PRECHARGE-TYPE CARRY CHAINED ADDER CIRCUIT

Abstract: In a precharge-type carry chained adder circuit, stages as represented by adders and treated thereby are grouped into a plurality of blocks (CG) each consisting of a plurality of stages when the propagating functions of the stages of each block are active, a carry signal reproduced from the preced­ ing block is transferred to the succeeding blocks through a bypass circuit.

Description: The present invention relates to a precharge-type carry chained adder circuit well adaptable for high speed addition. Fig. 1 shows a circuit diagram illustrating a one-stage part of a Manchester-type carry chained adder constructed with N type MOSFETs. The adder receives a precharge signal , a carry kill signal K , a carry propagation signal P , and a carry in signal Carry-in, and produces carry propagation signals P' and P ' and a carry-out signal Carry-out. One of the most important problems involved in such an adder circuit is how to quickly send a carry signal to a more significant digit position. To cope with this problem, various types of high speed adders have been proposed. The approach to speeding up the carry signal reproduction, however, suffers from a large increase in additional hardware. A known carry look-ahead circuit is advantageous in the reproduction speed of the carry signal, but is disadvantageous in the additional and complicated hardware. The carry look-ahead circuit further requires multi-input gates. The use of the multi-input gates is not recommandable for the MOSFET circuit, particularly CMOS circuit. For this background reasons, there is a great demand for realizing a high speed adder with a simple construction and well adaptable for the MOSFET circuit. One of the adder circuits thus far proposed to meet such a demand is a precharge type carry chained adder circuit. In this type of adder circuit, a carry is reproduced through N type transfer gates. With this arrangement, if each node in the transfer gate circuit is previously precharged to "H" level before the reproduction of a carry, a high speed reproduction is realized. In this arrangement, however, when no carry occurs, each node must be discharged through the gates connected in series between the carry-in and carry-out. In an extreme case, the gates corresponding in number to the word length of the adder must be connected in series. Therefore, as the word length of the adder is larger, the delay time caused when the carry signal reproduces is long and problematic. Prior art document US-A-3 728 532 discloses a carry skip-ahead network in which stages transmit a carry signal unidirectionally due to provision of one way gates. The carry signal is jumped over the gates in forward direction, i.e. the carry signal is jumped over ahead only. The carry signal which is jumped over a first gate, for example, is supplied forwardly to an inverter, without propagating backwardly to the stages. In this known network, transmission gates must be composed of both p-type and n-type channel FETs, since the signal level in the static-type adder is not defined fixedly in positive or negative polarity. Accordingly, in the known network, the number of elements is increased. Further, prior art document US-A-3 843 876 describes an electronic digital adder having a high speed carry propagation line in which a transmission gate is turned on and turned off by an addend signal A and an augend signal B, without using precharge signals  and  . In this digital adder a discharging operation of a carry propagation line is effected by applying signals the A, B to a FET, regardless of the precharge signals  and  , and a carry kill operation is performed by these signals A and B. To be more specific, when the FET is turned on only by the signals A and B, a discharge current flows from power supply terminal through further FETs to ground. It is an object of the present invention to provide a precharge-type carry chained adder circuit which is operable with a high speed carry reproduction and is remarkably improved in the operating speed and thus improved in addition speed, particularly when a lengthy word is treated. To solve this object the present invention provides a carry chained adder circuit as defined in claim 1. The precharge-type carry chained adder circuit of the present invention has a plurality of precharge-type carry chained adders connected in a cascade fashion. The adder circuit comprises AND gates for generating logical products of the carry-propagating functions of each of a plurality of blocks of the precharge-type carry chained adder circuit, and transfer control elements each for jumping, when the logic gate is active, a carry signal from the preceding block to the succeeding block over the present stage of the block. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 shows a circuit diagram of a one-bit portion of a Manchester-type carry chained adder circuit constructed with N type MOSFETs; Fig. 2 shows a circuit portion of one of a plurality of cascaded connected stages in an adder circuit according to an embodiment of the present invention which stages correspond to a word length; Fig. 3 shows a circuit diagram of a circuit for producing a carry propagation signal and a carry kill signal, which are applied to the adder of Fig. 2; Fig. 4 shows a set of waveforms illustrating the operation of the adder of Fig. 2; and Fig. 5 is a block diagram of a precharge-type carry chained adder circuit which is an embodiment of the present invention. An embodiment of a precharge-type carry chained adder circuit according to the present invention will be described referring to the accompanying drawings. Fig. 2 shows a circuit portion of one stage in a precharge-type carry chained adder circuit. An N type MOSFET 13 is inserted between a carry-in 11 and a carry-out 12. A precharge signal  and a carry propagation signal P are NORed by a NOR gate 14. The output signal of the NOR gate 14 is connected to the gate of the N type MOSFET 13. The carry-in 11 and the carry propagation signal P are exclusively ORed by an EX-OR circuit 15. The output signal from the EX-OR circuit 15 is output as an addition output S. A P type MOSFET 16 is inserted between a power source Vdd and the carry-out 12. The source-drain paths of two N type MOSFETs 17 and 18 are connected in series between the carry-out 12 and a power source Vss. The gates of the MOSFETs 17 and 18, to which a precharge signal  is applied, are interconnected with each other. A carry kill signal K is applied to the gate of the MOSFET 18. The carry propagation signals P and P , and the carry kill signal K can be produced by, for example, a logic circuit as shown in Fig. 3. The circuit of Fig. 3 is coupled with input signals corresponding to an addend Ai and an augend Bi in each adder stage of the adder circuit. To be more specific, the input signals Ai and Bi are applied to a NOR gate 19 and an EX-OR circuit 20, respectively. Then, the NOR gate 19 produces the carry kill signal K. The EX-OR circuit 20 produces the carry propagation signal P. The carry propagation signal P is inverted by an inverter 21 forming a carry propagation signal P . In operation, when the precharge signal  is in "H" level, the node of the carry-out 12 is precharged to "H" level. During the precharging, the carry propagation signal (function) P and the carry kill signal (function) K are produced from the input signals Ai and Bi. These functions P and K are given by the following logical expressions: Then, when the precharge signal  changes to be in "L" level, the adder operates in each stage according to the functions P and K in the following way. When the function P is active, no carry from the present stage to the upper order stage occurs. The node, which has been precharged to "H" level, is discharged to be in "L" level. When the functions P and K are both inactive, a carry is made from the adder stage to the next upper stage. Turning now to Fig. 4, there are shown waveforms for illustrating the operation of the Fig. 3 adder. Logical levels of the input signals Ai and Bi change with time as follows: "0" and "0" at time To; "0" and "1" at time T1; "1" and "0" at time T2; and "1" and "1" at time T3. With the change of the input signals Ai and Bi and the carry-in signal, the adder produces a carry kill signal K, a carry propagation signal P, and an output sum signal S. For an additional word length of n , n stages of the adders each arranged as shown in Fig. 2 must be connected in a cascade fashion. Accordingly, when in all of the stage the function P is active and the carry-in is "0", the nodes of all the adders must be discharged to be in "L" level. Delay times of the transistors connected in series considerably increase as the stages of the adders are increased. In the present invention, the adders of n stages are grouped into blocks each consisting auuers or j stages, from the lower to higher order stages successively. Using a carry-propagating function P of each of the j adders, the block carry propagating function Q is generated: Q = P1, P2···Pj (3), where P1, P2...Pj represent respectively carry-propagating functions of the adder stages, of each block consisting of adders of j stages. Fig. 5 shows a circuit arrangement of a 32-stage adder circuit which is an embodiment of the present invention. Each of adder groups CGo, CG1... CG7 has adders of 4 digits. Each adder is constructed with a CMOS circuit as shown in Fig. 2. In each adder group, a carry of one adder stage is reproduced to a carry input of the next adder stage from the lower order stage to the higher stage. The carry reproduction signals of the 4 stages adders in the adder groups CGo, CG1...CG7 are input to NAND gate Go, G1...G7, respectively. The output signals of the NAND gates Go, G1...G7 are applied through inverters Io, I1...I7 to transfer gates TG1, TG2...TG8, which are provided in association with the adder groups CGo, CG1...CG7. These transfer gates TG1, TG2...TG8 are connected in series with one another. The least significant end of the series connection of the transfer gates is used as a carry-in, while the most significant end of the series circuit is used as the carry-out. This series circuit of the transfer gates serves as a bypass circuit for the carry signal. The outputs of the NAND gates Go, G1...G7 are connected to a first NOR gate G8. The outputs of the succeeding NAND gates G4 - G7 are connected to a second NOR gate G9. The NOR outputs of the NOR gates G8 and G9 are connected to the gates of transfer gates TG9 and TG10, respectively. The transfer gates TG9 and TG10 are connected in series with each other. The least significant end of the series circuit is used as a carry-in, while the most significant end is used as a carry-out. The series circuit serves as a bypass circuit for the carry signal. Such an adder circuit has the longest reproducing time of the carry signal, for example, where a carry kill signal produces at the least significant digit and the carry reproduction is performed at the remaining stages. In such a case, in the conventional adder circuit as mentioned above, a "0" signal must pass through the number of the transfer gates corresponding to the number of stages of the adder circuit. On the other hand, in the Fig. 5 embodiment, when the carry kill signal produces at the least significant stage and the carry reproduction takes place at the remaining stages, the carry reproduces in the following way. A carry signal producing in the least significant adder group CGo passes through three stages of the transfer gates and through the transfer gates TG2, TG3 and TG4 in succession. Then, the carry signal is separated to travel through two routes. One of the separated carry signal travels through transfer gates TG5 and TG6. The other carry signal passes through the transfer gates TG10 and TG8. Therefore, the carries passed through the same number of stages of transfer gates reach the nodes on both sides of the transfer gate TG7, respectively. Accordingly, the adder group CG6 is applied with "0" from both the upper and the lower order stages. A value at each node is established by delays of two stages of the transfer gates. Accordingly, also in such a case, the carry passes through only ten stages of the transfer gates. This number of transfer gates indicates a remarkable reduction in the number of the transfer gates as required, when, even in the worst condition, comparing with 31 stages of the transfer gates of the conventional adder circuit with the same stages as those of the adder circuit of the present invention. Therefore, the reproduction delay of the carrier is considerably reduced. Accordingly, when the block carry reproduction function Q is active, that is, the carry reproducing function of the block is active, a carry signal from the preceding block may be reproduced to the succeeding block, bypassing the present block. At the same time, it is applied with a carry reproducing function also from the upper order side of the block which is bypassed. Therefore, the reproduction time of the carry signal in the block is approximately halved. Alternatively, the block carry reproducing functions of a plurality of blocks are gathered to form a large block. The by-passing operation of the carry signal is applied to these large blocks. If so, the number of transfer gates through which the carry signal passes can further be reduced.

Claims: A carry chained adder circuit having a plurality of carry chained adders connected in a cascade fashion, wherein said cascade of carry chained adders is divided into blocks (CG), and said carry chained adder circuit includes a plurality of NAND gates (G) for generating logical products of the carry reproduction functions of each block (CG), a bypass circuit for jumping a carry signal over a plurality of said blocks (CG) in response to the output signal from each of said NAND gates (G), each for jumping a carry signal over any block (CG) from the preceding block (CG) to the succeeding block (CG), and output terminals of said bypass circuit are connected to cascade-connected points between said blocks (CG) characterized in that said carry chained adders are precharge-type carry chained adders, each adder of said block include a bidirectional transmission gate (13) inserted between a carry-in terminal and a carry-out terminal, and a switching means (17) connected between said carry-out terminal and a power source (V SS ), said transmission gate (13) and said stitching means (17) are turned off during precharge period and turned on during discharge period, and said bypass circuit has at least a bidirectional transmission gate (TG) which is turned off during precharge period and turned on during discharge period, whereby a carry signal from a preceding block may be reproduced to the succeeding block, bypassing the present block, and at the same time it is applied with a carry reproducing function also from the upper order side of the block which is bypassed. The adder circuit according to claim 1, characterized by further comprising logic gate means (G) for logically multiplying the logic products from the plurality of said blocks (CG) applied thereto. The adder circuit according to claim 1, characterized in that said adder circuit is constructed with CMOS.

Inventors: IKUMI, NOBUYUKI C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1984
Main classification: G06F7
IPC codes: G06F7