5-333

FAST AND LS TTL DATA

SN54/74LS181

2

LOGIC DIAGRAM

S0 S1

S2 S3

Cn M

A = B

Cn+4

A0

B0 A1

A2

A3

B1

B2

B3

F0

F1

F2

F3

P

G

14

1

6

7

3

8

4

5

9

11

16

10

13

15

VCC = PIN 24

GND = PIN 12

= PIN NUMBERS

21

22

23

20

18

19

17

FUNCTIONAL DESCRIPTION

The SN54 / 74LS181 is a 4-bit high speed parallel Arithmetic

Logic Unit (ALU). Controlled by the four Function Select Inputs

(S0 . . . S3) and the Mode Control Input (M), it can perform all

the 16 possible logic operations or 16 different arithmetic

operations on active HIGH or active LOW operands. The

Function Table lists these operations.

When the Mode Control Input (M) is HIGH, all internal

carries are inhibited and the device performs logic operations

on the individual bits as listed. When the Mode Control Input is

LOW, the carries are enabled and the device performs

arithmetic operations on the two 4-bit words. The device

incorporates full internal carry lookahead and provides for

either ripple carry between devices using the Cn+4 output, or

for carry lookahead between packages using the signals P

(Carry Propagate) and G (Carry Generate), P and G are not

affected by carry in. When speed requirements are not

stringent, the LS181 can be used in a simple ripple carry mode

by connecting the Carry Output (Cn+4) signal to the Carry Input

(Cn) of the next unit. For high speed operation the LS181 is

used in conjunction with the 9342 or 93S42 carry lookahead

circuit. One carry lookahead package is required for each

group of the four LS181 devices. Carry lookahead can be

provided at various levels and offers high speed capability

over extremely long word lengths.

The A = B output from the LS181 goes HIGH when all four F

outputs are HIGH and can be used to indicate logic

equivalence over four bits when the unit is in the subtract

mode. The A = B output is open collector and can be

wired-AND with other A = B outputs to give a comparison for

more then four bits. The A = B signal can also be used with the

Cn+4 signal to indicate A>B and A<B.

The Function Table lists the arithmetic operations that are

performed without a carry in. An incoming carry adds a one to

each operation. Thus, select code LHHL generates A minus B

minus 1 (2s complement notation) without a carry in and

generates A minus B when a carry is applied. Because

subtraction is actually performed by complementary addition

(1s complement), a carry out means borrow; thus a carry is

generated when there is no underflow and no carry is

generated when there is underflow.

As indicated, the LS181 can be used with either active LOW

inputs producing active LOW outputs or with active HIGH

inputs producing active HIGH outputs. For either case the

table lists the operations that are performed to the operands

labeled inside the logic symbol.