74HC191 pdf datasheet

MM54HC190/MM74HC190 Synchronous
Decade Up/Down Counters with Mode
Control MM54HC191/MM74HC191
Synchronous Binary Up/Down Counters
with Mode Control
General Description
These high speed synchronous counters utilize advanced
silicon-gate CMOS technology. They possess the high noise
immunity and low power consumption of CMOS technology,
along with the speeds of low power Schottky TTL.
These circuits are synchronous, reversible, up/down counters.
The MM54HC191/MM74HC191 are 4-bit binary counters
and the MM54HC190/MM74HC190 are BCD counters.
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.
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.
These counters are 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.
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.