I have an assignment where where I have to design a FSM which outputs the following Hex number in binary, one digit at a time: 12498A3B. The FSM must use a clock signal and D flip-flops to do so.

I firstly made the state diagram, then I made the current/next state table, next I created 4 Karnaugh maps to determine the next state bit output for each DFF. I used the 4 Boolean expressions derived from these K-maps to create my FSM in the "Logisim" program.

My circuit cycles through each digit as it should in the correct order, however the initial output by my FSM isn't the initial digit of the Hex number, but rather one of the digits in the centre of the number. How is this possible? I am new to designing FSMs but I had the impression that when implementing the Boolean expressions obtained from the K-maps would produce the expected initial state?

I can provide screenshots of all my work if further clarity is needed.

  • \$\begingroup\$ K-maps are combinatorial logic. Outputting data in some order is sequential logic. \$\endgroup\$ Nov 10, 2015 at 0:06

1 Answer 1


When a flip flop powers up, the state it comes up in is effectively random. You have to force the FSM to go to the initial state by some other means. Otherwise it seems very probable that it starts the sequence in the middle somewhere.

If your flip flops have set and reset pins then you can use a power up detect circuit to set them all to an initial state. If they don't then you need something additional to push the initial state into them.


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