Honored Contributor
Honored ContributorRysc provides a good description of what I am seeing. Based on the State Machine Viewer I was expecting the "reset" state to be all 0's, but in SignalTap I was seeing a "1" for the reset state. So it would appear that the value that I am watching is the inverted one. Perhaps as FvM indicates it would make a difference in which type (pre vs. post synthesis, etc.) I choose in SignalTap.
Certainly as you both describe having an unknown or unstable value when the state machine is clocked can lead to all sorts of problems. However, I had thought that if I had combinatorial (A AND B) and A is 0, then it would not matter if B was unstable; the output would always be 0. Perhaps I am wrong here. This effectively describes my situation. The second state that becomes active has transitions that involve the unstable input but only from states other than the current state. I had thought that even though the input was unstable, the equation could never be TRUE since the state bit in the equation would be FALSE. In the State Machine Viewer and the Compiler report I can see the encoding and the transitions from each state (reset is all 0's). Is there any place that I can see the full equation for a state bit? i.e. if there are 4 ways to get into a state I can see the full optimized equation rather than just the 4 individual transition equations? FYI: The signal in question is bi-directional and I was driving inactive and disabling at the same time. I added an extra state so the the signal is driven inactive instead of weakly pulled inactive and the problem appears solved. Is there any easy way to indicate when more than one state is active so that you can trigger in SignalTap, light an LED, etc? Thanks Stefan
