HD74HC191FP Datasheet

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HD74HC190/HD74HC191

Synchronous Up/Down Decade Counter (Single Clock Line)

Synchronous Up/Donw 4-bit Binary Counter (Single Clock Line)

Description

The HD74HC190 and HD74HC191 are synchronous, reverside up/down counters. The HD74HC190 is a

4-bit decade counter and the HD74HC191 is a 4-bit binary counter. Synchronous counting operation is

provided by having all flip-flops clocked simultaneously so that the outputs change coincident with each

other when so 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 input if the

Enable G input is low. A high at Enable G 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, it

counts down.

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

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

function of the counter will be dictated solely by the condition meeting the stable setup and hold times.

These counters are fully programmable; that is, the outputs may each be preset to either level by placing a

low on the load input and entering the desired data at the data inputs. The output will change 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.

Two outputs have been made available to perform the cascading function. Ripple clock and

maximum/minimum count. The latter output produces a high-level output pulse with a duration

approximately qual to one complete cycle of the clock while the count is zero (all outputs low) counting

down or maximum (9 or 15) counting up. The ripple clock output produces a low-level output pulse under

those same conditions but only while the clock input is low. The counters can be easily cascaded by

feeding the ripple clock output to the enable input of the succeeding counter if parallel clocking is used, or

to the clock input if paralle enabling is used. The maximum/minimum count output can be used to

accomplish look-ahead for high-speed operation.

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