analysis of clocked sequential circuits. example of a sequential circuit d flip-flops
TRANSCRIPT
Analysis of Clocked Sequential Circuits
Example of a Sequential Circuit
D flip-flops
Example: Start with A=0, B=0, x=0.A(next)=0B(next)=0Y(next)=0
What are A(next), B(next) and y(next) given that A=1, B=1 and X=1?
D flip-flops
Alternate State Table
State Diagram
Each circle is a state
When x=1, y=0.
State Diagram
Each circle is a state
When x=0, y=1.
Detects 0 in the bit stream of data
Output is a 0 as long as input is a 1. The first 0 after a string of 1 transfers the circuit back to 00.
Summary
Model a Clocked Sequential Circuit with Verilog
1. Use parameter to define constants2. Form the next state from x and the current state3. Form the output4. fork.....join construction
S0
S1
S2
S3
Parameter
S0
S1
S2
S3
Define states with parameter
Update States
S0
S1
S2
S3
If reset is 0, set state to S0.
If reset is 1, update state with next_state.
Implement the States Using State Diagram
S0
S1
S2
S3
fork....join
Right click to add state and next_state to the waveform shown right
Valid Mealy Output
S0
S1
S2
S3
Mealy Glitches
S0
S1
S2
S3
Glitiches occur because x changes before the next rising clock edge
General Structure of a Mealy Machine
Output depends on input as well as the stateThe circuit is in. (variable: state)
Synchronize the input with clock when using Mealy machine.
Moore Machine
Output depends on the state, but not the input.
Synthesis of Sequential Circuit
• Start with a state diagram• Assign Binary Values to the States
State Diagram for Sequence Detector
• Problem statement:–Design a circuit that detects a
sequence of three or more consecutive 1’s in a string of bits coming through an input line
State Diagram of the Word Description
Start with S0.if input is 0, stay in S0.if input is 1, go to S1.
Once in S1if input is 1, go to S2.if input is 0, go to S0.
Once in S2if input is 0, go to S0.if input is 1, go to S3.
Once in S3,if input is 0, go to S0.if input is 1, stay in S3, set output to 1.
Output is 1 when the circuit is in S3, 0 otherwise.
Assign Binary Value to the States
Use two flip-flops to represent states.Let x be the inputAnd y be the output
01234567
Assign Binary Value to the States
01234567
Circuit Implementation
Sequence Detector Circuit
Random Numbergenerator
SequenceDetector
Measurement Results
A
B
x
y
clk