Microsoft Word - Digital Logic Design v_4_6a

when choices => sequential-statements

...

when choices => sequential_statements

when others sequential_statements -- do if none of choices match

end case;

Use case statement instead of if-then-else if possible since it is easier to synthesize.

 Example – Prime number detector using case statement

architecture prime6_arch of prime is

begin

process(N)

begin

case CONV_INTEGER(N) is

when 1 => F <= ‘1’;

when 3 | 5 | 7| 11 | 13 => F <= ‘1’;

when others => F <= ‘0’;

end case;

end process;

end prime6_arch;

 Sequential “Loop” Statements
There are three types of loops that are useful in synthesizing repeated structures.

 Sequential “Basic Loop” Statement syntax

This creates an infinite loop which is useful when doing modeling.

loop

sequential-statement

...
sequential-statement
end loop;

 Sequential “For Loop” Statement syntax

the identifier is implicitly declared and will have the same type as the range. The identifier may be

used inside the loop only.

for identifier in range loop

sequential-statement

...
sequential-statement
end loop;

The two sequential statements “exit” and “next” may be used in the loop body:

 “exit” terminates the loop and continues with the next statement after the loop.

 “next” starts the next iteration through the loop, bypassing the remaining statement in the

current iteration.

 Sequential “While Loop” statement syntax

The identifier is implicitly declared and will have the same type as the range. The identifier may

be used inside the loop only.

while Boolean_expression loop

sequential_statement

...