Integrated Circuit Three-Input Binary Adder Cell With High-Speed Sum Propagation

This invention concerns a three-input binary adder cell with high-speed sum propagation, of integrated circuit design.

It is well known that high-speed carry adder cells are commonly used in the design of integrated multipliers or adders. However, adder cells with high-speed sum propagation, that is to say in which propagation of the sum is favoured with respect to that of the carry, can be especially interesting for example in integrated multipliers of the "pipe-line" type, or serial adding type. This type of multiplier, controlled by a clock and therefore synchronous, produces the partial products from the right to the left, that is to say from the bits of the lowest order toward the highest-order bits, exactly as in manual multiplication, the shift of a supplementary power of two occurring on each clock pulse. This type of multiplier is much slower than the so-called parallel integrated multipliers in which information is propagated asynchronously, but it reduces by a factor of about ten the silicon surface required to produce it with respect to the silicon surface required to produce a parallel miltiplier of the same capacity.

The object of this invention is therefore to furnish a three-input two-output binary adder cell with high-speed propagation of the sums S=A.sym.B.sym.C, of integrated circuit design, which can be used for example in an integrated serial multiplier.

According to the invention, this adder cell contains three two-input complemented exclusive-OR gates and a carry (R=AB+AC+BC) generating circuit. The first gate receives the first two input variables A and B and furnishes the first intermediate variable F.sub.1 =A.sym.B to the second gate which receives, on the other hand, at its other input, the third input variable C. The output of this second gate furnishes the sum variable S=A.sym.B.sym.C. The third gate, receiving at its inputs the input variables A and C respectively, furnishes the second intermediate variable F.sub.3 =A.sym.C to the carry generating circuit which receives, on the other hand, from the first gate its output F.sub.1 =A.sym.B and from the second gate the sum output S. This carry generating circuit furnishes the carry variable R at its output.

The invention will be better understood and other characteristics will be brought out in the following description and attached drawing in which:

FIG. 1 shows a block diagram used to explain the principle of the adder cell according to this invention; and

FIG. 2 is a schematic diagram showing the detail of the elements of the cell shown in FIG. 1.

FIG. 1 is a diagram showing the adder cell with three inputs A, B and C and two outputs S and R in accordance with this invention. This cell includes three two-input complemented exclusive-OR gates P1, P2 and P3, and a circuit P0 generating the carry R from the output signals furnished by the complemented exclusive-OR gates. The first gate P1 receives as its first input the first binary variable A and at its second input the second binary variable B. It furnishes the first intermediate binary variable F.sub.1 =A.sym.B at its output. The second complemented exclusive-OR gate P2 receives at its first input this first intermediate variable F.sub.1 and at its second input the third input binary variable C. At its output, it furnishes the binary sum variable S which constitutes the first output variable of the adder cell. The third complemented exclusive-OR gate P3 receives at its first input the first binary variable A and at its second input the third binary variable C. It furnishes at its output the second intermediate variable F.sub.3 =A.sym.C. The circuit P0 generating the carry R receives at its first input the output S=A.sym.B.sym.C from the second complemented exclusive-OR gate P2, at its second input the output F.sub.1 =A.sym.B from the first complemented exclusive-OR gate P1 and at its third input the output F.sub.3 =A.sym.C from the third complemented exclusive-OR gate P3. It furnishes at its output the binary carry variable R=AB+AC+BC which constitutes the second binary output variable of the adder cell.

FIG. 2 represents an adder cell, according to this invention, of MOS integrated circuit design. It contains fourteen elements, ten of which are transistors and four are resistors. The carry R generating circit P0 contains four transistors T1, T2, T3 and T4 and one resistor T5. In order to simplify the description, the drain and the source of each of the transistors will be designated under the common designation "terminals", the difference being easily resolved by anyone skilled in the art by means of the direction of the current caused by the source utilized. Only the gate will be designated as such. The first two transistors T1 and T2, the resistor T5 and the dipole, consisting of transistors T3 and T4 in series, have a common point which constitutes the output of this carry generating circuit. The second terminal of the resistor T5 is connected to the power supply. The second terminal of transistor T1 is connected to the gate of transistor T4. The second terminal of transistor T2 is connected to the gate of transistor T3. And the second terminal of the dipole, consisting of transistors T3 and T4, is connected to the gate of transistor T1, which is itself connected to the gate of transistor T2. This common point of transistors T1 and T2 constitutes the first input to the carry R generating circuit, receiving the sum variable S from the second gate P2. The second input to this circuit consists of the gate of transistor T3 which receives the variable F.sub.1 =A.sym.B from the first gate P1. The third input to this circuit consists of the gate of transistor T4 which receives the variable F.sub.3 =A.sym.C from the third gate P3. This carry generating circuit performs the logic function:

This expression corresponds properly to the logical carry expression of the addition of three binary variables R=S(F.sub.1 +F.sub.1 F.sub.3)+SF.sub.1 F.sub.3, which can be derived from the addition truth table given below, for the case in which we choose to write the expression of this carry as a function of the three variables S, F.sub.1 =A.sym.B and F.sub.3 =A.sym.C.

The sum S and the intermediate variables F.sub.1 and F.sub.3 are generated by three identical MOS transistor networks, of well known type, used to constitute a complemented exclusive-OR gate by means of only two MOS transistors and a resistor.

The first gate P1 contains two transistors T6 and T7 and a resistor T8. These three components have a common terminal which constitutes the output of the gate furnishing the intermediate variable F.sub.1 =A.sym.B. The second terminal of the resistor T8 is connected to the power supply. The second terminal of the first transistor T6 is connected to the gate of the second transistor T7, both receiving the second input variable B. The second terminal of the second transistor T7 is connected to the gate of the first transistor T6, this terminal and this gate receiving the first input variable A.

The second gate P2 contains two transistors T9 and T10 and a resistor T11. These three components have a common terminal like the components T6, T7 and T8 of the first gate. This second gate P2 is identical with the first, transistor T9 replacing transistor T6, transistor T10 replacing transistor T7 and resistor T11 replacing resistor T8. The gate of the first transistor T9 receives the first input variable of this gate F.sub.1 =A.sym.B, and the gate of the second transistor T10 receives the second input variable of this gate which is the third input variable C of the adder cell.

The third gate P3 is identical with the first two gates P1 and P2 and contains as first transistor the transistor T12, as second transistor the transistor T13 and as resistor the resistor T14. The gate of the first transistor receives the first input variable A of the adder cell and the gate of the second transistor T13 receives its second input variable C which constitutes the third input variable of the cell.

Although this invention has been described in connection with a particular embodiment, it is clearly not limited to the said embodiment and is capable of modifications or variants still lying within its scope.