Digital Systems: Principles and Design

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Digital Systems

TABLE 18.14 State Table for Figure 18.3(b) asynchronous sequential circuit operating in funda-

mental mode

Present Excitation variables before the Present output

state transition (O

G0

, O

G1

, O

G2

) ;

0

(x

q1

, x

q2

) Input X = 0 Input X = 1 Input X = 0 Input X = 1

0, 0 S

0

S

2

S

0

00

0, 1 S

1

S

2

S

0

10

1, 0 S

2

S

3

S

1

11

1, 1 S

3

S

3

S

0

11

Example 18.4

Why is the flow table is not possible for a circuit with state table as Table 18.14?

Solution

This is because all entries are unstable due to JK FF. In flow table, at least one stable state

must be there in a row. State table shows that Y is changing in both the cases. When X input

is 1, S

2

® S

1

, S

1

® S

0

, S

2

is unstable. ...