中国·必威(bw·西汉姆联)中文官方网站-West Ham Unite

74HC19274系列产品详情

The 74HC/HCT192 are 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 74HC/HCT192 are synchronous BCD up/down counters. Separate up/down clocks, CPU and CPD respectively, simplify operation. The outputs change state synchronously with the LOW-to-HIGH transition of either clock input. If the CPU clock is pulsed while CPD is held HIGH, the device will count up. If the CPD clock is pulsed while CPU is held HIGH, the device will count down. Only one clock input can be held HIGH at any time, or erroneous operation will result. The device can be cleared at any time by the asynchronous master reset input (MR); it may also be loaded in parallel by activating the asynchronous parallel load input The “192” contains four master-slave JK flip-flops with the necessary steering logic to provide the asynchronous reset, load, and synchronous count up and count down functions. Each flip-flop contains JK feedback from slave to master, such that a LOW-to-HIGH transition on the CPD input will decrease the count by one, while a similar transition on the CPU input will advance the count by one. One clock should be held HIGH while counting with the other, otherwise the circuit will either count by two’s or not at all, depending on the state of the first flip-flop, which cannot toggle as long as either clock input is LOW. Applications requiring reversible operation must make the reversing decision while the activating clock is HIGH to avoid erroneous counts. The terminal count up and terminal count down outputs are normally HIGH. When the circuit has reached the maximum count state of 9, the next HIGH-to-LOW transition of CPU will cause to go LOW. will stay LOW until CPU goes HIGH again, duplicating the count up clock. Likewise, the output will go LOW when the circuit is in the zero state and the CPD goes LOW. The terminal count outputs can be used as the clock input signals to the next higher order circuit in a multistage counter, since they duplicate the clock wave forms. Multistage counters will not be fully synchronous, since there is a slight delay time difference added for each stage that is added.

The counter may be preset by the asynchronous parallel load capability of the circuit. Information present on the parallel data inputs (D0 to D3) is loaded into the counter and appears on the outputs (Q0 to Q3) regardless of the conditions of the clock inputs when the parallel load (PL) input is LOW. A HIGH level on the master reset (MR) input will disable the parallel load gates, override both clock inputs and set all outputs (Q0 to Q3) LOW. If one of the clock inputs is LOW during and after a reset or load operation, the next LOW-to-HIGH transition of that clock will be interpreted as a legitimate signal and will be counted.

FEATURES

Synchronous reversible counting

Asynchronous parallel load

Asynchronous reset

Expandable without external logic

Output capability: standard

ICC category: MSI