vhdl case in if

The case statement selects for execution one of several alternative sequences of statements; the alternative is chosen based on the value of the associated expression.

The following code illustrates an “if” statement with two assignments in each “then” branch.We can extend the conceptual implementation shown in Figure 1 to arrive at the implementation of the code in Listing 2. When we need to perform a choice or selection between two or more choices, we can use the VHDL conditional statement. This is simple to model, as shown in the first line of the second if statement.We then use an else statement to capture the case when the addr signal is 1b. This includes values other than 0 or 1 when using a type such as std_logic or std_logic_vector.However, we can also use the the case statement to model such components.To demonstrate this, let’s consider the example of a basic four to one multiplexor.The circuit diagram and code below show this implementation.As shown in the circuit diagram, the addr signal determines the value of the output q. Such a circuit is sequential because its output depends on both the input clock and the current value of the counter.Sequential VHDL allows us to easily describe both sequential circuits and combinational ones.The “if” statements of VHDL are similar to the conditional structures utilized in computer programming languages. When using this type of code, we should take care to limit the number of nested statements as it can lead to difficulties in meeting timing.We have already seen practical examples of the if statement when modelling flip flops in the post on To demonstrate the construct more thoroughly, let’s consider an example of a clocked The circuit diagram below shows the circuit which we will use in this example.There are several ways in which could model this circuit in VHDL. In the snippet above, these expressions are given by and .These expressions are sequentially evaluated and the code associated with a branch is executed when an expression evaluates as true.Only one branch of an if statement can ever be executed. Where an if statement is used to detect the clock edge in a "clocked process", certain conventions must be obeyed. These statements are collectively known as sequential statements and can only be used within a process block.In the rest of this post we look at the most commonly used sequential statement in more depth.Whenever a given condition evaluates as true, the code branch associated with that condition is executed.This statement is similar to conditional statements used in other programming languages such as C.The code shown below shows the basic syntax for the if statement.We can exclude the else and elseif branches from the statement if we don’t need them.We have seen this in previous posts where we used the if statement to detect a rising edge on a clock signal. Each of the branches represents a different multiplexer address, with the exception of the others branch. This is simple to model, as shown in the first line of the second if statement.We then use an else statement to capture the case when the addr signal is 1b. The BNF of the multiple VHDL conditional statement is reported below.When we use the CASE-WHEN statement no priority is implemented in the code and as consequence on the hardware instantiated.Here below we can see the same implementation of a 4-way mux using the VHDL code of 4-way mux using the sequential statement “VHDL code of 4-way mux using the sequential statement “Different RTL views can be translated in the same hardware structure!wait, wait… different RTL implementation can be translated in the same hardware circuit?Think about it: even if you are writing a VHDL code using Every time we write a VHDL code to implement some hardware circuit, we need to pay attention to which VHDL instruction or construct is better to use.As a rule of thumb, the selection of the RTL architecture is should be guided by the similarity of VHDL-RTL code to the final hardware.A very good practice is also to verify the RTL viewer implementation and eventually, the final technology implementation both on the output reports and the technology viewer.

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