What is a case statement in VHDL?
What is a case statement in VHDL?
The VHDL Case Statement works exactly the way that a switch statement in C works. Given an input, the statement looks at each possible condition to find one that the input signal satisfies. They are useful to check one input signal against many combinations. This is done via the “when others =>” statement.
Is case statement synthesizable in VHDL?
VHDL Reference Guide – Case Statement. The CASE statement is generally synthesisable.
What kind of statement is the IF statement in VHDL?
IF statement is a sequential statement and hence can be used in a process, function or procedure.
What is the problem with IF statement in VHDL?
Using an if statement without an else clause in a “combinational process” can result in latches being inferred, unless all signals driven by the process are given unconditional default assignments.
What is a VHDL process?
Process Statements include a set of sequential statements that assign values to signals. These statements allow you to perform step-by-step computations. Process Statements that describe purely combinational behavior can also be used to create combinational logic.
What is difference between case and if else?
Any kind of Boolean expression can be used in an If Then Statement. Each expression following an IF or ELSIF clause may be unrelated to the other expressions in the statement. In a Case Statement, however, a single Boolean expression is compared to a constant in each WHEN clause.
What kind of statement is if statement?
An if statement is a programming conditional statement that, if proved true, performs a function or displays information. Below is a general example of an if statement, not specific to any particular programming language.
Is there else if in VHDL?
You can use either sequential or concurrent conditional statement. It’s up to you. There is a total equivalence between the VHDL “if-then-else” sequential statement and “when-else” statement. Here below we can see the same circuit described using VHDL “if-then-else” or “when-else” syntax.
How do you use don’t care in VHDL?
What is the correct way to specify such “Don’t Care” values in VHDL in order to allow the synthesis tool more room for possible optimizations?
- Simply not assigning the signal.
- The std_logic_1164 package defines the value ‘-‘ — Don’t care for std_ulogic .
- Modelsim uses the value ‘X’ to display undefined signals.
What do VHDL stand for?
Hardware Description Language
The Very High Speed Integrated Circuit (VHSIC) Hardware Description Language (VHDL) is a language that describes the behavior of electronic circuits, most commonly digital circuits. VHDL is defined by IEEE standards.
What does => mean in VHDL?
A means to memorize when to use => and when to use <= is to think as follow. “<=” as an assignment for signal as target (for variable it is “:=” ). Examples: y <= a + b + c; –y is a signal v := a + b +c; –v is a variable. “=>” as mapping.
Why is switch better than if?
A switch statement is usually more efficient than a set of nested ifs. The compiler can do this because it knows that the case constants are all the same type and simply must be compared for equality with the switch expression, while in case of if expressions, the compiler has no such knowledge. …
How does a VHDL case statement in C work?
The VHDL Case Statement works exactly the way that a switch statement in C works. Given an input, the statement looks at each possible condition to find one that the input signal satisfies. They are useful to check one input signal against many combinations.
Can a branch include a valid VHDL code?
As with the if statement, the code associated with each branch can include any valid VHDL code. This includes further sequential statements, such as if or case statements. Again, we should try to limit the number of nested statements as it makes it easier to meet our timing requirements.
Can a default action be added to a VHDL?
Finally, a default action may be added to the case statement using an others (VHDL) or default (Verilog/SystemVerilog) clause. This removes the requirement to enumerate every option in the case statement. For clarity, the default should be the last clause of the case statement.
When to use level sensitive storage elements in VHDL?
This especially happens, if the “when others ➜” statement is present (no synthesis complaint), but empty. “If a signal or variable is assigned values in some branches of a case statement, but not in all cases, then level-sensitive storage elements may result (see 6.2).