The DM74LS191 circuit is a synchronous, reversible, up/
down 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
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.
The 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
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.
? Counts binary
? Single down/up count control line
? Count enable control input
? Ripple clock output for cascading
? Asynchronously presettable with load control
? Parallel outputs
? Cascadable for n-bit applications
? Average propagation delay 20 ns
? Typical clock frequency 25 MHz
? Typical power dissipation 100 mW