Event.....

[Jack]

I am puzzled by the "event" attribute. I am not the most experienced digital
designer yet, so apologize if my questions seems odd/simple.
If I write the simple VHDL code (not showing entity,
architecture-decleration etc.):

SIGNAL myvector1: std_logic_vector(15 DOWNTO 0);
SIGNAL mystate: std_logic_vector(2 DOWNTO 0);

PROCESS
BEGIN

IF(myvector'EVENT) THEN
mystate <= "001";
END IF;

END PROCESS

What does the "EVENT" attribute actually translate to in hardware? How do
you make an event detector. My problem is that I would like to have a
statemachine that has two states and jumps to either states when an event
occurs on two inputs. I could write the VHDL code but I am not sure how the
event statement is actually implemented in regards to logic...anyone knows
an answer to this question?

Best Regards
Jack

 

[Andrew Hall]

On Sat, 17 Apr 2004 13:42:53 +0200, Jack wrote:

I am puzzled by the "event" attribute. I am not the most experienced digital
designer yet, so apologize if my questions seems odd/simple.
snip

On its own the 'event attribute won't translate to anything as far as I'm
aware. It signifies that something has happened to that signal. I've seen
it used to detect rising edges of clocks like this:
foo: process(clk) is
begin
if (clk'event and clk = '1') then
--Something just happened to clk and it's now 1, so it was probably a
--rising edge
this_state <= next_state;
end if;
end process foo;

I'm sure some of our more experienced brethren can think of all sorts of
exciting stuff you can do with it.

HTH,

Andrew.

 

[Ralf Hildebrandt]

Jack wrote:

IF(myvector'EVENT) THEN
mystate <= "001";
END IF;

What does the "EVENT" attribute actually translate to in hardware?

Nothing. It is not synthesizeable.

Dual-edge behavior may come with the new VHDL standard in the future.

How do you make an event detector.

Take a clock, that is fast enough. Double buffer (2 flipflops in a
chain) the signal, you want to trigger on (input-signal). Compare the
two FFs: rising and falling edge of the input-signal is detectable.

Note: A fast clock will consume a lot of power, but with a slow clock
you may miss a too short event on the input-signal or get a big delay.


Ralf