Design mealy sequence detector to detect a sequence ----1010---- using D filpflop and logic
2 Answers
  • A sequence detector is a sequential state machine. In a Mealy machine, output depends on the present state and the external input (x). Hence in the diagram, the output is written outside the states, along with inputs. The state diagram of a Mealy machine for a 1010 detector is:

enter image description here

  • The state table for the above diagram:

enter image description here

  • State assignments: Let $S_0$ = 00

$S_1$ = 01

$S_2$ = 10

$S_3$ = 11

The above state table becomes:

enter image description here

  • Four states will require two flip flops. Consider two D flip flops. Their excitation table is shown below.

  • Excitation table:

enter image description here

  • K-maps to determine inputs to D Flip flop:

enter image description here

  • Circuit diagram for the sequence detector:

enter image description here

if i design it using 5 states, is the method wrong in case of mealy FSM?


for present state 11 ('S3') input X=1 changes it to next state 01 ('S1'). So there should be modification in excitation table in the last row in next state column.

Please log in to add an answer.