VHDL Synchronizer Function

[genogia...@gmail.com]

Is there a way to write a VHDL synchronizer function that could be called from a section of sequential code as:

if sync(clock, a) = '1' then...

where "a" is a signal from another clock domain and "clock" is the clock in the present domain?

I've tried

function sync(clock, in: std_logic) return std_logic is
variable med, result: std_logic;
begin
if clock'event and clock = '1' then
med := input;
result := med;
end if;
return result;
end sync;


The result in the Xilinx simulator is syntactically accepted but is always undefined, no matter how many variations of the above I tried.

Is there a technique which preserves the convenience of my approach but works?

Geno

[Dio Gratia]

Local variables in a function are created new each function call. Also note your sequential assignments of med and result don't imply a clock delay between them the value of med is updated immediately. While your simulation might work it doesn't synchronize.

You could try a procedure:

library ieee;
use ieee.std_logic_1164.all;

entity foo is
end entity;

architecture fum of foo is
procedure sync (
signal clk: in std_logic;
signal a: in std_logic;
signal med: inout std_logic;
signal result: inout std_logic
) is
begin
if clk'event and clk = '1' then
med <= a;

result <= med;
end if;

end ;

signal a: std_logic;
signal clk: std_logic := '0';
signal med: std_logic;
signal result: std_logic;
begin
SYNCHRO:
sync (clk,a,med,result);
EVALUATE:
process (result)
begin
if result = '1' then

end if;
end process;

end architecture;


Notice this is the equivalent of using a sync component because a procedure doesn't have a return value, with various restrictions on using signals in a procedure effectively limiting this to one level of hierarchy.

You might as well create a sync component and use and output signal name that's descriptive (e.g. a_sync).

[Dio Gratia]

You could note in your function, besides the formal 'in' in 'clock, in:' being illegal (appears to have been intended to be 'clock, input:'), the formal 'clock' hasn't been declared as class signal, meaning as class variable (the default) the expression 'clock'event' is illegal and should result in an error.