74LS168 National Semiconductor, 74LS168 Datasheet - Page 4

• Manufacturer Part Number: 74LS168
• Description: 54LS168 Synchronous Bi-Directional BCD Decade Counter
• Manufacturer: National Semiconductor

Functional Description
The ’LS168 uses edge-triggered D-type flip-flops and has
no constraints on changing the control or data input signals
in either state of the Clock The only requirement is that the
various inputs attain the desired state at least a setup time
before the rising edge of the clock and remain valid for the
recommended hold time thereafter The parallel load opera-
tion takes precedence over the other operations as indicat-
ed in the Mode Select Table When PE is LOW the data on
the P0 –P3 inputs enters the flip-flops on the next rising
edge of the Clock In order for counting to occur both CEP
and CET must be LOW and PE must be HIGH The U D
input then determines the direction of counting The Termi-
nal Count (TC) output is normally HIGH and goes LOW
provided that CET is LOW when a counter reaches zero in
the COUNT DOWN mode or reaches 9 in the COUNT UP
mode The TC output state is not a function of the Count
Enable Parallel (CEP) input level The TC output of the
’LS168 decade counter can also be LOW in the illegal
states 11 13 and 15 which can occur when power is turned
on or via parallel loading If an illegal state occurs the
’LS168 will return to the legitimate sequence within two
counts Since the TC signal is derived by decoding the flip-
flop states there exists the possibility of decoding spikes on
TC For this reason the use of TC as a clock signal is not
recommended (see logic equation below)
1 Count Enable
2 Up TC
3 Down TC
Logic Diagram
e
e
Q0
Q0
e
Q3
CEP
Q1
(U D)
CET
Q2
CET
Q3
PE
(U D)
CET
4
H
L
X
State Diagram
PE
e
H
H
H
H
e
e
L
LOW Voltage Level
Immaterial
HIGH Voltage Level
CEP
H
X
L
L
X
CET
H
X
L
L
X
’LS168 Mode Select Table
U D
H
X
L
X
X
Action on Rising Clock Edge
Load (P
Count Up (Increment)
Count Down (Decrement)
No Change (Hold)
No Change (Hold)
n
Q
n
)
TL F 10207– 3
TL F 10207– 4