These synchronous presettable counters feature an internal carry look-ahead
for cascading in high speed counting applications. The 'LS169B and 'S169 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 (ENP\, ENT\) 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 ENT\
is fed forward to enable the carry output. The carry output thus enabled will
produce a low-level output pulse with a duration approximately equal to the
high portion of the QA output when counting up and approximately
equal to the low portion of the QA output when counting down. This
low-level overflow carry pulse can be used to enable successive cascaded stages.
Transitions at the ENP\ or ENT\ 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 (ENP\, ENT\, LOAD\, U/D\) 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.