SCAS086 - D3200, OCTOBER 1989 - REVISED APRIL 1993
m Process EPIC is a trademark of Texas Instruments Incorporated.
The 74ACT11181 is an arithmetic logic unit (ALU)/functiongenerator that has a complexity of 75 equivalent gates on amonolithic chip. These circuits perform 16 binary arithmeticoperations on two 4-bit words as shown in Tables 1 and 2. Theseoperations are selected by the four function-select lines (S0, S1,S2, S3) and include addition, subtraction, decrement, and straighttransfer. When performing arithmetic manipulations, the internalcarries must be enabled by applying a low-level voltage to the modecontrol input (M). A full carry look-ahead scheme is made availablein these devices for fast, simultaneous carry generation by means oftwo cascade outputs G\ and P\ for the four bits in the package. Whenused in conjunction with the 'ACT11882 full carry look-aheadcircuits, high-speed arithmetic operations can be performed. Themethod of cascading 'ACT11882 circuits with these ALUs to providemultilevel full-carry look-ahead operation is illustrated undersignal designations.
If high speed is not important, a ripple-carry input(Cn) and a ripple-carry output (Cn + 4) areavailable. However, the ripple-carry delay has also been minimized sothat arithmetic manipulations for small word lengths can be performedwithout external circuitry.
The 74ACT11181 will accommodate active-high or active-low data ifthe pin designations are interpreted as follows:
Subtraction is accomplished by 1's complement addition where the1's complement of the subtrahend is generated internally. Theresultant output is A - B - 1, which requires an end-around or forcedcarry to provide A - B.
The 74ACT11181 can also be used as a comparator. The A = B outputis internally decoded from the function outputs (F0, F1, F2, F3) sothat when two words of equal magnitude are applied at the A and Binputs, it will assume a high level to indicate equality (A = B).When performing this comparison, the ALU must be in the subtract modewith Cn = H. The A = B output is open drain so that it canbe wired-AND connected to give a comparison for more than four bits.The carry output (Cn + 4) can also be used to supplyrelative magnitude information. Again, the ALU must be placed in thesubtract mode by placing the function select inputs S3, S2, S1, S0 atL, H, H, L, respectively.
These circuits have been designed not only to incorporate all ofthe designer's requirements for arithmetic operation but also toprovide 16 possible functions of two Boolean variables without usingexternal circuitry. These logic functions are selected using the fourfunction-select inputs (S0, S1, S2, S3) with the mode-control input(M) at a high level to disable the internal carry. The 16 logicfunctions are detailed in Tables 1 and 2 and include exclusive-OR,NAND, AND, NOR, and OR functions.
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