1. VHDL Programming (08 Marks)
CH.4
VHDL Programming
(08 Marks)
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2. Comparison os concurrent and sequential statements

3. Sequential statements
Sequential statements are not event triggered that is they are executed in sequence one after other statement inside the process.

4. Concurrent statements
Concurrent statements are event triggered i.e. they are executed whenever there is event on the input signal .
Concurrent Statements appear anywhere in architecture.
These statements are executed in an asynchronous manner. i.e. they are order independent.
Concurrent Statements-
Process
when
with

5. Process Statement The ‘process’ statement
is the primary concurrent VHDL statement used in sequential behavior.
within process statement is sequential statement.
can exist anywhere in architecture and define region in architecture where all statements are sequential.
can have explicit sensitivity list.
Sensitivity list is the list of the signals that the process is sensitive to change on it.
should have either sensitivity list or wait statement at the end.

6. Continued…. Syntax process ( sensitivity_list) begin
sequential_ statements;
end process;
For example
process( b,c,d)
a<= b;
b<= c;
c<= d;
These statements should be executed in the given sequence otherwise the signal values will change.

7. When Statement
When- else statement evaluates choice and selects the choice depending on expression.
Syntax
Target<= exp1 when choice_exp= choice1;
exp2 when choice_exp= choice2;
else
exp3;

8. With statement
‘With’ statement evaluates choice and compare that value to each choice value.
Syntax
With choice_exp select
Target <= exp1 when choice1;
<= exp2 when choice2;
<= exp3 when choice3;
<= exp4 when choiceN;
The matching choice value has its expression assigned to target in ‘when’ statement.
Each value in the range of the choice_expression type must be covered by one choice.

9. Sequential statements
Sequential statements are not event triggered that is they are executed in sequence one after other statement inside the process.
Sequential Statements- If
case
loop
assert
wait

10. If Statement
If statement checks a condition and selects a set of statements for execution based on the value of condition.
Syntax
if (condition 1) then
sequential statement;
else if (condition 2) then
Sequential statement;
else
end if;

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Case statement
The case statement is a series of parallel checks on the condition.
It selects one of the branch for the execution based on the value of the expression.
The expression value must be a discrete type or one dimensional.
The equivalent hardware of case is Multilexer.
Syntax
Case expression is
when choice 1 => sequential statement;
when choice 2 => sequential statement;
when choice 3 => sequential statement;
end case;
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