Chapter D Finite State Machines

output (R) from the present state. Note carefully that we use the present state and notthe next state to get the output. This is because it’s the present state that generates theoutput. Repeating this for the Odd state and its inputs generates the whole table shownin Fig.13(b) This table is easy to convert to a computer program, and also into adigital electronic circuit. We’ll discuss such a program in section?? Below and you’llhave opportunity to play with the program in the Activities at the end of this chapter.D13 Table-Based FSM Representation – Door LockHere we produce a Symbolic State Transition Table for a cut-down version of thedoor lock we met in Section D3. The simplified door lock opens when the string oftwo bits “01” is entered at the keypad. The state diagram is shown in Fig. 14(a) wherethe terminal state Q3 outputs R=1 to trigger the opening mechanism, the starting andintermediate states Q1 and Q2 both output R=0. The table shown in Fig.14(b) is easilyconstructed, one only has to remember that the output R is derived from the presentstate PS and not from the next state NS.Q1[0]0Q2[0]1Q3[1]1 0Present State(PS)Input (S)Q1 0Next State Output (R)(NS)Q2 0Q11Q10Q20Q20Q21Q30Q30Q31Q31Q31Figure 14. Simplified door lock FSM (based on the example shown in Fig.5). This acceptsthe string “0 1” to unlock the door, the output signal “r=1” being generated in Q3. (a) showsthe State Diagram and (b) the Symbolic State Transition Table. The next state (NS) isgenerated from the present state (PS) by the input stimulus S, and each PS generates itsoutput response R.D14Setting a FSM into Digital Electronic Hardware. Parity Checker