Hello, I was curious if anyone can comment on writing safe state machines in AHDL. I often use One-hot encoding in my state machines and am curious how safe it is. I set the State Machine setting in Quartus for One-hot Encoding, does this force all my state machines to be safe. If I just add the line to my state machine:
When Others => StateMachine = StateA; -- go back to start
will the state machine be considered safe?
thanks, joe
Yes and no. The real questions is: "safe from what?". State machines just don't go wild out of blue. In fact, this question is very hard to answer. Consider this: you detect an invalid code in the middle of some complex situation, should you only reset the module? Reset your chip? Reset the whole board? Reset the system? In your module, go to a default state? What can you do in the default state? What about the state of all of the output ports in your module just before the failure? Can you just set them back to a default state? What about the other modules that are connected these outputs? What about their internal state?....
It all depends on the situation, the objectives of the whole system design, the probabilities of such failures and the cost you're willing to pay to protect your system from them. Sometimes it is going to be very complex and in some simpler cases, a reset would do the trick...
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