How to measure distance between two data

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well just a clocked process will do.

process(clk)
begin
if rising_edge(clk) then
   if clear = '1' then
       sum <= accum;
       accum <= (others => '0');
   else
       accum <= accum +distance;
   end if;
end if;
end process;

assuming distance is an incoming stream of samples, one per clock period. you control the clear signal using a counter for your samples.

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sorry kaz . i didn't undersatand that .if you remember that distance is output ,i make a signal instead of output but it every clk increment the accum.

could you plz illustrate.