Subversion Repositories forth-cpu

-------------------------------------------------------------------------------

--| @file timer.vhd

--| @brief General Purpose Timer. It is of customizable length,

--|        the minimum being 4-bits, one for the actual timing, the other

--|        three for control. (timer.vhd, original file name)

--|

--| @author         Richard James Howe.

--| @copyright      Copyright 2017 Richard James Howe.

--| @license        MIT

--| @email          howe.r.j.89@gmail.com

--|

--| The control register contains both the value to compare the timer against

--| as well as three control bits. Given a "timer_length" value of eight the

--| control bits are:

--|

--| Bit     Input Description

--| 7       Clock enable

--| 6       Timer reset

--| 5       Interrupt enable

--| 4 - 0   Timer compare value

--|

-------------------------------------------------------------------------------

library ieee,work,std;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

entity timer is

        generic(

                timer_length: positive := 16);

        port(

                clk:          in  std_ulogic;

                rst:          in  std_ulogic;

                we:           in  std_ulogic; -- write enable for control register

                control_i:    in  std_ulogic_vector(timer_length - 1 downto 0); -- control register

                counter_o:    out std_ulogic_vector(timer_length - 4 downto 0);

                irq:          out std_ulogic); -- generate interrupt

end entity;

architecture behav of timer is

        constant highest_bit:         positive := timer_length - 1;

        constant control_enable_bit:  positive := highest_bit;

        constant timer_reset_bit:     positive := highest_bit - 1;

        constant irq_enable_bit:      positive := highest_bit - 2;

        constant timer_highest_bit:   positive := highest_bit - 3;

        signal control_c, control_n:  std_ulogic_vector(highest_bit downto 0) := (others => '0');

        signal reset_timer:           std_ulogic  := '0';

        signal enabled:               std_ulogic  := '0';

        signal irq_en:                std_ulogic  := '0';

        signal timer_reset:           std_ulogic  := '0';

        signal compare:               std_ulogic_vector(timer_highest_bit downto 0) := (others => '0');

        signal count:                 unsigned(timer_highest_bit downto 0)         := (others => '0');

begin

        assert (timer_length >= 4) report "Timer needs to be at least 4 bits wide: 3 bits for control - 1 for counter" severity failure;

        enabled     <= control_c(control_enable_bit);

        reset_timer <= control_c(timer_reset_bit);

        irq_en      <= control_c(irq_enable_bit);

        compare     <= control_c(timer_highest_bit downto 0);

        counter_o   <= std_ulogic_vector(count);

        clockRegisters: process(clk, rst)

        begin

                if rst = '1' then

                        control_c <= (others => '0');

                elsif rising_edge(clk) then

                        control_c <= control_n;

                end if;

        end process;

        counter: process (clk, rst)

        begin

                if rst = '1' then

                        count <= (others => '0');

                elsif rising_edge(clk) then

                        if reset_timer = '1' or timer_reset = '1' then

                                count <= (others => '0');

                        elsif enabled = '1' then

                                count <= count + 1;

                        else

                                count <= count;

                        end if;

                end if;

        end process;

        output: process(count, we, control_i, control_c, compare, irq_en, enabled)

        begin

                irq         <= '0';

                control_n   <= control_c;

                timer_reset <= '0';

                control_n(timer_reset_bit)  <= '0'; -- reset

                if we = '1' then

                        control_n <= control_i;

                end if;

                if count = unsigned(compare) and enabled = '1' then

                        if irq_en = '1' then

                                irq <= '1';

                        end if;

                        timer_reset <= '1';

                end if;

        end process;

end architecture;