April 1984
Revised February 2000
DM74ALS161B • DM74ALS162B • DM74ALS163B
Synchronous Four-Bit Counter
The carry look ahead circuitry provides for cascading
General Description
counters for n bit synchronous application without addi-
tional gating. Instrumental in accomplishing this function
are two count enable inputs (P and T) and a ripple carry
output. Both count enable inputs must be HIGH to count.
The T input is fed forward to enable the ripple carry output.
The ripple carry output thus enabled will produce a high
level output pulse with a duration approximately equal to
the high level portion of QA output. This high level overflow
ripple carry pulse can be used to enable successive cas-
caded stages. HIGH-to-LOW level transitions at the enable
These synchronous presettable counters feature an inter-
nal carry look ahead for application in high speed counting
designs. The DM74ALS162B is a four-bit decade counter,
while the DM74ALS161B and DM74ALS163B are four-bit
binary counters. The DM74ALS161B clears asynchro-
nously, while the DM74ALS162B and DM74ALS163B clear
synchronously. The carry output is decoded to prevent
spikes during normal counting mode of operation. Synchro-
nous operation is provided by having all flip-flops clocked
simultaneously so that outputs change coincident with
each other when so instructed by count enable inputs and
internal gating. This mode of operation eliminates the out-
put 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.
P
or
T
inputs of the DM74ALS161B through
DM74ALS163B may occur regardless of the logic level on
the clock.
The DM74ALS161B through DM74ALS163B feature a fully
independent clock circuit. changes made to control inputs
(enable P or T, or load) that will modify the operating mode
will 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.
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 set up data after the
next clock pulse regardless of the levels of enable input.
LOW-to-HIGH transitions at the load input are perfectly
acceptable regardless of the logic levels on the clock or
enable inputs.
Features
■ Switching specifications at 50 pF
■ Switching specifications guaranteed over full tempera-
ture and VCC range
The DM74ALS161B clear function is asynchronous. 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. These two counters are provided with a clear on
power-up feature. The DM74ALS162B and DM74ALS163B
clear function is synchronous; and a low level at the clear
input sets all four of the flip-flop outputs LOW after the next
clock pulse, regardless of the levels of 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 out-
put is connected to the clear input to synchronously clear
the counter to all low outputs. LOW-to-HIGH transitions at
the clear input of the DM74ALS162B and DM74ALS163B
are also permissible regardless of the levels of logic on the
clock, enable or load inputs.
■ Advanced oxide-isolated, ion-implanted Schottky TTL
process
■ Functionally and pin-for-pin compatible with Schottky
and low power Schottky TTL counterpart
■ Improved AC performance over Schottky and low power
Schottky counterparts
■ Synchronously programmable
■ Internal look ahead for fast counting
■ Carry output for n-bit cascading
■ Synchronous counting
■ Load control line
■ ESD inputs
Ordering Code:
Order Number Package Number
Package Description
DM74ALS161BM
DM74ALS161BN
DM74ALS162BM
DM74ALS162BN
DM74ALS163BM
DM74ALS163BN
M16A
N16E
M16A
N16E
M16A
N16E
16-Lead Small Outline Integrated Circuit (SOIC), JEDEC MS-012, 0.150 Narrow
16-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300 Wide
16-Lead Small Outline Integrated Circuit (SOIC), JEDEC MS-012, 0.150 Narrow
16-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300 Wide
16-Lead Small Outline Integrated Circuit (SOIC), JEDEC MS-012, 0.150 Narrow
16-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300 Wide
Devices also available in Tape and Reel. Specify by appending the suffix letter “X” to the ordering code.
© 2000 Fairchild Semiconductor Corporation
DS006206
www.fairchildsemi.com
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