• Though in principle the presence of ELSE in the IF/ELSE statement might infer the implementation of a priority decoder (which would never occur with CASE), this will generally not happen.
• When IF (a sequential statement) is used to implement a fully combinational circuit, a multiplexer might be inferred instead.
• Therefore, after optimization, the general tendency is for a circuit synthesized from a VHDL code based on IF not to differ from that based on CASE.
Same inference !
---- With IF: --------------
IF (sel="00") THEN x<=a;
ELSIF (sel="01") THEN x<=b;
ELSIF (sel="10") THEN x<=c;
ELSE x<=d;
---- With CASE: ------------
CASE sel IS
WHEN "00" => x<=a;
WHEN "01" => x<=b;
WHEN "10" => x<=c;
WHEN OTHERS => x<=d;
END CASE;
CASE versus WHEN
• CASE and WHEN are very similar. However, while one is concurrent (WHEN), the other is sequential (CASE).
Same functionality !
---- With WHEN: ----------------
WITH sel SELECT
x <= aWHEN "000",
b WHEN "001",
c WHEN "010",
UNAFFECTED WHEN OTHERS;
---- With CASE: ----------------
CASE sel IS
WHEN "000" => x<=a;
WHEN "001" => x<=b;
WHEN "010" => x<=c;
WHEN OTHERS => NULL;
END CASE;
information shared by www.irvs.info
