产品描述

SN74HC161DR/AIP74HC161 和74LS161都是常用的四位二进制可预置的同步加法计数器，74HC161是CMOS型，74LS161是TTL型。它可以灵活的运用在各种数字电路，以及单片机系统中实现分频器等很多重要的功能。

The AiP74HC/HCT161 is a synchronous presettable binary counter with an internal look-head carry. Synchronous operation is provided by having all flip-flops clocked simultaneously on the positive-going edge of the clock (CP). The outputs (Q0 to Q3) of the counters may be preset HIGH or LOW. A LOW at the parallel enable input (PˉEˉ) disables the counting action and causes the data at the data inputs (D0 to D3)to be loaded into the counter on the positive-going edge of the clock. Preset takes place regardless of the levels at count enable inputs (CEP and CET).

A LOW at the master reset input (MˉRˉ) sets Q0 to Q3 LOW regardless of the levels at input pins CP, PˉEˉ, CET and CEP (thus providing an asynchronous clear function). The look-ahead carry simplifies serial cascading of the counters. Both CEP and CET must be HIGH to count. The CET input is fed forward to enable the terminal count output (TC). The TC output thus enabled will produce a HIGH output pulse of a duration approximately equal to a HIGH output of Q0.

This pulse can be used to enable the next cascaded stage. The maximum clock frequency for the cascaded counters is determined by the CP to TC propagation delay and CEP to CP set-up time, according to the following formula:

fmax=1/(tP(max)(CP to TC)+tSU(CEP to CP))

Inputs include clamp diodes. This enables the use of current limiting resistors to interface inputs to voltages in excess of VCC.