SDAS209B - DECEMBER 1982 - REVISED DECEMBER 1994
The SN54AS181B and SN74AS181A arithmetic logic units(ALUs)/function generators have a complexity of 75 equivalent gateson a monolithic chip. These circuits perform 16 binary arithmeticoperations on two 4-bit words as shown in Tables1 and 2. Theseoperations are selected by the four function-select (S0, S1, S2, andS3) lines and include addition, subtraction, decrement, and straighttransfer. When performing arithmetic manipulations, the internalcarries are enabled by applying a low-level voltage to themode-control (M) input. A full carry look-ahead scheme is used togenerate fast, simultaneous carry by means of two cascade (G\ and P\)outputs for the four bits in the package.
If high speed is not important, a ripple-carry (Cn)input and a ripple-carry (Cn + 4) output are available.The ripple-carry delay is minimized so that arithmetic manipulationsfor small word lengths can be performed without external circuitry.
The SN54AS181B and SN74AS181A accommodate active-high oractive-low data if the 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 SN54AS181B and SN74AS181A also can be used as comparators. TheA = B output is internally decoded from the function (F0, F1, F2, F3)outputs so that when two words of equal magnitude are applied at theA and B inputs, the output assumes a high level to indicate equality(A = B). The ALU must be in the subtract mode with Cn = Hwhen performing this comparison. The A = B output is open collectorso that it can be wire-AND connected to give a comparison for morethan four bits. Cn + 4 also can be used to supply relativemagnitude information. The ALU must be placed in the subtract mode byplacing the function-select inputs S3, S2, S1, and S0 at L, H, H, andL, respectively.
These circuits not only incorporate all of the designer'srequirements for arithmetic operations, but also provide 16 possiblefunctions of two Boolean variables without using external circuitry.These logic functions are selected by the four function-select inputswith 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.
The SN54AS181B is characterized for operation over the fullmilitary temperature range of -55°C to 125°C. TheSN74AS181A is characterized for operation from 0°C to 70°C.
View more information about generic part numbers:SN54AS181B, SN74AS181A
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