SIGNAL Xo appears in a propagation delay time required for input signal propagation through a total of four gate-stages, i.e., the gates 204 (205), 206 (207), 208 and 210.

Accordingly, the processing time of the prior art full subtracters of FIGS. 1 and 2 is long.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an improved full subtracter which is much faster than conventional full subtracters. A fast divider (can be formed of full subtracters made in accordance with the present invention.

More specifically, four composite logic gates are employed in the present invention to provide a fast full subtracter suitable for integrated circuits of carry save type array dividers. The present invention is described by making reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a logic diagram of a conventional full subtracter.

FIG. 2 is a logic diagram of another conventional full subtracter.

FIG. 3 shows a truth table for the full subtracters shown in FIGS. 1 and 2.

FIG. 4 is a circuit diagram of a full subtracter in accordance with an embodiment of the present invention.

FIG. 5 shows a truth table for the upper half section of the full subtracter shown in FIG. 4.

FIG. 6 shows a truth table for the lower half section of the full subtracter shown in FIG. 4.

FIG. 7 is a circuit diagram of a full subtracter in accordance with another embodiment of the present invention.

FIG. 8 shows a truth table for the upper half section of the full subtracter shown in FIG. 7.

FIG. 9 shows a truth table for the lower half section of the full subtracter shown in FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of the present invention is described. FIG. 4 illustrates a circuit of a CMOS full subtracter of the first embodiment which is configured so that the logical expressions (1) and (2) hold. More specifically, this circuit is composed of a first logic gate 1, a second logic gate 2, a third logic gate 3, a fourth logic gate 4 and three CMOS inverters 5, 6, 7 in order that the circuit receives three different signals (a minuend signal A having a weight of +1, a subtrahend signal B having a weight of -1, and a borrow input signal Xi having a weight of -1) and provides two different signals (a difference output signal D having a weight of +1 and a borrow output signal Xo having a weight of -2).

The first logic gate 1 has five input terminals a, b, c, d, e and a single output terminal f. The first logic gate 1 provides, at the output terminal f, a low-logic-level output signal at least either in a first state or in a second state. The first state is the state in which the input terminals a, b, c are in the state of high logic level. The second state is the state in which the input terminals a, d, e are in the state of high logic level. On the other hand, the first logic gate 1 provides, at the output terminal f, a high-logic-level output signal at least either in a third state or in a fourth state. The third state is the state in which the input terminals a, b, c are in the state of low logic level. The fourth state is the state in which the input terminals a, d, e are in the state of low logic level. In different states other than the first to fourth states, the output terminal f is placed into the state of high impedance. The first logic gate 1 is composed of five p-channel MOS (PMOS) transistors 10-14 and five n-channel MOS (NMOS) transistors 15-19. The PMOS transistors 10 and 11 are connected in series to make a first series connection. The PMOS transistors 12 and 13 are connected in series to make a second series connection. These first and second series connections are connected in parallel to form a first parallel circuit. The NMOS transistors 16 and 17 are connected in series to make a third series connection. The NMOS transistors 18 and 19 are connected in series to make a fourth series connection. These third and fourth series connections are connected in parallel to form a second parallel circuit. The PMOS transistor 14 and the NMOS transistor 15 are connected in series to make a fifth series connection. The first parallel circuit, the fifth series connection, and the second parallel circuit are serially connected between a positive power supply terminal V.sub.DD and a negative power supply terminal, i.e., ground. The gates of the PMOS transistor 14 and the NMOS transistor 15 are connected in common to the input terminal a and the drains thereof are connected in common to the output terminal f. The input terminals b, c, d and e are connected to the gates of the PMOS transistor 10 and the NMOS transistor 17, to the gates of the PMOS transistor 11 and the NMOS transistor 16, to the gates of the PMOS transistor 12 and the NMOS transistor 19 and to the gates of the PMOS transistor 13 and the NMOS transistor 18, respectively.

The second logic gate 2 is identical in configuration with the first logic gate 1. The second logic gate 2 with five input terminals a, b, c, d, e and a single output terminal f' is composed of five PMOS transistors 20-24 and five NMOS transistors 25-29. The second logic gate 2 functions in substantially the same way as the first logic gate 1.

The third logic gate 3 with two input terminals g, h and a single output terminal i provides a low-logic-level output signal at the output terminal i in a fifth state in which the input terminals g and h are in the state of high logic level. Conversely, the third logic gate 3 provides a high-logic-level output signal at the output terminal i in a sixth state in which the input terminals g and h a:e in the state of low logic level. In different states other than the fifth and sixth states, the output terminal i is placed into the state of high impedance. The third Logic gate 3 is composed of two PMOS transistors 30 and 31 and two NMOS transistors 32 and 33. These four MOS transistors 30, 31, 32 and 33 are serially connected between V.sub.DD and ground. The gates of the PMOS transistor 30 and the NMOS transistor 33 are connected to the input terminal g. The gates of the PMOS transistor 31 and the NMOS transistor 32 are connected to the input terminal h. The drains of the PMOS transistor 31 and the NMOS transistor 32 are connected in common to the output terminal i.

The fourth logic gate 4 has three input terminals j, m, n and a single output terminal k. The fourth logic gate 4 provides, at the output terminal k, a low-logic-level output signal, at least either in a seventh state or in an eighth state. The seventh state is the state in which the input terminals j and m are in the state of high logic level. The eighth state is the state in which the input terminals j and n are in the state of high logic level. On the other hand, the fourth logic gate 4 provides, at the output terminal k, a high-logic-level output signal, at least either in a ninth state or in a tenth state. The ninth state is the state in which the input terminal j and m are in the state of low logic level. The tenth state is the state in which the input terminals j and n are in the state of low logic level. In different states other than the seventh to tenth states, the output terminal k is placed into the state of high impedance. The fourth logic gate 4 is composed of three PMOS transistors 40-42 and three NMOS transistors 43-45. The PMOS transistors 40 and 41 make a parallel connection. The NMOS transistors 44 and 45 make a parallel connection. The PMOS transistor 42 and the NMOS transistor 43 make a series connection. These parallel connections and the series connection are serially connected between V.sub.DD and ground. The gates of the PMOS transistor 42 and the NMOS transistor 43 are connected in common to the input terminal j and the drains thereof are connected in common to the output terminal k. The input terminals m and n are connected to the gates of the PMOS transistor 40 and the NMOS transistor 44 and to the gates of the PMOS transistor 41 and the NMOS transistor 45, respectively.

With reference to FIG. 4, SIGNAL A, which is a minuend signal with a weight of +1, is supplied to the input terminal b of the first logic gate 1, to the input terminal b of the second logic gate 2, to the input terminal g of the third logic gate 3 and to the input terminal m of the fourth logic gate 4. An inverted signal of SIGNAL A is supplied to the input terminal e of the first logic gate 1 as well as to the input terminal d of the second logic gate 2. SIGNAL B, which is a subtrahend signal with a weight of -1, is supplied to the input terminal d of the first logic gate 1 and to the input terminal c of the second logic gate 2. An inverted (signal of SIGNAL B is supplied to the input terminal c of the first logic gate 1, to the input terminal e of the second logic gate 2, to the input terminal h of the third logic gate 3 and to the input terminal n of the fourth logic gate 4. SIGNAL Xi, which is a borrow input signal with a weight of -1, is supplied to the input terminal a of the first logic gate 1. An inverted signal of SIGNAL Xi is supplied to the input terminal a of the second logic gate 2 and to the input terminal j of the fourth logic gate 4. The output terminal f of the first logic gate 1 and the output terminal f' of the second logic gate 2 are connected together in such a way as to produce a SIGNAL D which is a difference output signal with a weight of +1. The output terminal i of the third logic gate 3 and the output terminal k of the fourth logic gate 4 are connected together in such a way as to produce a SIGNAL Xo which is a borrow output signal with a weight of -2.

The truth table of FIG. 5 shows the relation between outputs of the first and second logic gates 1 and 2 in response to the input signals (SIGNALS A, B, Xi) and corresponding logic levels of SIGNAL D (the difference output signal identical with SIGNAL D of FIG. 3). The truth table of FIG. 6 shows the relation between outputs of the third and fourth logic gates 3 and 4 in response to the input signals (SIGNALS A, B, Xi) and corresponding logic levels of SIGNAL Xo (the borrow output signal identical with SIGNAL Xo of FIG. 3). "0" represents the state of low logic level and "1" represents the state of high logic level in these truth tables. Suppose an input situation where SIGNAL A="0", SIGNAL B="0" and SIGNAL Xi="0". The PMOS transistors 11, 13 and the NMOS transistors 15, 17, 19 are in the off state and, as a result, the output of the first logic gate 1 is in the state of high impedance. Each NMOS transistor 25, 28, 29 is placed into the on state and the PMOS transistor 24 turns off, as a result of which the second logic gate 2 gives an output of "0". SIGNAL D therefore becomes "0". Meanwhile, both the PMOS transistor 31 and the NMOS transistor 33 turn off and, as a result, the output of the third logic gate 3 is in the state of high impedance. Both the NMOS transistors 43 and 45 are placed into the on state and the PMOS transistor 42 turns off, so that the fourth logic gate 4 provides a "0" as its output. SIGNAL Xo becomes "0".

The length of processing time taken in a case where input signals (SIGNALS A, B, Xi) are simultaneously supplied to the full subtracter shown in FIG. 4, is explained. Inverted signals of SIGNALS A, B and Xi are produced at the same time in the inverters 5, 6 and 7, respectively. SIGNALS A, B and Xi and their inverted signals are supplied to the first to fourth logic gates 1-4. SIGNAL D is obtained in a propagation delay time required for input signal propagation through a total of two gate-stages, i.e., one inverter and one logic gate (the logic gate 1 or logic gate 2). Likewise, SIGNAL Xo is obtained in a propagation delay time required for input signal propagation through a total of two gate-stages, i.e., one inverter and one logic gate (the logic gate, 3 or logic gate 4). With SIGNALS D and Xo, the processing time of the present full subtracter of FIG. 4 is about 1/2 of that of the FIG. 1 prior art logic circuit. The processing rate of the FIG. 4 full subtracter is about twice that of the FIG. 1 logic circuit. One of advantages provided by the FIG. 4 configuration is that no through current flows from V.sub.DD to ground in any input state.

A second embodiment of the present invention is described. FIG. 7 illustrates a circuit of a CMOS full subtracter which is constructed such that the logical expressions (1) and (2) hold. Like the FIG. 4 circuit, this circuit comprises the first to fourth logic gates 1-4 and the CMOS inverters 5, 6, 7. In addition to these components, two CMOS output inverters 8, 9 are provided in the present embodiment. The CMOS output inverters 8 and 9 are inverting buffers for high drive performance.

With reference to FIG. 7, SIGNAL A with a weight of +1 is supplied to the input terminal b of the first logic gate 1 and to the input terminal b of the second logic gate 2. An inverted signal of SIGNAL A is supplied to the input terminal d of the first logic gate 1, to the input terminal e of the second logic gate 2, to the input terminal h of the third logic gate 3 and to the input terminal m of the fourth logic gate 4. SIGNAL B with a weight of -1 is supplied to the input terminal c of the first logic gate 1, to the input terminal d of the second logic gate 2, to the input terminal g of the third logic gate 3 and to the input terminal n of the fourth logic gate 4. An inverted signal of SIGNAL B is supplied to the input terminal e of the first logic gate 1 and to the input terminal c of the second logic gate 2. SIGNAL Xi with a weight of -1 is supplied to the input terminal a of the first logic gate 1 and to the input terminal j of the fourth logic gate 4. An inverted signal of SIGNAL Xi is supplied to the input terminal a of the second logic gate 2. The output terminal f of the first logic gate 1 and the output terminal f' of the second logic gate 2 are connected in common to the input terminal of the output inverter 8 so that the output inverter 8 provides a SIGNAL D with a weight of +1 as its output. The output terminal i of the third logic gate 3 and the output terminal k of the fourth logic gate 4 are connected in common to the input terminal of the output inverter 9 so that the output inverter 9 provides a SIGNAL Xo with a weight of -2 as its output.

The truth table of FIG. 8 shows the relation between outputs of the first and second logic gates 1 and 2 in response to the input signals (SIGNALS A, B, Xi) and corresponding logic levels of SIGNAL D (the difference output signal identical with SIGNAL D of FIG. 3). The truth table of FIG. 9 shows the relation between outputs of the third and fourth logic gates 3 and 4 in response to the input signals (SIGNALS A, B, Xi) and corresponding logic levels of SIGNAL Xo (the borrow output signal identical with SIGNAL Xo of FIG. 3). "0" represents the low logic level and "1" represents the high logic level in these truth tables. Suppose an input situation where SIGNAL A="0", SIGNAL B="0" and SIGNAL Xi="0". The PMOS transistors 21, 23, 24 and the NMOS transistors 27, 29 are placed into the off state and, as a result, the output of the second logic gate 2 is in the state of high impedance. Each PMOS transistor 10, 11, 14 is placed into the on state and the NMOS transistor 15 turns off, as a result of which the first logic gate 1 gives an output of "1". The signal at the output terminal to which the first and second logic gates 1 and 2 are connected in common, becomes "1". This signal is inverted by the output inverter 8 to become a SIGNAL D of "0". Meanwhile, both the PMOS transistor 31 and the NMOS transistor 33 turn off and, as a result, the output of the third logic gate 3 is in the state of high impedance. Both the PMOS transistors 41 and 42 are placed into the on state and the NMOS transistor 43 turns off, so that the fourth logic gate 4 provides a "1" as its output. The signal at the output terminal to which both the third and fourth logic gates 3 and 4 are connected in common, becomes "1". This signal is inverted by the output inverter 9 to become a SIGNAL Xo of "0".

With SIGNALS D and Xo, the processing time of the. present full subtracter of FIG. 7 is reduced in comparison with the FIG. 2 prior art logic circuit. The present invention provides a fast full subtracter with buffers. One of advantages provided by the FIG. 7 configuration is that no through current flows from V.sub.DD to ground in any input state.

The configuration of the logic gates 1-4 is not necessarily limited to the configuration of FIGS. 4 and 7. Any other circuit configuration may be utilized as far as the same functions are performed.