锯齿波分段函数程序仿真结果不对，坐等高手指点

hapday1

library IEEE;
use IEEE.Std_logic_1164.all;
USE ieee.std_logic_unsigned.ALL;

entity adclk_pwm_gen is
   port( clock : in Std_logic;   

datain:in std_logic_vector(15 downto 0);
   
data_us_testut std_logic_vector(15 downto 0)

);   

end adclk_pwm_gen;

architecture V2 of adclk_pwm_gen is


SIGNAL data_us: std_logic_vector(15 downto 0);

SIGNAL data_us_2: std_logic_vector(15 downto 0);



begin
process(clock)
begin
if clock='1' and clock'event then

if (datain>=x"0000" and datain<x"0800")then

data_us<=datain;

else if (datain>=x"0800" and datain<x"1000") then

data_us<=datain-x"0800";

else if (datain>=x"1000" and datain<x"1800")then

data_us<=datain-x"1000";

else if (datain>=x"1800" and datain<x"2000")then

data_us<=datain-x"1800";


end if;

end if;


end if;

end if;

end if;
end process;
data_us_test<=data_us;
end v2;

hapday1

仿真结果：，data_us在datain大于等于0800以后一直不变