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ELE 2120
DigitalDigital Circuits & SystemsCircuits&Systems
Lecture #14Lecture #14
Part III: Combinational and Sequential Circuits
Synchronous Sequential Circuits
Synchronous Sequential Circuits
We have considered combinational logic
circuits in which outputsp are determined
fully by the present values of inputs. We
also discussed how simple storage
elements can be implemented in the form
of flip‐flops. The output of a flip
‐flop
depends on the stateof the flip‐flop rather
than the value of its inputs at any given
time; the inputs cause changes in the statestate..
© 2008 香港中文大學電子工程學系
ELE 2120数字电路与系统
1
Synchronous Sequential Circuits
Now we will deal with a general class of circuits
in which the outputs depend on the past
bebehaviorao oof thee ccircuit,cu, as welle as oon thee ppresentese
values of inputs. They are called sequential
circuits. In most cases a clock signal is used to
control the operation of a sequential circuit; such
a circuit is called a synchronous sequential circuit.
The alternative,alternative in which no clock signal is used,used
is called an asynchronous sequential circuit.
Synchronous circuits are easier to design and are
used in a vast majority of practical applications.
© 2008 香港中文大學電子工程學系
ELE 2120数字电路与系统
Synchronous Sequential Circuits
Synchronous sequential circuits are realized using
combinational logic and one or more flip‐flops. The
generalg structure of such a circuit is shown. The
circuit has a set of primary inputs, W, and produces
a set of outputs, Z. The values of the outputs of the
f‐fsare referred to as thestate, Q, of the circuit.
WW
Combinational
Flip-flops
CCombinational biil
circuit
Q
circuit
Z
Clock
© 2008 香港中文大學電子工程學系
ELE 2120数字电路与系统
2
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