SN74F168

This SN74F168 synchronous, presettable, 4-bit up/down decade counter features an internal carry look-ahead circuitry for cascading in high-speed counting applications. Synchronous operation is provided by having all flip-flops clocked simultaneously so that the outputs change coincident with each other when so instructed by the count-enable (ENP, ENT) 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 (CLK) input triggers the four flip-flops on the rising (positive-going) edge of the clock waveform.

This SN74F168 counter is fully programmable; that is, it may be preset to any number between 0 and its maximum count. The load-input circuitry allows loading with the carry-enable output of cascaded counters. As loading is synchronous, setting up a low level at the load (LOAD) input disables the counter and causes the outputs to agree with the data inputs after the next clock pulse.

The carry look-ahead circuitry SN74F168 provides for cascading counters for n-bit synchronous application without additional gating. Instrumental in accomplishing this function are two count-enable (ENP, ENT) inputs and a ripple-carry (RCO) output. Both ENP and ENT must be low to count. The direction of the count is determined by the level of the up/down (U/D) input. When U/D is high, the counter counts up; when low, it counts down. Input ENT is fed forward to enable the RCO. RCO thus enabled will produce a low-level pulse while the count is zero (all inputs low) counting down or maximum (9 or 15) counting up. This low-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 the clock input. All inputs are diode clamped to minimize transmission-line effects, thereby simplifying system design.

The SN74F168 features a fully independent clock circuit. Changes at control inputs (ENP, ENT, LOAD or U/D) that will 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) will be dictated solely by the conditions meeting the setup and hold times.

The SN74F168 is characterized for operation from 0°C to 70°C.