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Single Down/Up Count-Control Line

Look-Ahead Circuitry Enhances Speed of

Cascaded Counters

Fully Synchronous in Count Modes

Asynchronously Presettable With Load

Control

Package Options Include Plastic

Small-Outline (D) Packages, Ceramic Chip

Carriers (FK), and Standard Plastic (N) and

Ceramic (J) 300-mil DIPs

description

The ’ALS191A are synchronous 4-bit reversible

up/down binary counters. Synchronous counting

operation is provided by having all flip-flops

clocked simultaneously so that the outputs

change coincidentally with each other when

instructed by the steering logic. This mode of

operation eliminates the output counting spikes

normally associated with asynchronous

(ripple-clock) counters.

The outputs of the four flip-flops are triggered on

a low-to-high-level transition of the clock (CLK)

input if the count enable (CTEN) input is low. A

high at CTEN inhibits counting. The direction of

the count is determined by the level of the

down/up (D/U) input. When D/U is low, the counter

counts up, and when D/U is high, the counter

counts down.

These counters feature a fully independent clock circuit. Changes at the control inputs (CTEN and D/U) that

modify the operating mode have no effect on the contents of the counter until clocking occurs. The function of

the counter is dictated solely by the conditions meeting the stable setup and hold times.

These counters are fully programmable. Each output can be preset to either level by placing a low on the LOAD

input and entering the desired data at the data inputs. The output changes to agree with the data inputs

independently of the level of the clock input. This feature allows the counters to be used as modulo-N dividers

by simply modifying the count length with the preset inputs.

CLK, D/U, and LOAD are buffered to lower the drive requirement, which significantly reduces the loading on

(current required by) clock drivers, for long parallel words.