clk'event vs rising_edge()

Question

I had always used this for detecting a rising edge:

if (clk'event and clk='1') then

but this can also be used:

if rising_edge(clk) then

Reading this post, rising_edge(clk) is recommended, but there is also a comment indicating that rising_edge(clk) could lead to wrong behaviour.

I can't decide which one to choose for the future, going on with (clk'event and clk='1') or adopting rising_edge(clk).

Any real-world expereince on these two? Any preferences?

Thanks!

Answer 1

rising_edge is defined as:

FUNCTION rising_edge  (SIGNAL s : std_ulogic) RETURN BOOLEAN IS BEGIN     RETURN (s'EVENT AND (To_X01(s) = '1') AND                         (To_X01(s'LAST_VALUE) = '0')); END;  FUNCTION To_X01  ( s : std_ulogic ) RETURN  X01 IS BEGIN     RETURN (cvt_to_x01(s)); END;  CONSTANT cvt_to_x01 : logic_x01_table := (                      'X',  -- 'U'                      'X',  -- 'X'                      '0',  -- '0'                      '1',  -- '1'                      'X',  -- 'Z'                      'X',  -- 'W'                      '0',  -- 'L'                      '1',  -- 'H'                      'X'   -- '-'                     ); 

If your clock only goes from 0 to 1, and from 1 to 0, then rising_edge will produce identical code. Otherwise, you can interpret the difference.

Personally, my clocks only go from 0 to 1 and vice versa. I find rising_edge(clk) to be more descriptive than the (clk'event and clk = '1') variant.