SN54HC191, SN74HC191

4BIT SYNCHRONOUS UP/DOWN BINARY COUNTERS

SCLS121D − DECEMBER 1982 − REVISED OCTOBER 2003

2

POST OFFICE BOX 655303

description/ordering information (continued)

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

modifies 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 condition meeting the stable setup and hold times.

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

low on the load (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 CLK. This feature allows the counters to be used as modulo-N

dividers simply by modifying the count length with the preset inputs.

Two outputs are available to perform the cascading function: ripple clock (RCO) and maximum/minimum

(MAX/MIN) count. MAX/MIN produces a high-level output pulse with a duration approximately equal to one

complete cycle of the clock while the count is zero (all outputs low) counting down, or maximum (9 or 15)

counting up. RCO produces a low-level output pulse under those same conditions, but only while CLK is low.

The counters can be cascaded easily by feeding RCO to CTEN of the succeeding counter if parallel clocking

is used, or to CLK if parallel enabling is used. MAX/MIN can be used to accomplish look ahead for high-speed

operation.