SCLS370A - MAY 1997 - REVISED JUNE 1997
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These synchronous, presettable counters feature an internal carrylook-ahead for application in high-speed counting designs. The'AHCT163 are 4-bit binary counters. Synchronous operation is providedby having all flip-flops clocked simultaneously so that the outputschange coincident with each other when instructed by the count-enable(ENP, ENT) inputs and internal gating. This mode of operationeliminates the output counting spikes normally associated withsynchronous (ripple-clock) counters. A buffered clock (CLK) inputtriggers the four flip-flops on the rising (positive-going) edge ofthe clock waveform.
These counters are fully programmable; that is, they can be presetto any number between 0 and 9 or 15. As presetting is synchronous,setting up a low level at the load input disables the counter andcauses the outputs to agree with the setup data after the next clockpulse, regardless of the levels of the enable inputs.
The clear function for the 'AHCT163 is synchronous. A low level atthe clear (
) input setsall four of the flip-flop outputs low after the next low-to-hightransition of CLK, regardless of the levels of the enable inputs.This synchronous clear allows the count length to be modified easilyby decoding the Q outputs for the maximum count desired. Theactive-low output of the gate used for decoding is connected to
The carry look-ahead circuitry provides for cascading counters forn-bit synchronous applications without additional gating. ENP, ENT,and a ripple-carry output (RCO) are instrumental in accomplishingthis function. Both ENP and ENT must be high to count, and ENT is fedforward to enable RCO. Enabling RCO produces a high-level pulse whilethe count is maximum (9 or 15 with QA high). Thishigh-level overflow ripple-carry pulse can be used to enablesuccessive cascaded stages. Transitions at ENP or ENT are allowed,regardless of the level of CLK.
These counters feature a fully independent clock circuit. Changesat control inputs (ENP, ENT, or 
) that modify the operating mode have no effect on thecontents of the counter until clocking occurs. The function of thecounter (whether enabled, disabled, loading, or counting) is dictatedsolely by the conditions meeting the stable setup and hold times.
The SN54AHCT163 is characterized for operation over the fullmilitary temperature range of -55°C to 125°C. TheSN74AHCT163 is characterized for operation from -40°C to85°C.
View more information about generic part numbers:SN74AHCT163
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