What is the difference between signals and variables in vhdl

The example below demonstrates how signals behave differently than variables. Variables can be a bit tricky to display in simulation. If you are using Modelsim, read more about how to see your variables in Modelsim's waveform window. Look carefully at the waveform above. Using signals and variables to store data generates very different behavior.

Make sure you clearly understand what you code will be generating and make sure that you simulate your code to check that behaves like you want! Help Me Make Great Content! Support me on Patreon! The following table shows the value of the two variables, the current and next values of the signals after executing each instruction of each process:.

With the same initial conditions, if the scheduler decides to run P6 first and P5 next:. As we can see, after the execution of our two processes, the result is the same whatever the order of execution. This counter-intuitive signal assignment semantics is the reason of a second type of problems that VHDL beginners frequently encounter: the assignment that apparently does not work because it is delayed by one delta cycle. Note: the same signal can be assigned several times during the same execution phase.

In this case, it is the last assignment that decides the next value of the signal. The other assignments have no effect at all, just like if they never had been executed. It is time to check our understanding: please go back to our very first swapping example and try to understand why:.

Try to understand also why, if s is an integer signal and its current value is 15, and we execute:. In order to model hardware it is very useful to be able to model the physical time taken by some operation. Here is an example of how this can be done in VHDL.

The example models a synchronous counter and it is a full, self-contained, VHDL code that could be compiled and simulated:. In process P1 the wait instruction is not used to wait until the value of a signal changes, like we saw up to now, but to wait for a given duration.

This process models a clock generator. Signal clk is the clock of our system, it is periodic with period 20 ns 50 MHz and has duty cycle.

Process P2 models a register that, if a rising edge of clk just occurred, assigns the value of its input nc to its output c and then waits for the next value change of clk. Process P3 models an incrementer that assigns the value of its input c , incremented by one, to its output nc It then waits until the value of its input c changes.

This is also new. Up to now we always assigned signals with:. There is one important aspect of the VHDL simulation that we did not discuss yet: after an execution phase all processes are in suspended state. We informally stated that the scheduler then updates the values of the signals that have been assigned.

But, in our example of a synchronous counter, shall it update signals clk , c and nc at the same time? What about the physical delays? And what happens next with all processes in suspended state and none in run-able state? To conclude, let us now manually exercise the simplified simulation algorithm on the synchronous counter presented above. The following tables represent the evolution of the state of the system during the initialization and the first simulation cycles. Each signal has its own column in which the current value is indicated.

When a signal assignment is executed, the scheduled value is appended to the current value, e. The 3 last columns indicate the condition to resume the suspended processes name of signals that must change or time at which the process shall resume. Note: during the first simulation cycle there is no execution phase because none of our 3 processes has its resume condition satisfied. P2 is waiting for a value change of clk and there has been a transaction on clk , but as the old and new values are the same, this is not a value change.

Note: again, there is no execution phase. As soon as the physical time changes, the delta cycles can be reset. Example This example deals with one of the most fundamental aspects of the VHDL language: the simulation semantics.

Signals and variables Most classical imperative programming languages use variables. Let us illustrate this difference on a concrete example: the swapping. These sequential programs are called processes: P1: process begin instruction1; instruction2; Scheduling The scheduler maintains a list of all processes and, for each of them, records its current state which can be running , run-able or suspended.

Example: wait on s; means suspend me until the value of signal s changes. Signals and inter-process communication VHDL avoids non determinism using two specific characteristics: Processes can exchange information only through signals signal r, s: integer; -- Common to all processes The 10 nanoseconds is an arbitrary value, it could be anything, even 0 nanoseconds.

Try it! Another important observation is that event though the signal was incremented twice before the wait , its value only incremented once. Therefore, at the second increment of MySignal , 1 is added to its old value. When it is incremented again, the first increment is completely lost.

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