CD74HC161

The 'HC161, 'HCT161, 'HC163, and 'HCT163 are presettable synchronous counters that feature look-ahead carry logic for use in high-speed counting applications. The 'HC161 and 'HCT161 are asynchronous reset decade and binary counters, respectively; the 'HC163 and 'HCT163 devices are decade and binary counters, respectively, that are reset synchronously with the clock. Counting and parallel presetting are both accomplished synchronously with the negative-to-positive transition of the clock.

A CD74HC161 low level on the synchronous parallel enable input, SPE, disables counting operation and allows data at the P0 to P3 inputs to be loaded into the counter (provided that the setup and hold requirements for SPE are met).

All CD74HC161 counters are reset with a low level on the Master Reset input, MR. In the 'HC163 and 'HCT163 counters (synchronous reset types), the requirements for setup and hold time with respect to the clock must be met.

Two CD74HC161 count enables, PE and TE, in each counter are provided for n-bit cascading. In all counters reset action occurs regardless of the level of the SPE, PE and TE inputs (and the clock input, CP, in the 'HC161 and 'HCT161 types).

If a decade counter is preset to an illegal state or assumes an illegal state when power is applied, CD74HC161 will return to the normal sequence in one count as shown in state diagram.

The look-ahead carry feature simplifies serial cascading of the counters. CD74HC161 both count enable inputs (PE and TE) must be high to count. The TE input is gated with the Q outputs of all four stages so that at the maximum count the terminal count (TC) output goes high for one clock period. This TC pulse is used to enable the next cascaded stage.