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Sequential Design

Sequential Design. Sequential code. Code written within the following statements execute sequentially. Process Functions Procedures. Process. A process is a sequential section of VHDL code. It is characterized by the presence of following statements: IF Wait Case Loop.

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Sequential Design

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  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 assignments; ………………….. ……… else assignments; end if;

  6. Case statements The format of a case statement is caseexpressionis whenchoices => sequential-statements -- branch #1 whenchoices => 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; begin 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 in2 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 • 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

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