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Sequential Design

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Sequential code Code written within the following

statements execute sequentially.

Process

Functions

Procedures

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Process A process is a sequential section of VHDL

code.

It is characterized by the presence offollowing statements:

IF

Wait Case

Loop

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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 isfulfilled).

Syntax:

[label:] Process (sensitivity list)

[variable name type [range] [:=initial value;]]Begin

(sequential code)

End Process [label];

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If statementIf condition then

assignments;

elsif condition then

assignments;

..

else assignments;

end if;

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Case statementsThe 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 ] -- lastbranch

end case;

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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;

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entity MUX is

port (A, B, C, D: in BIT; CTRL: in BIT_VECTOR(0 to 1);

Z: out BIT);

end MUX;

architecture MUX_BEHAVIORofMUX is

constant MUX_DELAY: TIME := 10 ns;

begin

PMUX: process (A, B, C, D, CTRL)

variable TEMP: BIT;

begin

case CTRL is

when "00" => TEMP := A:

when "01" => TEMP := B;

when "10" => TEMP := C;

when "11" => TEMP := D;

end case;

Z

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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];

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Example of For/loopFACTORIAL := 1;

forNUMBERin 2 toN loop

FACTORIAL := FACTORIAL * NUMBER;

end loop;

NOTE: Range must be static.

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Loop (cont.) WHILE/LOOP : The loop is repeated

until a condition no longer holds.

[label:] WHILE condition LOOP

(sequential statements);

end LOOP [label];

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Example WHILE/Loop

While (I

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Other statements EXIT

Used for ending the loop

[label:] EXIT [label] [WHEN condition]

NEXT

Used for skipping loop steps.

[label:] NEXT [loop_label] [WHEN condition]

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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;

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Example (next)For I in 0 to 15 loop

next when I= skip; -- jump to next iteration