DM74161

    These DM74161 synchronous presettable counters feature an inter-nal carry look-ahead for application in high-speed counting designs The DM74161 and DM74163  are 4-bit binary counters The carry output is decoded by means of a NOR gate thus pre-venting spikes during the normal counting mode of opera-tion Synchronous operation is provided by having all flip-flops clocked simultaneously so that the outputs change co-incident with each other when so instructed by the count-enable inputs and internal gating This mode of operation eliminates the output counting spikes which are normally associated with asynchronous (ripple clock) counters A buffered clock input triggers the four flip-flops on the rising(positive-going) edge of the clock input waveform

   These DM74161 counters are 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 input The clear function for the DM74161 is asynchronous and a low level at the clear input sets all four of the flip-flop out-puts low regardless of the levels of clock load or enable inputs The clear function for the DM74163 is synchronous and a low level at the clear input sets all four of the flip-flop out-puts low after the next clock pulse regardless of the levels of the enable inputs This synchronous clear allows the count length to be modified easily as decoding the maxi-mum count desired can be accomplished with one external NAND gate The gate output is connected to the clear input to synchronously clear the counter to all low out-puts Low-to-high transitions at the clear input of the DM74163 are also permissible regardless of the logic levels on the clock enable or load inputs

  The carry look-ahead circuitry provides for cascading coun-ters for n-bit synchronous applications without additional gating Instrumental in accomplishing this function are two count-enable inputs and a ripple carry output Both count-enable inputs (P and T) must be high to count and input T is fed forward to enable the ripple carry output The ripple car-ry output thus enabled will produce a high-level output pulse with a duration approximately equal to the high-level portion of the QA output This high-level overflow ripple carry pulse can be used to enable successive cascaded stages High-to-low-level transitions at the enable P or T inputs of the DM74161 through DM74163 may occur regardless of the logic level on the clock