These synchronous, presettable counters feature an internal carry look-ahead for application in high-speed counting
designs. The DM74LS161A and DM74LS163A are 4-bit
binary counters. The carry output is decoded by means of
a NOR gate, thus preventing spikes during the normal
counting mode of operation. 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 which are normally associated with asynchronous
(ripple clock) counters. A buffered clock input triggers the
four flip-flops on the rising (positive-going) edge of the
clock input waveform.
These counters are fully programmable; that is, the outputs
may be preset to either level. As presetting is synchronous,
setting up a low level at the load input disables the counter
and causes the outputs to agree with the setup data after
the next clock pulse, regardless of the levels of the enable
input. The clear function for the DM74LS161A is asynchronous; and a low level at the clear input sets all four of the
flip-flop outputs LOW, regardless of the levels of clock,
load, or enable inputs. The clear function for the
DM74LS163A is synchronous; and a low level at the clear
inputs sets all four of the flip-flop outputs LOW after the
next clock pulse, regardless of the levels of the enable
inputs. This synchronous clear allows the count length to
be modified easily, as decoding the maximum count
desired can be accomplished with one external NAND
gate. The gate output is connected to the clear input to
synchronously clear the counter to all low outputs.
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 ripple carry output.
Both count-enable inputs (P and T) must be HIGH to count,
and input T is fed forward to enable the ripple carry output.
The ripple carry output thus enabled will produce a highlevel output pulse with a duration approximately equal to
the high-level portion of the QA output. This high-level overflow ripple carry pulse can be used to enable successive
cascaded stages. HIGH-to-LOW level transitions at the
enable P or T inputs may occur, regardless of the logic
level of the clock.
These counters feature a fully independent clock circuit.
Changes made to control inputs (enable P or T or load) 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 set-up and hold times.
? Synchronously programmable
? Internal look-ahead for fast counting
? Carry output for n-bit cascading
? Synchronous counting
? Load control line
? Diode-clamped inputs
? Typical propagation time, clock to Q output 14 ns
? Typical clock frequency 32 MHz
? Typical power dissipation 93 mW