Modelsim altera signal assignment
Although we are still at 0 ns simulation time, the signal now has a new value. The signal was '0' at the start of the simulation but changes to '1' before any time has passed at all.
Modelsim altera signal assignment simulator#
In practice, the simulator will schedule the signal to have the new value in the next delta cycle. In VHDL, signal values are only updated when the process hits a Wait statement. Then, the simulator will run the process and stop at the wait for 5 ns line.
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Modelsim altera signal assignment code#
Simulators use a technique called event scheduling to precisely model the behavior of VHDL code while doing the least amount of work.Īt the start of the simulation, Sig1 has its initial value of '0'. But how does this work internally in the simulator? Every time a signal changes, it is an event. Unsurprisingly, Sig1 changes between '0' and '1' with a frequency of 100 MHz. This is the code from the previous example, but with an additional process that inverts the value of Sig1 every 5 nanoseconds: It is easy to know what the output will look like in a second or an hour from now, because it is exactly the same as now.įrom the simulator’s point of view, there are no events in this VHDL code. The Sig1 signal is '0' right from the start, and it doesn’t change at all. There is not a lot going on in this code. How can it be that this very long simulation completed so fast? VSIM 1> run 200 hr VSIM 2> When I tell ModelSim to simulate this design for 200 hours, that works just fine, in fact, the simulation completes almost instantly, and the command prompt gives me a new line where I can enter the next command. Let’s first look at what happens when we run this trivial VHDL code in the ModelSim VHDL simulator: To precisely model the behavior of digital logic, simulators use an event-based approach for executing VHDL code. While in VHDL, there can be multiple sequences of logic that react to each other in ways that are not compatible with the standard computer architecture. When a normal programming language is run, the CPU executes one instruction after the other. VHDL is a parallel programming language, while computers and CPUs work in a sequential manner.
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They are events that happen in zero simulation time after a preceding event. Delta cycles are non-time-consuming timesteps used by VHDL simulators for modeling events during execution of VHDL code.