Using the watch and turning it on

Question

I was provided with code for D-Flip-Flop with support.

process(clk, en)
    begin
        if rising_edge(clk) then
             if en = ‘1’ then
                 Q <= D;
        end if;
     end if;
end process;

• I was told that I should not use if rising_edge(clk) and en = ‘1’ then .... Why?
  
• Why is there no if for en = '1'before if, if for hours, since the clock changes more often?
  
• And is it necessary to indicate what is enin the process brackets process(clk, en)?

Answer 1

• Some people believe that VHDL compilers and synthesizers cannot understand that this is the same as what you showed here. I never compared the result directly, but I would be sad if that mattered.

• . . , , , .

• .

Answer 2

:

, . reset, :

process(clock)  -- nothing else should go in the sensitivity list
begin
    -- never put anything here
    if rising_edge(clock) then  -- or falling_edge(clock)
        -- put the synchronous stuff here
        -- ie the stuff that happens on the rising or falling edge of the clock
    end if;
     -- never put anything here
end process;        

en senstivity, if, .

, , en : reset ; D.