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

entity steering_servo is
   generic (
      -- pwd_period controls the output peroid
      -- of the pulse width modulator,
      -- pwm_duty_cycle and pwm_counter width
      -- should be adjusted if this generic is altered
      trim        : positive := 18;
      pwm_period  : positive := 272
   );
   port (
      -- clk is rising edge sensitive
      -- reset and enable are active high
      clk, enable, reset : in std_logic;
      servo_pos   : in std_logic_vector(3 downto 0);
      servo_out   : buffer std_logic
   );
end steering_servo;

architecture behavioral of steering_servo is
signal pwm_counter : std_logic_vector(8 downto 0);
signal slow_clk : std_logic;
signal slow_clk_count : std_logic_vector(10 downto 0);
signal duty_cycle : std_logic_vector(8 downto 0);
signal zero : std_logic_vector(3 downto 0);

begin
	zero <= "0111";

   -- provides a clock of 16.3262 kHz
   process (clk)
   begin
	  if reset = '0' then
			slow_clk_count <= (others => '0');
			slow_clk <= '0';
      elsif clk'event and clk = '1' then
         if slow_clk_count >= 771 then
            slow_clk_count <= (others => '0');
            slow_clk <= not slow_clk;
         else
            slow_clk_count <= slow_clk_count + 1;
         end if;
      end if;
   end process;

   process (slow_clk)
   begin
		if reset = '0' then
			pwm_counter <= (others => '0');
			duty_cycle <= zero + trim;
      elsif slow_clk'event and slow_clk = '1' then
            -- if our counter has exceeded the desired
            -- period then restart it
            if pwm_counter >= pwm_period then
               pwm_counter <= (others => '0');
            else
               pwm_counter <= pwm_counter + 1;
            end if;
			-- if enabled adjust the servo position
			-- as required
			-- else set it to zero position
			if enable = '1' then
	            duty_cycle <= (servo_pos + trim);
			end if;

            -- output a 1 for the duty cycle and
            -- output 0 otherwise
            if pwm_counter < duty_cycle then
               servo_out <= '1';
            else
               servo_out <= '0';
            end if;
      end if;
   end process;
end behavioral;