calculate 64 bit unsigned

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the problem arises when N is 64 as a integer is not enough to represent it.

How could i resolve?

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Indeed..

try this since Fo*2^64/clk = Fo*2^32/clk * 2*32/clk.

function phase_inc(Fout,clk, N : natural) return std_logic_vector is 
variable phase : integer;
variable temp1,temp2:unsigned(N/2-1 downto 0);
begin
  phase := Fout*(2**N/2)/clk;
  temp1 := to_unsigned(phase,32);
  phase := (2**N/2)/clk;
  temp2 := to_unsigned(phase,32);
  return std_logic_vector(temp1*temp2);
end function;