74LS161

SL74LS161 Synchronous 4 Bit Counters; Binary, Direct Reset This synchronous, presettable counter features 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 conicident with each other when so instructed by the count-enable inputs and internal gating. This mode of operation eliminates the output counting spikes that are normally associated with asynchronous (ripple clock) counters. A buffered clock input triggers the four flip-flops on the rising (positivegoing) edge of the clock input wave form. This counter is fully programmable; that is the outputs may be preset to either level. As presetting is synchronous 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 carry look-ahead circuitry provides for cascading counters for n-bit synchronous applications without additional gating. Instrumental in accomplishiing this function are two counter-enable inputs and a ripple carry output. Both countenable inputs (ENABLE P and ENABLE T) must be high to count, and ENABLE T is fed forward to enable the ripple carry output. The ripple carry output thus enabled will produce a high-level output pulse with a duration approximately equal to the high level portion of the Q A output. The high-level overflow ripple carry puls...