SDFS089 - MARCH 1987 - REVISED OCTOBER 1993
This synchronous, presettable, 4-bit up/down binary 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 (
, 
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 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 (
) 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 application without additional gating. Instrumental
in accomplishing this function are two count-enable (, ) inputs and a ripple-carry (
) output. Both and 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 is fed forward
to enable the . 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 or 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 SN74F169 features a fully independent clock circuit. Changes
at control inputs (,
, 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 SN74F169 is characterized for operation from 0°C to 70°C.