SCLS368A - 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'AHCT161 are 4-bit binary counters. Synchronous operation is providedby having all flip-flops clocked simultaneously so that the outputschange coincident with each other when so instructed by thecount-enable (ENP, ENT) inputs and internal gating. This mode ofoperation eliminates the output counting spikes that are normallyassociated 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.
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 'AHCT161 is asynchronous. A low levelat the clear (
) input setsall four of the flip-flop outputs low, regardless of the levels ofthe CLK, load (
), or enableinputs.
The carry look-ahead circuitry provides for cascading counters forn-bit synchronous applications without additional gating.Instrumental in accomplishing this function are ENP, ENT, and aripple-carry (RCO) output. Both ENP and ENT must be high to count,and ENT is fed forward to enable RCO. Enabling RCO produces ahigh-level pulse while the count is maximum (9 or 15 withQA high). This high-level overflow ripple-carry pulse canbe used to enable successive cascaded stages. Transitions at ENP orENT 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 SN54AHCT161 is characterized for operation over the fullmilitary temperature range of -55°C to 125°C. TheSN74AHCT161 is characterized for operation from -40°C to85°C.
View more information about generic part numbers:SN74AHCT161
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