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Synchronous Sequential Logic

Set up the Truth Table

Truth Table

Set up the Truth Table

Truth Table

Eliminates the indeterminate S=R=1 state of the NAND Latch in addition to a control input C.

Note: 74LS75 is D Latch

Flip-Flops are triggered by a clock transition in order to make the operation reliable

Latch

FF

FF

Master reads while the clock is high but Q records the last data when the clock is low.

Edge-triggered design is superior to master-slave because reading and recording occur in a flash during the clock transition.

Determine the Truth Table of the T FF

Characteristic Tables

D Flip-Flop T Flip-Flop

D Q(t+1) T Q(t+1)

0 0 0 No Change

1 1 1 Toggles

T Flip-Flop can be used to divide the frequency of a clock by 2. Sketch the circuit. How can you divide the frequency by 4?

Draw the Characteristic Table

What’s wrong with this picture?

Connect a wire fom the AND gate to the D Flip-Flop.

P.S. This is figure 5-15 in your textbook!

P.P.S. Analyze the given sequential circuit. In other words, write the equations for A(t+1), B(t+1), and y, draw a state table, and sketch a state diagram.

Present State Next State Output

x = 0 x =1 x = 0 x =1

A B A B A B y y

00 00 01 0 0

01 00 11 1 0

10 00 10 1 0

11 00 10 1 0

Design a circuit that detects three or more consecutive 1’s in a string of bits coming through an input line

Our circuit should start off in a “state” S(0). If a 0 comes along it should stay put in S(0). If a 1 comes along it should jump to state S(1). Now if a 0 comes along it should go right back to S(0) but if a second 1 comes along it should jump to S(2). At this point if a third 1 comes along it should jump to S(3) and also set a flag. Otherwise start all over again in S(0).

State Table for Sequence Detector

Present State Next State Output

x = 0 x =1 x = 0 x =1

A B A B A B y y

00 00 01 0 0

01 00 10 0 0

10 00 11 0 0

11 00 11 1 1

FSM:A Sequential Circuit is also called a Finite State Machine (FSM)

Mealy Model: The output (y) of an FSM depends on the input (x) as well as the present state of A and B [e.g. Fig 5-15 where y = (A+B)x\']

Moore Model: The output (y) of an FSM depends on the present state of A and B but not on the input (x). [e.g. Sequence Detector where y = AB]

// Functional description of JK flip-flop

module My_JKFlipFlop (J,K,CLK,Q,Qnot);

output Q,Qnot;

input J,K,CLK;

reg Q;

assign Qnot = ~ Q ;

always @ (posedge CLK)

case ({J,K})

2\'b00: Q = Q;

2\'b01: Q = 1\'b0;

2\'b10: Q = 1\'b1;

2\'b11: Q = ~ Q;

endcase

endmodule

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