Embed Size (px)
DESCRIPTION
Sequential Circuits. Chapter 5. Sequential Circuits. Combinational Circuits + Storage element output depends both on previous state and input. Fig. 5-1. Storage element. (a): a buffer t G the delay the information enters the buffer at t and output at t + t G - PowerPoint PPT Presentation
Citation preview
Sequential Circuits
• Combinational Circuits + Storage element• output depends both on previous state and input
Fig. 5-1
Fig. 5-2
• (a): a buffer• tG the delay• the information enters the buffer at t and output at t+ tG
• the stored information only retained in buffer by tG • longer storage time is necessary in most applications
Fig. 5-3
Use flip-flop
•A storage element can maintain a binary state indefinitely, until directed by an input signal to switch states.•The most basic storage elements are latches.
Fig. 5-5
RS
RS
RS
Fig. 5-7
Fig. 5-8
A change in value on the control input allows the state of a latch in a flip-flop to switch.
This change is called a trigger The trigger enable (trigger) the flip-flops See Fig. 5-3 for sequential circuits A present (original) and next (new) state occur in
flip-flop before and after the trigger, respectively The most important element in sequential circuits Can be derived from latch
Fig. 5-9
Fig. 5-10
•Pulse trigger •Pulse in the inputs SR will result wrong output
Initially unknown
Unknown due to R=1 S=1
Pulse input results wrong output
Fig. 5-11
Fig. 5-13
Fig. 5-14
17
5-4 Sequential Circuit Analysis
The output and the next state are a function of the inputs and the present state.
An example
XBAY )( Fig. 5-15
input equations
output equation
XAD
XBXAD
B
A
State table
XADtB
XBXADtA
B
A
)1(
)1( XBAY )(
Two-dimensional state table
Mealy model/Moore model
Mealy model circuits Sequential circuits in which the outputs depend
on the input, as well as on the states The circuits in Fig. 5-15
Moore model circuits Sequential circuits in which the outputs depend
only on the states The circuits in Fig. 5-16
A Moore model circuit (Fig. 5-16)
State diagram(a): for Fig. 5-15 (b): for Fig. 5-16
Example 5-1 States reduction
equivalent
Example 5-1 States reduction
Example 5-1 States reduction
equivalent
Example 5-1 States reduction
• Reduce from 4 states, 2 flip-flops to 2 states, 1 flip-flop
• may or may not result in reduced cost
Sequential Circuit Simulation
A simulator for the input/output of a designed circuit
Functional simulation Timing simulation
