1. Sequential Design

2. Sequential code
Code written within the following statements execute sequentially.
Process
Functions
Procedures

3. Process A process is a sequential section of VHDL code.
It is characterized by the presence of following statements: IF
Wait
Case
Loop

4. Process (cont..)
A Process must be installed in the main code, and is executed every time a signal in the sensitivity list changes (or the condition related to WAIT is fulfilled).
Syntax:
[label:] Process (sensitivity list)
[variable name type [range] [:=initial value;]]
Begin
(sequential code)
End Process [label];

5. If statement If condition then assignments; elsif condition then
…………………..
………
else assignments;
end if;

6. Case statements The format of a case statement is case expression is
when choices => sequential-statements -- branch #1
when choices => sequential-statements -- branch #2
-- Can have any number of branches.
[ when others => sequential-statements ] -- last branch
end case;

7. Case (cont.) The case statement is very similar to when statement.
All the permutation must be tested, so the keyword OTHERS may be used.
NULL may be used, when no action is required to take place.
e.g. When OTHERS => NULL;

8. entity MUX is
port (A, B, C, D: in BIT; CTRL: in BIT_VECTOR(0 to 1);
Z: out BIT);
end MUX;
architecture MUX_BEHAVIOR of MUX is
constant MUX_DELAY: TIME := 10 ns;
begin
PMUX: process (A, B, C, D, CTRL)
variable TEMP: BIT;
case CTRL is
when "00" => TEMP := A:
when "01" => TEMP := B;
when "10" => TEMP := C;
when "11" => TEMP := D;
end case;
Z <= TEMP after MUX_DELAY;
end process PMUX;
end MUX_BEHAVIOR;

9. Loop
Loop is useful when a piece of code must be instantiated several times.
Loop is intended exclusively for sequential code.
For/loop : The loop is repeated a fixed number of times.
[label:] FOR identifier IN range LOOP
(sequential statements)
END LOOP [label];

10. Example of For/loop FACTORIAL := 1; for NUMBER in 2 to N loop
FACTORIAL := FACTORIAL * NUMBER;
end loop;
NOTE: Range must be static.

11. Loop (cont.)
WHILE/LOOP : The loop is repeated until a condition no longer holds.
[label:] WHILE condition LOOP
(sequential statements);
end LOOP [label];

12. Example WHILE/Loop While (I <10) Loop
wait until clk’event and clk=‘1’;
(other statement)
End loop;

13. Other statements EXIT NEXT Used for ending the loop
[label:] EXIT [label] [WHEN condition]
NEXT
Used for skipping loop steps.
[label:] NEXT [loop_label] [WHEN condition]

14. Example (exit and Next)
SUM := 1; J := 0;
L3: loop
J:=J+21;
SUM := SUM* 10;
if (SUM > 100) then
exit L3; "exit;" also would have been sufficient.
end if;
end loop L3;

15. Example (next) For I in 0 to 15 loop
next when I= skip; -- jump to next iteration