There are two largely accepted ways to detect clock edges, and many style books prescribe or prefer one over the other. Let's investigate.

This article deals with edge detection in synthesizable code, not with behavioral code as found in test benches and behavioral models.

Old style

What I call the old style is:

if clk'event and clk='1' then -- old style: outdated

This coding style is accepted by the IEEE Standard for VHDL Register Transfer Level (RTL) Synthesis (IEEE 1076.6-1999), so all compliant synthesis tools should be able to handle this. However, there are some loopholes for simulators. This condition will also trigger when the clk signal changes from 'U' to '1', or even from 'H' to '1'. These events have no meaning in physical hardware, but they are perfectly legal in simulated VHDL code. Hence, this coding style introduces a discrepancy between simulation and synthesized hardware.

Another argument against the old style edge detection is that it does not clearly express the intent of the VHDL designer. Checking for an event makes sense in the domain of discrete event simulators, but is utter nonsense both in the (more low-level) domain physical electronics and in the (more high-level) domain of interacting state machines. Code is more readable when you use the rising_edge and falling_edge functions for edge detection.

In VHDL 87, this was the only way to detect edges. VHDL 93 has introduced the rising_edge and falling_edge functions for edge detection. For more than twenty years, the old style edge detection has been outdated.

Rising_edge and Falling_edge

In the VHDL 93 standard, the IEEE introduced two functions which are meant to be used for clock edge detection: rising_edge and falling_edge.

if rising_edge(clk) then -- recommended since 1993

These function can be applied to signals of type std_ulogic (and hence, std_logic). Since VHDL-2008, these functions are also available for bit and boolean (IEEE 1076-2008 paragraph 5.2.6). There are two benefits to using these functions: they check for meta values and they clearly express the intent fo the designer. For your information, the ieee.std_logic_1164 package implements rising_edge like this:

FUNCTION rising_edge(SIGNAL s : std_ulogic) RETURN BOOLEAN IS
        RETURN(s'EVENT AND(To_X01(s) = '1') AND(To_X01(s'LAST_VALUE) = '0'));

Rising or falling?

If you use a single edge, by convention, you should use the rising edge. This is arbitrary, but universally accepted. If you make a different choice, people will be surprised and confused. Of course, if you have a design with two clock edges, you also need the falling edge. If your process uses both edges (to instantiate dual edge triggered flip-flops), you should write:

if rising_edge(clk) or falling_edge(clk) then

If you just check for an event on the clock like this:

if clk'event then --DO NOT use this!

you end up with the same problems as in the old-style. Moreover, this is not a valid edge detection according to the IEEE VHDL RTL standard.

Other rules

Here are some more coding rules that are required by IEEE 1076.6-1999, and hence should be considered imperative.

No enable conditions in the same if statement. Do not write:

if rising_edge(clk) and enable='1' then -- BAD!

but write:

if rising_edge(clk) then
  if enable='1' then -- good.

Only use one clock per process (IEEE 1076.6-1999 paragraph 6.1.3).