Interview Question!

Hello All:
In fact it has been a long time since we had posted any sort of interview questions or puzzles and so here we are with a very practical problem.

Lets see how many of you can crack it!

I will post the solutions depending on the responses. Thanks.

The problem:
The Sheel lock Company has recently received an order from Mr. Manmohan Singh for their all-digital Perfectly Perplexing Padlock(P3). The P3 has two buttons ("0" and "1") that when pressed cause the FSM controlling the lock to advance to a new state. In addition to advancing the FSM, each button press is encoded on the B signal (B=0 for button "0", B=1 for button "1").

The padlock unlocks when the FSM sets the UNLOCK output signal to 1, which it does whenever the last N button presses correspond to the N-digit combination. Unfortunately the design notes for the P3 are incomplete. Using the specification above and clues gleaned from the partially completed diagrams below fill in the information that is missing from the state transition diagram with its accompanying truth table.

When done
  • each state in the transition diagram should be assigned a 2-bit state name S1S0 (note that in this design the state name is not derived from the combination that opens the lock),
  • the arcs leaving each state should be mutually exclusive and collectively exhaustive,
  • the value for UNLOCK should be specified for each state, and
  • the truth table should be completed.

What is the combination for the lock?

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