www.fairchildsemi.com

2

Unit Loading/Fan Out

Note 1: OC-Open Collector

Functional Description

The 74F181 is a 4-bit high-speed parallel Arithmetic Logic

Unit (ALU). Controlled by the four Function Select inputs

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 car-

ries 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 per-

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

device incorporates full internal carry lookahead and pro-

vides for either ripple carry between devices using the

using the signals P (Carry Propagate) and G (Carry Gener-

ate). In the Add mode, P indicates that F is 15 or more,

while G indicates that F is 16 or more. In the Subtract mode

P indicates that F is zero or less, while G indicates that F is

less than zero. P and G are not affected by carry in. When

speed requirements are not stringent, the 74F181 can be

used in a simple Ripple Carry mode by connecting the

next unit. For high speed operation the device is used in

conjunction with a carry lookahead circuit. One carry looka-

head package is required for each group of four 74F181

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 device 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 than four bits. The A

= B signal can also be used with

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 complemen-

tary 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 indi-

cated, this device 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 oper-

ands labeled inside the logic symbol.

Pin Names

Description

U.L.

HIGH/LOW

A Operand Inputs (Active LOW)

1.0/3.0

20

µA/−1.8 mA

B Operand Inputs (Active LOW)

1.0/3.0

20

µA/−1.8 mA

Function Select Inputs

1.0/4.0

20

µA/−2.4 mA

M

Mode Control Input

1.0/1.0

20

µA/−0.6 mA

Carry Input

1.0/5.0

20

µA/−3.0 mA

Function Outputs (Active LOW)

50/33.3

−1 mA/20 mA

A

= B

Comparator Output

OC (Note 1)/33.3

(Note 1)/20 mA

G

Carry Generate Output (Active LOW)

50/33.3

−1 mA/20 mA

P

Carry Propagate Output (Active LOW)

50/33.3

−1 mA/20 mA

Carry Output

50/33.3

−1 mA/20 mA