SN54LS669

These synchronous presettable counters feature an internal carry look-ahead for cascading in high-speed counting applications. The 'LS668 are decade counters and the 'LS669 are 4-bit binary counters. 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 inputs and internal gating. This mode of operation helps eliminate the output counting spikes that are normally associated with asynchronous (ripple-clock) counters. A buffered clock input triggers the four master-slave flip-flops on the rising (positive-going) edge of the clock waveform.

These counters are fully programmable; that is, the outputs may each be preset to either level. 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 input disables the counter and causes the outputs to agree with the data inputs after the next clock pulse.

The carry look-ahead circuitry provides for cascading counters for n-bit synchronous applications without additional gating. Instrumental in accomplishing this function are two count-enable inputs and a carry output. Both count enable inputs (P\ and T\) must be low to count. The direction of the count is determined by the level of the up/down input. When the input is high, the counter counts up; when low, it counts down. Input T\ is fed forward to enable the carry output. The carry output thus enabled will produce a low-level output pulse when the count is maximum counting up or zero counting down. This low-level overflow carry pulse can be used to enable successive cascaded stages. Transitions at the enable P\ or T\ inputs are allowed regardless of the level of the clock input. All inputs are diode-clamped to minimize transmission-line effects, thereby simplifying system design.

These counters feature a fully independent clock circuit. Changes at control inputs (enable P\, enable T\, load, up/down) that will modify the operating mode have no effect until clocking occurs. The function of the counter (whether enabled, disabled, loading, or counting) will be dictated solely by the conditions meeting the stable setup and hold times.

The 'LS668 and 'LS669 are completely new designs. Compared to the original 'LS168 and 'LS169, they feature 0-nanosecond minimum hold time, reduced input currents I_IH and I_IL, and all buffered outputs.