------------------------------------------------------------------------------
----                                                                      ----
----  64 bits clock counter                                               ----
----                                                                      ----
----  http://www.opencores.org/                                           ----
----                                                                      ----
----  Description:                                                        ----
----  This is a peripheral used by the PHI I/O layout. It just counts the ----
----  elapsed number of clocks.                                           ----
----                                                                      ----
----  To Do:                                                              ----
----  -                                                                   ----
----                                                                      ----
----  Author:                                                             ----
----    - Øyvind Harboe, oyvind.harboe zylin.com                          ----
----    - Salvador E. Tropea, salvador inti.gob.ar                        ----
----                                                                      ----
------------------------------------------------------------------------------
----                                                                      ----
---- Copyright (c) 2008 Øyvind Harboe <oyvind.harboe zylin.com>           ----
---- Copyright (c) 2008 Salvador E. Tropea <salvador inti.gob.ar>         ----
---- Copyright (c) 2008 Instituto Nacional de Tecnología Industrial       ----
----                                                                      ----
---- Distributed under the BSD license                                    ----
----                                                                      ----
------------------------------------------------------------------------------
----                                                                      ----
---- Design unit:      Timer(Behave) (Entity and architecture)            ----
---- File name:        timer.vhdl                                         ----
---- Note:             None                                               ----
---- Limitations:      None known                                         ----
---- Errors:           None known                                         ----
---- Library:          zpu                                                ----
---- Dependencies:     IEEE.std_logic_1164                                ----
----                   IEEE.numeric_std                                   ----
----                   zpu.zpupkg                                         ----
---- Target FPGA:      Spartan 3 (XC3S1500-4-FG456)                       ----
---- Language:         VHDL                                               ----
---- Wishbone:         No                                                 ----
---- Synthesis tools:  Xilinx Release 9.2.03i - xst J.39                  ----
---- Simulation tools: GHDL [Sokcho edition] (0.2x)                       ----
---- Text editor:      SETEdit 0.5.x                                      ----
----                                                                      ----
------------------------------------------------------------------------------

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;
 
entity Timer is
   port(
      clk_i    : in  std_logic;
      reset_i  : in  std_logic;
      we_i     : in  std_logic;
      data_i   : in  unsigned(31 downto 0);
      addr_i   : in  unsigned(0 downto 0);
      data_o   : out unsigned(31 downto 0));
end entity Timer;

architecture Behave of Timer is
   signal sample   : std_logic;
   signal reset    : std_logic;
   
   signal cnt      : unsigned(63 downto 0);
   signal cnt_smp  : unsigned(63 downto 0);
begin
   reset  <= '1' when (we_i='1' and data_i(0)='1') else '0';
   sample <= '1' when (we_i='1' and data_i(1)='1') else '0';

   -- Carry generation
   do_timer:
   process (clk_i)
   begin
      if rising_edge(clk_i) then
         if reset_i='1' or reset='1' then
            cnt <= (others => '0');
            cnt_smp <= (others => '0');
         else
            cnt <= cnt+1;
            if sample='1' then
               -- report "sampling" severity failure;
               cnt_smp <= cnt;
            end if;
         end if; -- else reset_i='1'
      end if; -- rising_edge(clk_i)
   end process do_timer;

   data_o <= cnt_smp(31 downto  0) when to_01(addr_i)="0" else
             cnt_smp(63 downto 32);
end architecture Behave; -- Entity: Timer