This circuit is a synchronous, reversible, up/down counter.
The 191 is a 4-bit binary counter. Synchronous operation is
provided by having all flip-flops clocked simultaneously so
that the outputs change simultaneously when so instructed
by the steering logic. This mode of operation eliminates the
output counting spikes normally associated with asynchronous
(ripple clock) counters.
The outputs of the four master-slave flip-flops are triggered
on a low-to-high level transition of the clock input, if the
enable input is low. A high at the enable input inhibits counting.
Level changes at either the enable input or the down/
up input should be made only when the clock input is high.
The direction of the count is determined by the level of the
down/up input. When low, the counter counts up and when
high, it counts down.
This counter is fully programmable; that is, the outputs may
be preset to either level by placing a low on the load input
and entering the desired data at the data inputs. The output
will change independent of the level of the clock input. This
feature allows the counters to be used as modulo-N dividers
by simply modifying the count length with the preset inputs.
The clock, down/up, and load inputs are buffered to lower
the drive requirement; which significantly reduces the number
of clock drivers, etc., required for long parallel words.
Two outputs have been made available to perform the cascading
function: ripple clock and maximum/minimum count.
The latter output produces a high-level output pulse with a
duration approximately equal to one complete cycle of the
clock when the counter overflows or underflows. The ripple
clock output produces a low-level output pulse equal in
width to the low-level portion of the clock input when an
overflow or underflow condition exists. The counters can be
easily cascaded by feeding the ripple clock output to the
enable input of the succeeding counter if parallel clocking is
used, or to the clock input if parallel enabling is used. The
maximum/minimum count output can be used to accomplish
look-ahead for high-speed operation.