measure period of impulse!

Here's a basic code template. Signal and port definitions are left up to you.

process (clk)
begin
if rising_edge(clk) then
  pulse_inp_sync <= pulse_inp;
  pulse_inp_prev <= pulse_inp_sync;
  if (not pulse_inp_prev and pulse_inp_sync ) = '1' then
    counter <= (others => '0');
    period <=  counter;
  elsif counter < MAXCOUNT then
    counter <= counter + 1;
  end if;
end if;
end process;