© 2000 Fairchild Semiconductor Corporation

DS006291

www.fairchildsemi.com

April 1984

Revised March 2000

DM74AS161 • DM74AS163

Synchronous 4-Bit Counter with Asynchronous Clear •

Synchronous 4-Bit Counter

General Description

These synchronous presettable counters feature an inter-

nal carry look ahead for application in high speed counting

designs. The DM74AS161 and DM74AS163 are 4-bit

binary counters. The DM74AS161 clear asynchronously,

while the DM74AS163 clear synchronously. The carry out-

put is decoded to prevent spikes during normal counting

mode of operation. Synchronous operation is provided by

having all flip-flops clocked simultaneously so that outputs

change coincident with each other when so instructed by

count enable inputs and internal gating. This mode of oper-

ation eliminates the output counting spikes which are nor-

mally

associated

with

asynchronous

(ripple

clock)

counters. A buffered clock input triggers the four flip-flops

on the rising (positive-going) edge of the clock input wave-

form.

These counters are fully programmable, that is, the outputs

may each be preset to either level. As presetting is syn-

chronous, setting up a low level at the LOAD input disables

the counter and causes the outputs to agree with set up

data after the next clock pulse regardless of the levels of

enable input. LOW-to-HIGH transitions at the LOAD input

are perfectly acceptable regardless of the logic levels on

the clock or enable inputs.

The DM74AS161 clear function is asynchronous. A low

level at the clear input sets all four of the flip-flop outputs

LOW regardless of the levels of clock, load or enable

inputs. This counter is provided with a clear on power-up

feature. The DM74AS163 clear function is synchronous;

and a low level at the clear input sets all four of the flip-flop

outputs LOW after the next clock pulse, regardless of the

levels of enable inputs. This synchronous clear allows the

count length to be modified easily, as decoding the maxi-

mum count desired can be accomplished with one external

NAND gate. The gate output is connected to the clear input

to synchronously clear the counter to all LOW outputs.

LOW-to-HIGH

transitions

at

the

clear input

of the

DM74AS163 is also permissible regardless of the levels of

logic on the clock, enable or load inputs.

The carry look ahead circuitry provides for cascading

counters for n bit synchronous application without addi-

tional gating. Instrumental in accomplishing this function

are two count-enable inputs (P and T) and a ripple carry

output. Both count-enable inputs must be HIGH to count.

The T input is fed forward to enable the ripple carry output.

The ripple carry output thus enabled will produce a high

level output pulse with a duration approximately equal to

the high level portion of QA output. This high level overflow

ripple carry pulse can be used to enable successive cas-

caded stages. HIGH-to-LOW level transitions at the enable

P or T inputs of the DM74AS161 and DM74AS163, may

occur regardless of the logic level on the clock.

The DM74AS161 and DM74AS163 feature a fully indepen-

dent clock circuit. Changes made to control inputs (enable

P or T, or load) that will modify the operating mode will

have no effect until clocking occurs. The function of the

counter (whether enabled, disabled, loading or counting)

will be dictated solely by the conditions meeting the stable

set-up and hold times.

Features

s Switching specifications at 50 pF

s Switching specifications guaranteed over full tempera-

s Advanced oxide-isolated, ion-implanted Schottky TTL

process

s Functionally and pin-for-pin compatible with Schottky

and low power Schottky TTL counterpart

s Improved AC performance over Schottky and low power

Schottky counterparts

s Synchronously programmable

s Internal look ahead for fast counting

s Carry output for n-bit cascading

s Synchronous counting

s Load control line

s ESD inputs

Ordering Code:

Devices also available in Tape and Reel. Specify by appending the suffix letter “X” to the ordering code.

Order Number

Package Number

Package Description

DM74AS161M

M16A

16-Lead Small Outline Integrated Circuit (SOIC), JEDEC MS-012, 0.150 Narrow

DM74AS161N

N16E

16-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300 Wide

DM74AS163M

M16A

16-Lead Small Outline Integrated Circuit (SOIC), JEDEC MS-012, 0.150 Narrow

DM74AS163N

N16E

16-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300 Wide