Measuring Frequency help

[Altera_Forum]

I need to measuring a signal frequency 

i have a reference frequency of 100 MHz  i used the following code bu it didn't work properly  it counter right at the first but after that it got wrong:  process(n_reset,clk_ref100,clk)  begin  if n_reset='0' then  s_clk_ref100_counter<=(others=>'0');  elsif rising_edge(clk_ref100) then  if s_clk_ref100_counter>=10000 then   s_freq<=s_sys_clk_counter;  s_clk_ref100_counter<=(others=>'0');  else  s_clk_ref100_counter<=s_clk_ref100_counter+1;  end if;  end if;  if n_reset='0' then  s_sys_clk_counter<= (others=>'0');  elsif rising_edge(clk) then  if ((s_freq<=s_sys_clk_counter-5) and (s_sys_clk_counter/=x"000"))   then  s_sys_clk_counter<= (others=>'0');  else  s_sys_clk_counter<=s_sys_clk_counter+1;   end if;  end if;  end process;   Please advice me.

[Altera_Forum]

how fast is clk? the fact you are samplining a 100MHz counter in another clock domain and vice versa is probably your problem. You will need to transfer the counters across the clock domains safely, either with a double register or a dual clock fifo.

[Altera_Forum]

I tiried with triple register and it worked 

thanks