6 Detailed Description

6.1 Overview

The 'AC161 devices are 4-bit binary counters. These synchronous, presettable counters feature an internal carry

look-ahead for application in high-speed counting These devices are fully programmable; that is, they can be

preset to any number between 0 and 9 or 15. Presetting is synchronous; therefore, setting up a low level at the

load input disables the counter and causes the outputs to agree with the setup data after the next clock pulse,

regardless of the levels of the enable inputs.

The clear function is asynchronous. A low level at the clear (CLR) input sets all four of the flip-flop outputs low,

regardless of the levels of the CLK, load (LOAD), or enable inputs.

The carry look-ahead circuitry provides for cascading counters for n-bit synchronous applications without

additional gating. Instrumental in accomplishing this function are ENP, ENT, and a ripple-carry output (RCO).

Both ENP and ENT must be high to count, and ENT is fed forward to enable RCO. Enabling RCO produces a

high-level pulse while the count is maximum (9 or 15, with QA high). This high-level overflow ripple-carry pulse

can be used to enable successive cascaded stages. Transitions at ENP or ENT are allowed, regardless of the

level of CLK.

The counters feature a fully independent clock circuit. Changes at control inputs (ENP, ENT, or LOAD) that

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

the counter (whether enabled, disabled, loading, or counting) is dictated solely by the conditions meeting the

stable setup and hold times.

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CD54AC161, CD74AC161

SCHS239D – NOVEMBER 1998 – REVISED MAY 2024

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Product Folder Links: CD54AC161 CD74AC161