TIMER.vhd

-- TIMER.VHD (a peripheral module for SCOMP)
-- 2003.04.24
--
-- Timer returns a 16 bit counter value with a resolution of the CLOCK period.
-- Writing any value to timer resets to 0x0000, but the timer continues to run.
-- The counter value rolls over to 0x0000 after a clock tick at 0xFFFF.

LIBRARY IEEE;
LIBRARY LPM;

USE IEEE.STD_LOGIC_1164.ALL;
USE IEEE.STD_LOGIC_ARITH.ALL;
USE IEEE.STD_LOGIC_UNSIGNED.ALL;
USE LPM.LPM_COMPONENTS.ALL;


ENTITY TIMER IS
  PORT(CLOCK,
       RESETN,
       CS,
       IO_WRITE : IN    STD_LOGIC;
       IO_DATA  : INOUT STD_LOGIC_VECTOR(15 DOWNTO 0) );
END TIMER;


ARCHITECTURE a OF TIMER IS
  SIGNAL COUNT  : STD_LOGIC_VECTOR(15 DOWNTO 0);
  SIGNAL IO_OUT : STD_LOGIC;


  BEGIN
    -- Use LPM function to create bidirection I/O data bus
    IO_BUS: lpm_bustri
    GENERIC MAP (
      lpm_width => 16
    )
    PORT MAP (
      data     => COUNT,
      enabledt => IO_OUT,
      tridata  => IO_DATA
    );


    IO_OUT <= (CS AND NOT(IO_WRITE));


    PROCESS (CLOCK, RESETN, CS, IO_WRITE)
      BEGIN
        IF (RESETN = '0' OR (CS AND IO_WRITE) = '1') THEN
          COUNT <= x"0000";
        ELSIF (FALLING_EDGE(CLOCK)) THEN
          COUNT <= COUNT + 1;
        END IF;
      END PROCESS;
  END a;