October 1992
DM54161 DM74161 DM74163
Synchronous 4-Bit Counters
General Description
These synchronous presettable counters feature an inter-
nal carry look-ahead for application in high-speed counting
designs The 161 and 163 are 4-bit binary counters The
carry output is decoded by means of a NOR gate thus pre-
venting spikes during the normal counting mode of opera-
tion Synchronous operation is provided by having all flip-
flops clocked simultaneously so that the outputs change co-
incident 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 161 is asynchronous and a
low level at the clear input sets all four of the flip-flop out-
puts low regardless of the levels of clock load or enable
inputs The clear function for the 163 is synchronous and a
low level at the clear input sets all four of the flip-flop out-
puts 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 maxi-
mum 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 out-
puts Low-to-high transitions at the clear input of the 163 are
also permissible regardless of the logic levels on the clock
enable or load inputs
The carry look-ahead circuitry provides for cascading coun-
ters 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 car-
ry output thus enabled will produce a high-level 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 of the 161
through 163 may occur regardless of the logic level on the
clock
Features
Y Synchronously programmable
Y Internal look-ahead for fast counting
Y Carry output for n-bit cascading
Y Synchronous counting
Y Load control line
Y Diode-clamped inputs
Connection Diagram
Dual-In-Line Package
Order Number DM54161J DM54161W
DM74161N or DM74163N
See NS Package Number J16A N16E or W16A
TL F 6551 – 1
C1995 National Semiconductor Corporation TL F 6551
RRD-B30M105 Printed in U S A
