source: firmware/FAD/FACT_FAD_lib/hdl/timer_beha.vhd

--
-- VHDL Architecture FACT_FAD_lib.timer.beha
--
-- Created:
--          by - dneise.UNKNOWN (E5B-LABOR6)
--          at - 13:44:41 22.02.2011
--
-- using Mentor Graphics HDL Designer(TM) 2009.2 (Build 10)

LIBRARY ieee;
USE ieee.std_logic_1164.all;
USE ieee.std_logic_arith.all;
USE ieee.std_logic_unsigned.all;

ENTITY timer IS
        generic(
                TIMER_WIDTH : integer := 32;
                PRESCALER : integer := 5000             
        );
        port (
                clk : in std_logic; -- assumed to be 25MHz, if not 25MHz adjust PRESCALER
                time_o : out std_logic_vector ( TIMER_WIDTH-1 downto 0);
                synch_i : in std_logic ; 
                synched_o : out std_logic := '0';
                reset_synch_i : in std_logic;
                enable_i : in std_logic
        );
END ENTITY timer;

--
ARCHITECTURE beha OF timer IS
        signal prescale_counter : integer range 0 to PRESCALER - 1 := 0;
        
        --signal time_s : integer range 0 to 2**(TIMER_WIDTH-1);
        signal time_s : std_logic_vector ( TIMER_WIDTH-1 downto 0);
        
        signal en_sr : std_logic_vector(1 downto 0) := "00";
        signal sy_sr : std_logic_vector(1 downto 0) := "00";
        signal reset_synch_sr : std_logic_vector(1 downto 0) := "00";
        
        signal timer_proc_enabled : std_logic := '0';
        signal synched : std_logic := '0';
        
BEGIN
        --time_o <= conv_std_logic_vector(time_s, TIMER_WIDTH);
        time_o <= time_s;
        synched_o <= synched;
        
                
        main_proc: process (clk)
        begin
                if rising_edge(clk) then
                        en_sr <= en_sr(0) & enable_i;
                        sy_sr <= sy_sr(0) & synch_i;
                        reset_synch_sr <= reset_synch_sr(0) & reset_synch_i; 
                        
                        if ( reset_synch_sr = "01" ) then
                                synched <= '0';
                        end if;
                        
                        if (sy_sr = "01" and synched = '0') then -- rising edge on synchronizstion_input detected AND if not already synched
                                time_s <= conv_std_logic_vector(0,TIMER_WIDTH);
                                prescale_counter <= 1;
                                synched <= '1';
                        end if;

                        if (en_sr = "01") then -- rising edge on enable_input detected
                                time_s <= conv_std_logic_vector(0,TIMER_WIDTH);
                                prescale_counter <= 1;
                                timer_proc_enabled <= '1';
                        elsif (en_sr = "10") then -- falling edge on enable_input detected
                                time_s <= conv_std_logic_vector(0,TIMER_WIDTH); 
                                prescale_counter <= 0;
                                timer_proc_enabled <= '0';
                        end if;
                        
                        -- PRESCALER PART OF PROCESS
                        if (timer_proc_enabled = '1') then
                                if (prescale_counter < PRESCALER - 1) then 
                                        prescale_counter <= prescale_counter + 1;
                                else 
                                        prescale_counter <= 0;
                                end if;
                                if (prescale_counter = PRESCALER - 1) then 
                                        if ( time_s < conv_std_logic_vector(2**TIMER_WIDTH-1 ,TIMER_WIDTH) ) then 
                                                time_s <= time_s + conv_std_logic_vector(1,TIMER_WIDTH);
                                        else
                                                time_s <= conv_std_logic_vector(0,TIMER_WIDTH);
                                        end if;
                                end if;
                        else  -- not timer_proc_enabled
                                time_s <= conv_std_logic_vector(0,TIMER_WIDTH);
                        end if; -- if timer_proc_enabled                
                end if; -- rising_edge(clk)
        end process main_proc;
END ARCHITECTURE beha;