The state-machine appears to be not changing states cleanly/

[EDA_hg81]

I put a couple extra signals in the state-machine process in order to use chip scope to check which state I am in. After a reset is pulled low, the state is supposed to change, and my test signals are supposed to go from “10” to “01”... however on chip scope, I see the change from “10” to “11” and stays that way. This problem occurs about 1 in 10 trials.

What is the possible reasons?

Thanks for your all suggestions.

 

[Iouri]

are you sure you have all states diskribe all the states in the machine something like this

when S1 =>

if signal = active then
current_state <= S2;
else
current_state <= S1;
end if;

 

[tkbits]

Re: The state-machine appears to be not changing states clea

Be careful how you handle external inputs.

if ext_signal = '0' then ...

The external signal needs to be synchronized. If it's from a mechanical switch, it will also need to be debounced (filtered).

 

[Iouri]

and also make sure external signals are longer then just one clock period, they needs to be actually triggered, and when machine change state they get reset