CD74ACT163

具有同步复位的同步可预设的二进制计数器

The ’ACT163 devices are 4-bit binary counters. These synchronous, presettable counters feature an internal carry look-ahead for application in high-speed counting designs. Synchronous operation is provided by having all flip-flops clocked simultaneously so that the outputs change, coincident with each other, when instructed by the count-enable ENP, ENT inputs and internal gating. This mode of operation eliminates the output counting spikes normally associated with synchronous ripple-clock counters. A buffered clock CLK input triggers the four flip-flops on the rising positive-going edge of the clock waveform.

The counters 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 synchronous. A low level at the clear CLR\ input sets all four of the flip-flop outputs low after the next low-to-high transition of CLK, regardless of the levels of the enable inputs. This synchronous clear allows the count length to be modified easily by decoding the Q outputs for the maximum count desired. The active-low output of the gate used for decoding is connected to CLR\ to synchronously clear the counter to 0000 LLLL.

The carry look-ahead circuitry provides for cascading counters for n-bit synchronous applications without additional gating. ENP, ENT, and a ripple-carry output RCO are instrumental in accomplishing this function. 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.

These devices 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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Inputs Are TTL-Voltage Compatible

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Internal Look-Ahead for Fast Counting

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Carry Output for n-Bit Cascading

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Synchronous Counting

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Synchronously Programmable