SN54AS181B, SN74AS181A

ARITHMETIC LOGIC UNITS/FUNCTION GENERATORS

SDAS209B – DECEMBER 1982 – REVISED DECEMBER 1994

2

POST OFFICE BOX 655303

description (continued)

The SN54AS181B and SN74AS181A also can be used as comparators. The A = B output is internally decoded

from the function (F0, F1, F2, F3) outputs so that when two words of equal magnitude are applied at the A and

B inputs, the output assumes a high level to indicate equality (A = B). The ALU must be in the subtract mode

information. The ALU must be placed in the subtract mode by placing the function-select inputs S3, S2, S1, and

S0 at L, H, H, and L, respectively.

INPUT

Cn

OUTPUT

Cn + 4

ACTIVE-LOW DATA

(Figure 1)

ACTIVE-HIGH DATA

(Figure 2)

H

H

A

≥ B

A

≤ B

H

L

A < B

A > B

L

H

A > B

A < B

L

L

A

≤ B

A

≥ B

These circuits not only incorporate all of the designer’s requirements for arithmetic operations, but also provide

16 possible functions of two Boolean variables without using external circuitry. These logic functions are

selected by the four function-select inputs with M at a high level to disable the internal carry. The 16 logic

functions are detailed in Tables 1 and 2 and include exclusive-OR, NAND, AND, NOR, and OR functions.

TYPICAL ADDITION TIME

(CL = 15 pF, RL = 280 Ω, TA = 25°C)

ADDITION

PACKAGE COUNT

NUMBER

OF BITS

TIME USING

′S181 AND

′S182

ALUs

LOOK-AHEAD

CARRY

GENERATORS

CARRY METHOD

BETWEEN ALUs

1 to 4

11 ns

1

None

5 to 8

18 ns

2

Ripple

9 to 16

19 ns

3 or 4

1

Full look ahead

17 to 64

28 ns

5 to 16

2 to 5

Full look ahead

The SN54AS181B is characterized for operation over the full military temperature range of – 55

°C to 125°C. The

SN74AS181A is characterized for operation from 0

°C to 70°C.

application note

An application-specific problem has been identified in the SN54AS181B device. The F0 – F4 outputs exhibit

voltage transients when one or more B-data inputs transition from a high to a low state. The resultant voltage

transient pulse occurs coincidentally with the high-to-low transition of the B-data input(s) and appears to be

caused by internal coupling.

In system operations in which this device is used, it is likely that transmission-line effects minimize this anomaly.

Narrow width of the voltage transient makes the pulse transparent to most circuitry; however, in certain

applications, the transients can cause system errors.