求基于VHDL的控制舵机程序？

这是控制180度舵机转到90度的程序，比较简单！

library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;
use ieee.std_logic_unsigned.all;

entity yan is
port(clk:in std_logic;
po:out std_logic);
end yan;

architecture zheng of yan is
signal co:integer range 0 to 1000000;
begin
process(clk)
if clk'event and clk='1' then
co<=co+1;
if co<75000 then
po<='1';
else
po<='0';
end if;
end if;
end process;
end zheng;