Hello all! I'm new to VHDL and am looking to learn it as fast as possible. I've been reading all I can on the topic and think I have a fairly good grasp of what's going on. However, something bothers me about the simultaneousness of evaluation. Take, for example, two processes. The first process assigns a binary '1' to a signal. Another process assigns '0' to the same signal. Given that both processes are converted to hardware which do not execute sequentially, both processes will simultaneously be trying to pull the signal in a conflicting manner. What happens in this situation? Have I misunderstood something that prevents this situation from occurring in the first place? Thank you! Alexandicity
> What happens in this situation?
Where?
In simulation: you will get a collision on this signal. When the
assigned value is different it depends on the signals type how the
simulator copes up with this problem. For std_logic a resolution table
exists. With that the colliding signals will be resolved (and result
usually in a 'X').
In hardware: you will not get synthesized such a description.
(Because todays fpgas don't have "wires" to which more than 1 output can
be connected to...)
Ah, so this would be an invalid description that would cause errors, be they handled errors (as with your std_logic example), simulation errors (other binary types) or synthesis errors?
You can and will get all three of them. And maybe even if the simulation does not report an error (because your stimuli always drives the same value from both outsputs to the signal), the syntheses will not be able to resolve it...
Understood. Thanks for your clear and succinct response!