74HCT161

The 74HCT161 is high-speed Si-gate CMOS devices and are pin compatible with low power Schottky TTL (LSTTL). They are specified in compliance with JEDEC standard no. 7A.

The 74HCT161 is synchronous presettable binary counters which feature an internal look-ahead carry and can be used for high-speed counting. 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 of the 74HCT161 may be preset to a HIGH or LOW level. A LOW level at the parallel enableinput (PE) 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 (providing that the set-up and hold time requirements for PE are met). Preset takes place regardless of the levels at count enable inputs (CEP and CET).

A LOW level at the master reset input (MR) of the 74HCT161 sets all four outputs of the flip-flops (Q₀ to Q₃) to LOW level regardless of the levels at CP, PE, CET and CEP inputs (thus providing an asynchronous clear function).

The look-ahead carry simplifies serial cascading of the counters of the 74HCT161. Both count enable inputs (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 level output of Q₀. This pulse can be used to enable the next cascaded stage.

The maximum clock frequency of the 74HCT161 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) + t SU (CEP to CP)