Subversion Repositories System09

library IEEE, UNISIM;

use IEEE.std_logic_1164.all;

package sdram is

  -- SDRAM controller

  component sdramCntl

    generic(

      FREQ                 :     natural := 100_000;  -- operating frequency in KHz

      IN_PHASE             :     boolean := true;  -- SDRAM and controller work on same or opposite clock edge

      PIPE_EN              :     boolean := false;  -- if true, enable pipelined read operations

      MAX_NOP              :     natural := 10000;  -- number of NOPs before entering self-refresh

      MULTIPLE_ACTIVE_ROWS :     boolean := false;  -- if true, allow an active row in each bank

      DATA_WIDTH           :     natural := 16;  -- host & SDRAM data width

      NROWS                :     natural := 8192;  -- number of rows in SDRAM array

      NCOLS                :     natural := 512;  -- number of columns in SDRAM array

      HADDR_WIDTH          :     natural := 24;  -- host-side address width

      SADDR_WIDTH          :     natural := 13  -- SDRAM-side address width

      );

    port(

      -- host side

      clk                  : in  std_logic;  -- master clock

      lock                 : in  std_logic;  -- true if clock is stable

      rst                  : in  std_logic;  -- reset

      rd                   : in  std_logic;  -- initiate read operation

      wr                   : in  std_logic;  -- initiate write operation

      earlyOpBegun         : out std_logic;  -- read/write/self-refresh op has begun (async)

      opBegun              : out std_logic;  -- read/write/self-refresh op has begun (clocked)

      rdPending            : out std_logic;  -- true if read operation(s) are still in the pipeline

      done                 : out std_logic;  -- read or write operation is done

      rdDone               : out std_logic;  -- read operation is done and data is available

      hAddr                : in  std_logic_vector(HADDR_WIDTH-1 downto 0);  -- address from host to SDRAM

      hDIn                 : in  std_logic_vector(DATA_WIDTH-1 downto 0);  -- data from host       to SDRAM

      hDOut                : out std_logic_vector(DATA_WIDTH-1 downto 0);  -- data from SDRAM to host

      status               : out std_logic_vector(3 downto 0);  -- diagnostic status of the FSM         

      -- SDRAM side

      cke                  : out std_logic;  -- clock-enable to SDRAM

      ce_n                 : out std_logic;  -- chip-select to SDRAM

      ras_n                : out std_logic;  -- SDRAM row address strobe

      cas_n                : out std_logic;  -- SDRAM column address strobe

      we_n                 : out std_logic;  -- SDRAM write enable

      ba                   : out std_logic_vector(1 downto 0);  -- SDRAM bank address

      sAddr                : out std_logic_vector(SADDR_WIDTH-1 downto 0);  -- SDRAM row/column address

      sDIn                 : in  std_logic_vector(DATA_WIDTH-1 downto 0);  -- data from SDRAM

      sDOut                : out std_logic_vector(DATA_WIDTH-1 downto 0);  -- data to SDRAM

      sDOutEn              : out std_logic  -- true if data is output to SDRAM on sDOut

      );

  end component;

  -- dual-port interface to the SDRAM controller

  component dualport

    generic(

      PIPE_EN         :    boolean                       := false;  -- enable pipelined read operations

      PORT_TIME_SLOTS :    std_logic_vector(15 downto 0) := "1111000011110000";

      DATA_WIDTH      :    natural                       := 16;  -- host & SDRAM data width

      HADDR_WIDTH     :    natural                       := 23  -- host-side address width

      );

    port(

      clk             : in std_logic;   -- master clock

      -- host-side port 0

      rst0          : in  std_logic;    -- reset

      rd0           : in  std_logic;    -- initiate read operation

      wr0           : in  std_logic;    -- initiate write operation

      earlyOpBegun0 : out std_logic;    -- read/write op has begun (async)

      opBegun0      : out std_logic;    -- read/write op has begun (clocked)

      rdPending0    : out std_logic;    -- true if read operation(s) are still in the pipeline

      done0         : out std_logic;    -- read or write operation is done

      rdDone0       : out std_logic;    -- read operation is done and data is available

      hAddr0        : in  std_logic_vector(HADDR_WIDTH-1 downto 0);  -- address from host to SDRAM

      hDIn0         : in  std_logic_vector(DATA_WIDTH-1 downto 0);  -- data from host to SDRAM

      hDOut0        : out std_logic_vector(DATA_WIDTH-1 downto 0);  -- data from SDRAM to host

      status0       : out std_logic_vector(3 downto 0);  -- diagnostic status of the SDRAM controller FSM         

      -- host-side port 1

      rst1          : in  std_logic;

      rd1           : in  std_logic;

      wr1           : in  std_logic;

      earlyOpBegun1 : out std_logic;

      opBegun1      : out std_logic;

      rdPending1    : out std_logic;

      done1         : out std_logic;

      rdDone1       : out std_logic;

      hAddr1        : in  std_logic_vector(HADDR_WIDTH-1 downto 0);

      hDIn1         : in  std_logic_vector(DATA_WIDTH-1 downto 0);

      hDOut1        : out std_logic_vector(DATA_WIDTH-1 downto 0);

      status1       : out std_logic_vector(3 downto 0);

      -- SDRAM controller port

      rst          : out std_logic;

      rd           : out std_logic;

      wr           : out std_logic;

      earlyOpBegun : in  std_logic;

      opBegun      : in  std_logic;

      rdPending    : in  std_logic;

      done         : in  std_logic;

      rdDone       : in  std_logic;

      hAddr        : out std_logic_vector(HADDR_WIDTH-1 downto 0);

      hDIn         : out std_logic_vector(DATA_WIDTH-1 downto 0);

      hDOut        : in  std_logic_vector(DATA_WIDTH-1 downto 0);

      status       : in  std_logic_vector(3 downto 0)

      );

  end component;

end package sdram;

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

-- Company : XESS Corp.

-- Engineer : Dave Vanden Bout

-- Creation Date : 05/17/2005

-- Copyright : 2005, XESS Corp

-- Tool Versions : WebPACK 6.3.03i

--

-- Description:

-- SDRAM controller

--

-- Revision:

-- 1.5.0

--

-- Additional Comments:

-- 1.5.0:

-- John Kent 2008-03-28

-- added upper and lower data strobes.

-- 1.4.0:

-- Added generic parameter to enable/disable independent active rows in each bank.

-- 1.3.0:

-- Modified to allow independently active rows in each bank.

-- 1.2.0:

-- Modified to allow pipelining of read/write operations.

-- 1.1.0:

-- Initial release.

-- 2.0.0 John Kent

-- removed uds, lds, dmqu, dmql for XuLA implementation

--

-- License:

-- This code can be freely distributed and modified as long as

-- this header is not removed.

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

library IEEE, UNISIM;

use IEEE.std_logic_1164.all;

use IEEE.std_logic_unsigned.all;

use IEEE.numeric_std.all;

use WORK.common.all;

entity sdramCntl is

  generic(

    FREQ                 :     natural := 100_000;  -- operating frequency in KHz

    IN_PHASE             :     boolean := true;  -- SDRAM and controller work on same or opposite clock edge

    PIPE_EN              :     boolean := false;  -- if true, enable pipelined read operations

    MAX_NOP              :     natural := 10000;  -- number of NOPs before entering self-refresh

    MULTIPLE_ACTIVE_ROWS :     boolean := false;  -- if true, allow an active row in each bank

    DATA_WIDTH           :     natural := 16;  -- host & SDRAM data width

    NROWS                :     natural := 4096;  -- number of rows in SDRAM array

    NCOLS                :     natural := 512;  -- number of columns in SDRAM array

    HADDR_WIDTH          :     natural := 24;  -- host-side address width

    SADDR_WIDTH          :     natural := 13  -- SDRAM-side address width

    );

  port(

    -- host side

    clk                  : in  std_logic;  -- master clock

    lock                 : in  std_logic;  -- true if clock is stable

    rst                  : in  std_logic;  -- reset

    rd                   : in  std_logic;  -- initiate read operation

    wr                   : in  std_logic;  -- initiate write operation

    earlyOpBegun         : out std_logic;  -- read/write/self-refresh op has begun (async)

    opBegun              : out std_logic;  -- read/write/self-refresh op has begun (clocked)

    rdPending            : out std_logic;  -- true if read operation(s) are still in the pipeline

    done                 : out std_logic;  -- read or write operation is done

    rdDone               : out std_logic;  -- read operation is done and data is available

    hAddr                : in  std_logic_vector(HADDR_WIDTH-1 downto 0);  -- address from host to SDRAM

    hDIn                 : in  std_logic_vector(DATA_WIDTH-1 downto 0);  -- data from host       to SDRAM

    hDOut                : out std_logic_vector(DATA_WIDTH-1 downto 0);  -- data from SDRAM to host

    status               : out std_logic_vector(3 downto 0);  -- diagnostic status of the FSM         

    -- SDRAM side

    cke     : out std_logic;            -- clock-enable to SDRAM

    ce_n    : out std_logic;            -- chip-select to SDRAM

    ras_n   : out std_logic;            -- SDRAM row address strobe

    cas_n   : out std_logic;            -- SDRAM column address strobe

    we_n    : out std_logic;            -- SDRAM write enable

    ba      : out std_logic_vector(1 downto 0);  -- SDRAM bank address

    sAddr   : out std_logic_vector(SADDR_WIDTH-1 downto 0);  -- SDRAM row/column address

    sDIn    : in  std_logic_vector(DATA_WIDTH-1 downto 0);  -- data from SDRAM

    sDOut   : out std_logic_vector(DATA_WIDTH-1 downto 0);  -- data to SDRAM

    sDOutEn : out std_logic             -- true if data is output to SDRAM on sDOut

    );

end sdramCntl;

architecture arch of sdramCntl is

  constant OUTPUT : std_logic := '1';   -- direction of dataflow w.r.t. this controller

  constant INPUT  : std_logic := '0';

  constant NOP    : std_logic := '0';   -- no operation

  constant READ   : std_logic := '1';   -- read operation

  constant WRITE  : std_logic := '1';   -- write operation

  -- SDRAM timing parameters

  constant Tinit : natural := 200;      -- min initialization interval (us)

  constant Tras  : natural := 45;       -- min interval between active to precharge commands (ns)

  constant Trcd  : natural := 20;       -- min interval between active and R/W commands (ns)

  constant Tref  : natural := 64_000_000;  -- maximum refresh interval (ns)

  constant Trfc  : natural := 66;       -- duration of refresh operation (ns)

  constant Trp   : natural := 20;       -- min precharge command duration (ns)

  constant Twr   : natural := 15;       -- write recovery time (ns)

  constant Txsr  : natural := 75;       -- exit self-refresh time (ns)

  -- SDRAM timing parameters converted into clock cycles (based on FREQ)

  constant NORM        : natural := 1_000_000;  -- normalize ns * KHz

  constant INIT_CYCLES : natural := 1+((Tinit*FREQ)/1000);  -- SDRAM power-on initialization interval

  constant RAS_CYCLES  : natural := 1+((Tras*FREQ)/NORM);  -- active-to-precharge interval

  constant RCD_CYCLES  : natural := 1+((Trcd*FREQ)/NORM);  -- active-to-R/W interval

  constant REF_CYCLES  : natural := 1+(((Tref/NROWS)*FREQ)/NORM);  -- interval between row refreshes

  constant RFC_CYCLES  : natural := 1+((Trfc*FREQ)/NORM);  -- refresh operation interval

  constant RP_CYCLES   : natural := 1+((Trp*FREQ)/NORM);  -- precharge operation interval

  constant WR_CYCLES   : natural := 1+((Twr*FREQ)/NORM);  -- write recovery time

  constant XSR_CYCLES  : natural := 1+((Txsr*FREQ)/NORM);  -- exit self-refresh time

  constant MODE_CYCLES : natural := 2;  -- mode register setup time

  constant CAS_CYCLES  : natural := 3;  -- CAS latency

  constant RFSH_OPS    : natural := 8;  -- number of refresh operations needed to init SDRAM

  -- timer registers that count down times for various SDRAM operations

  signal timer_r, timer_x       : natural range 0 to INIT_CYCLES;  -- current SDRAM op time

  signal rasTimer_r, rasTimer_x : natural range 0 to RAS_CYCLES;  -- active-to-precharge time

  signal wrTimer_r, wrTimer_x   : natural range 0 to WR_CYCLES;  -- write-to-precharge time

  signal refTimer_r, refTimer_x : natural range 0 to REF_CYCLES;  -- time between row refreshes

  signal rfshCntr_r, rfshCntr_x : natural range 0 to NROWS;  -- counts refreshes that are neede

  signal nopCntr_r, nopCntr_x   : natural range 0 to MAX_NOP;  -- counts consecutive NOP operations

  signal doSelfRfsh : std_logic;        -- active when the NOP counter hits zero and self-refresh can start

  -- states of the SDRAM controller state machine

  type cntlState is (

    INITWAIT,                           -- initialization - waiting for power-on initialization to complete

    INITPCHG,                           -- initialization - initial precharge of SDRAM banks

    INITSETMODE,                        -- initialization - set SDRAM mode

    INITRFSH,                           -- initialization - do initial refreshes

    RW,                                 -- read/write/refresh the SDRAM

    ACTIVATE,                           -- open a row of the SDRAM for reading/writing

    REFRESHROW,                         -- refresh a row of the SDRAM

    SELFREFRESH                         -- keep SDRAM in self-refresh mode with CKE low

    );

  signal state_r, state_x : cntlState;  -- state register and next state

  -- commands that are sent to the SDRAM to make it perform certain operations

  -- commands use these SDRAM input pins (ce_n,ras_n,cas_n,we_n)

--  subtype sdramCmd is unsigned(5 downto 0);

--  constant NOP_CMD    : sdramCmd := "011100";

--  constant ACTIVE_CMD : sdramCmd := "001100";

--  constant READ_CMD   : sdramCmd := "010100";

--  constant WRITE_CMD  : sdramCmd := "010000";

--  constant PCHG_CMD   : sdramCmd := "001011";

--  constant MODE_CMD   : sdramCmd := "000011";

--  constant RFSH_CMD   : sdramCmd := "000111";

  -- commands that are sent to the SDRAM to make it perform certain operations

  -- commands use these SDRAM input pins (ce_n,ras_n,cas_n,we_n)

  subtype sdramCmd is unsigned(3 downto 0);

  constant NOP_CMD    : sdramCmd := "0111";

  constant ACTIVE_CMD : sdramCmd := "0011";

  constant READ_CMD   : sdramCmd := "0101";

  constant WRITE_CMD  : sdramCmd := "0100";

  constant PCHG_CMD   : sdramCmd := "0010";

  constant MODE_CMD   : sdramCmd := "0000";

  constant RFSH_CMD   : sdramCmd := "0001";

  -- SDRAM mode register

  -- the SDRAM is placed in a non-burst mode (burst length = 1) with a 3-cycle CAS

  subtype sdramMode is std_logic_vector(12 downto 0);

  constant MODE : sdramMode := "000" & "0" & "00" & "011" & "0" & "000";

  -- the host address is decomposed into these sets of SDRAM address components

  constant ROW_LEN : natural := log2(NROWS);  -- number of row address bits

  constant COL_LEN : natural := log2(NCOLS);  -- number of column address bits

  signal   bank    : std_logic_vector(ba'range);  -- bank address bits

  signal   row     : std_logic_vector(ROW_LEN - 1 downto 0);  -- row address within bank

  signal   col     : std_logic_vector(sAddr'range);  -- column address within row

  -- registers that store the currently active row in each bank of the SDRAM

  constant NUM_ACTIVE_ROWS            : integer := int_select(MULTIPLE_ACTIVE_ROWS = false, 1, 2**ba'length);

  type activeRowType is array(0 to NUM_ACTIVE_ROWS-1) of std_logic_vector(row'range);

  signal   activeRow_r, activeRow_x   : activeRowType;

  signal   activeFlag_r, activeFlag_x : std_logic_vector(0 to NUM_ACTIVE_ROWS-1);  -- indicates that some row in a bank is active

  signal   bankIndex                  : natural range 0 to NUM_ACTIVE_ROWS-1;  -- bank address bits

  signal   activeBank_r, activeBank_x : std_logic_vector(ba'range);  -- indicates the bank with the active row

  signal   doActivate                 : std_logic;  -- indicates when a new row in a bank needs to be activated

  -- there is a command bit embedded within the SDRAM column address

  constant CMDBIT_POS    : natural   := 10;  -- position of command bit

  constant AUTO_PCHG_ON  : std_logic := '1';  -- CMDBIT value to auto-precharge the bank

  constant AUTO_PCHG_OFF : std_logic := '0';  -- CMDBIT value to disable auto-precharge

  constant ONE_BANK      : std_logic := '0';  -- CMDBIT value to select one bank

  constant ALL_BANKS     : std_logic := '1';  -- CMDBIT value to select all banks

  -- status signals that indicate when certain operations are in progress

  signal wrInProgress       : std_logic;  -- write operation in progress

  signal rdInProgress       : std_logic;  -- read operation in progress

  signal activateInProgress : std_logic;  -- row activation is in progress

  -- these registers track the progress of read and write operations

  signal rdPipeline_r, rdPipeline_x : std_logic_vector(CAS_CYCLES+1 downto 0);  -- pipeline of read ops in progress

  signal wrPipeline_r, wrPipeline_x : std_logic_vector(0 downto 0);  -- pipeline of write ops (only need 1 cycle)

  -- registered outputs to host

  signal opBegun_r, opBegun_x             : std_logic;  -- true when SDRAM read or write operation is started

  signal hDOut_r, hDOut_x                 : std_logic_vector(hDOut'range);  -- holds data read from SDRAM and sent to the host

  signal hDOutOppPhase_r, hDOutOppPhase_x : std_logic_vector(hDOut'range);  -- holds data read from SDRAM   on opposite clock edge

  -- registered outputs to SDRAM

  signal cke_r, cke_x           : std_logic;  -- clock enable 

  signal cmd_r, cmd_x           : sdramCmd;  -- SDRAM command bits

  signal ba_r, ba_x             : std_logic_vector(ba'range);  -- SDRAM bank address bits

  signal sAddr_r, sAddr_x       : std_logic_vector(sAddr'range);  -- SDRAM row/column address

  signal sData_r, sData_x       : std_logic_vector(sDOut'range);  -- SDRAM out databus

  signal sDataDir_r, sDataDir_x : std_logic;  -- SDRAM databus direction control bit

begin

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

  -- attach some internal signals to the I/O ports 

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

  -- attach registered SDRAM control signals to SDRAM input pins

  (ce_n, ras_n, cas_n, we_n) <= cmd_r;  -- SDRAM operation control bits

  cke                        <= cke_r;  -- SDRAM clock enable

  ba                         <= ba_r;  -- SDRAM bank address

  sAddr                      <= sAddr_r;  -- SDRAM address

  sDOut                      <= sData_r;  -- SDRAM output data bus

  sDOutEn                    <= YES when sDataDir_r = OUTPUT else NO;  -- output databus enable

  -- attach some port signals

  hDOut   <= hDOut_r;                   -- data back to host

  opBegun <= opBegun_r;                 -- true if requested operation has begun

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

  -- compute the next state and outputs 

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

  combinatorial : process(rd, wr, hAddr, hDIn, hDOut_r, sDIn, state_r, opBegun_x,

                          activeFlag_r, activeRow_r, rdPipeline_r, wrPipeline_r,

                          hDOutOppPhase_r, nopCntr_r, lock, rfshCntr_r, timer_r, rasTimer_r,

                          wrTimer_r, refTimer_r, cmd_r, cke_r, activeBank_r, ba_r )

  begin

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

    -- setup default values for signals 

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

    opBegun_x    <= NO;                 -- no operations have begun

    earlyOpBegun <= opBegun_x;

    cke_x        <= YES;                -- enable SDRAM clock

    cmd_x        <= NOP_CMD;            -- set SDRAM command to no-operation

    sDataDir_x   <= INPUT;              -- accept data from the SDRAM

    sData_x      <= hDIn(sData_x'range);  -- output data from host to SDRAM

    state_x      <= state_r;            -- reload these registers and flags

    activeFlag_x <= activeFlag_r;       --              with their existing values

    activeRow_x  <= activeRow_r;

    activeBank_x <= activeBank_r;

    rfshCntr_x   <= rfshCntr_r;

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

    -- setup default value for the SDRAM address 

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

    -- extract bank field from host address

    ba_x                    <= hAddr(ba'length + ROW_LEN + COL_LEN - 1 downto ROW_LEN + COL_LEN);

    if MULTIPLE_ACTIVE_ROWS = true then

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

      bankIndex             <= CONV_INTEGER(ba_x);

    else

      bank                  <= ba_x;

      bankIndex             <= 0;

    end if;

    -- extract row, column fields from host address

    row                     <= hAddr(ROW_LEN + COL_LEN - 1 downto COL_LEN);

    -- extend column (if needed) until it is as large as the (SDRAM address bus - 1)

    col                     <= (others => '0');  -- set it to all zeroes

    col(COL_LEN-1 downto 0) <= hAddr(COL_LEN-1 downto 0);

    -- by default, set SDRAM address to the column address with interspersed

    -- command bit set to disable auto-precharge

    sAddr_x                 <= col(col'high downto CMDBIT_POS+1) & AUTO_PCHG_OFF

                               & col(CMDBIT_POS-1 downto 0);

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

    -- manage the read and write operation pipelines

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

    -- determine if read operations are in progress by the presence of

    -- READ flags in the read pipeline 

    if rdPipeline_r(rdPipeline_r'high downto 1) /= 0 then

      rdInProgress <= YES;

    else

      rdInProgress <= NO;

    end if;

    rdPending      <= rdInProgress;     -- tell the host if read operations are in progress

    -- enter NOPs into the read and write pipeline shift registers by default

    rdPipeline_x    <= NOP & rdPipeline_r(rdPipeline_r'high downto 1);

    wrPipeline_x(0) <= NOP;

    -- transfer data from SDRAM to the host data register if a read flag has exited the pipeline

    -- (the transfer occurs 1 cycle before we tell the host the read operation is done)

    if rdPipeline_r(1) = READ then

      hDOutOppPhase_x <= sDIn(hDOut'range);  -- gets value on the SDRAM databus on the opposite phase

      if IN_PHASE then

        -- get the SDRAM data for the host directly from the SDRAM if the controller and SDRAM are in-phase

        hDOut_x       <= sDIn(hDOut'range);

      else

        -- otherwise get the SDRAM data that was gathered on the previous opposite clock edge

        hDOut_x       <= hDOutOppPhase_r(hDOut'range);

      end if;

    else

      -- retain contents of host data registers if no data from the SDRAM has arrived yet

      hDOutOppPhase_x <= hDOutOppPhase_r;

      hDOut_x         <= hDOut_r;

    end if;

    done   <= rdPipeline_r(0) or wrPipeline_r(0);  -- a read or write operation is done

    rdDone <= rdPipeline_r(0);          -- SDRAM data available when a READ flag exits the pipeline 

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

    -- manage row activation

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

    -- request a row activation operation if the row of the current address

    -- does not match the currently active row in the bank, or if no row

    -- in the bank is currently active

    if (bank /= activeBank_r) or (row /= activeRow_r(bankIndex)) or (activeFlag_r(bankIndex) = NO) then

      doActivate <= YES;

    else

      doActivate <= NO;

    end if;

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

    -- manage self-refresh

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

    -- enter self-refresh if neither a read or write is requested for MAX_NOP consecutive cycles.

    if (rd = YES) or (wr = YES) then

      -- any read or write resets NOP counter and exits self-refresh state

      nopCntr_x  <= 0;

      doSelfRfsh <= NO;

    elsif nopCntr_r /= MAX_NOP then

      -- increment NOP counter whenever there is no read or write operation 

      nopCntr_x  <= nopCntr_r + 1;

      doSelfRfsh <= NO;

    else

      -- start self-refresh when counter hits maximum NOP count and leave counter unchanged

      nopCntr_x  <= nopCntr_r;

      doSelfRfsh <= YES;

    end if;

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

    -- update the timers 

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

    -- row activation timer

    if rasTimer_r /= 0 then

      -- decrement a non-zero timer and set the flag

      -- to indicate the row activation is still inprogress

      rasTimer_x         <= rasTimer_r - 1;

      activateInProgress <= YES;

    else

      -- on timeout, keep the timer at zero     and reset the flag

      -- to indicate the row activation operation is done

      rasTimer_x         <= rasTimer_r;

      activateInProgress <= NO;

    end if;

    -- write operation timer            

    if wrTimer_r /= 0 then

      -- decrement a non-zero timer and set the flag

      -- to indicate the write operation is still inprogress

      wrTimer_x    <= wrTimer_r - 1;

      wrInPRogress <= YES;

    else

      -- on timeout, keep the timer at zero and reset the flag that

      -- indicates a write operation is in progress

      wrTimer_x    <= wrTimer_r;

      wrInPRogress <= NO;

    end if;

    -- refresh timer            

    if refTimer_r /= 0 then

      refTimer_x <= refTimer_r - 1;

    else

      -- on timeout, reload the timer with the interval between row refreshes

      -- and increment the counter for the number of row refreshes that are needed

      refTimer_x <= REF_CYCLES;

      rfshCntr_x <= rfshCntr_r + 1;

    end if;

    -- main timer for sequencing SDRAM operations               

    if timer_r /= 0 then

      -- decrement the timer and do nothing else since the previous operation has not completed yet.

      timer_x <= timer_r - 1;

      status  <= "0000";

    else

      -- the previous operation has completed once the timer hits zero

      timer_x <= timer_r;               -- by default, leave the timer at zero

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

      -- compute the next state and outputs 

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

      case state_r is

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

        -- let clock stabilize and then wait for the SDRAM to initialize 

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

        when INITWAIT =>

          if lock = YES then

                                        -- wait for SDRAM power-on initialization once the clock is stable

            timer_x <= INIT_CYCLES;     -- set timer for initialization duration

            state_x <= INITPCHG;

          else

                                        -- disable SDRAM clock and return to this state if the clock is not stable

                                        -- this insures the clock is stable before enabling the SDRAM

                                        -- it also insures a clean startup if the SDRAM is currently in self-refresh mode

            cke_x   <= NO;

          end if;

          status    <= "0001";

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

          -- precharge all SDRAM banks after power-on initialization 

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

        when INITPCHG =>

          cmd_x               <= PCHG_CMD;

          sAddr_x(CMDBIT_POS) <= ALL_BANKS;  -- precharge all banks

          timer_x             <= RP_CYCLES;  -- set timer for precharge operation duration

          rfshCntr_x          <= RFSH_OPS;  -- set counter for refresh ops needed after precharge

          state_x             <= INITRFSH;

          status              <= "0010";

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

          -- refresh the SDRAM a number of times after initial precharge 

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

        when INITRFSH =>

          cmd_x      <= RFSH_CMD;

          timer_x    <= RFC_CYCLES;     -- set timer to refresh operation duration

          rfshCntr_x <= rfshCntr_r - 1;  -- decrement refresh operation counter

          if rfshCntr_r = 1 then

            state_x  <= INITSETMODE;    -- set the SDRAM mode once all refresh ops are done

          end if;

          status     <= "0011";

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

          -- set the mode register of the SDRAM 

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

        when INITSETMODE =>

          cmd_x   <= MODE_CMD;

          sAddr_x <= MODE;              -- output mode register bits on the SDRAM address bits

          timer_x <= MODE_CYCLES;       -- set timer for mode setting operation duration

          state_x <= RW;

          status  <= "0100";

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

          -- process read/write/refresh operations after initialization is done 

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

        when RW                                      =>

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

          -- highest priority operation: row refresh 

          -- do a refresh operation if the refresh counter is non-zero

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

          if rfshCntr_r /= 0 then

                                        -- wait for any row activations, writes or reads to finish before doing a precharge

            if (activateInProgress = NO) and (wrInProgress = NO) and (rdInProgress = NO) then

              cmd_x                       <= PCHG_CMD;  -- initiate precharge of the SDRAM

              sAddr_x(CMDBIT_POS)         <= ALL_BANKS;  -- precharge all banks

              timer_x                     <= RP_CYCLES;  -- set timer for this operation

              activeFlag_x                <= (others => NO);  -- all rows are inactive after a precharge operation

              state_x                     <= REFRESHROW;  -- refresh the SDRAM after the precharge

            end if;

            status                        <= "0101";

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

            -- do a host-initiated read operation 

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

          elsif rd = YES then

                                          -- Wait one clock cycle if the bank address has just changed and each bank has its own active row.

            -- This gives extra time for the row activation circuitry.

            if (ba_x = ba_r) or (MULTIPLE_ACTIVE_ROWS=false) then

                                        -- activate a new row if the current read is outside the active row or bank

              if doActivate = YES then

                                        -- activate new row only if all previous activations, writes, reads are done

                if (activateInProgress = NO) and (wrInProgress = NO) and (rdInProgress = NO) then

                  cmd_x                   <= PCHG_CMD;  -- initiate precharge of the SDRAM

                  sAddr_x(CMDBIT_POS)     <= ONE_BANK;  -- precharge this bank

                  timer_x                 <= RP_CYCLES;  -- set timer for this operation

                  activeFlag_x(bankIndex) <= NO;  -- rows in this bank are inactive after a precharge operation

                  state_x                 <= ACTIVATE;  -- activate the new row after the precharge is done

                end if;

                                        -- read from the currently active row if no previous read operation

                                        -- is in progress or if pipeline reads are enabled

                                        -- we can always initiate a read even if a write is already in progress

              elsif (rdInProgress = NO) or PIPE_EN then

                cmd_x                     <= READ_CMD;  -- initiate a read of the SDRAM

                                        -- insert a flag into the pipeline shift register that will exit the end

                                        -- of the shift register when the data from the SDRAM is available

                rdPipeline_x              <= READ & rdPipeline_r(rdPipeline_r'high downto 1);

                opBegun_x                 <= YES;  -- tell the host the requested operation has begun

              end if;

            end if;

            status                        <= "0110";

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

            -- do a host-initiated write operation 

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

          elsif wr = YES then

                                          -- Wait one clock cycle if the bank address has just changed and each bank has its own active row.

            -- This gives extra time for the row activation circuitry.

            if (ba_x = ba_r) or (MULTIPLE_ACTIVE_ROWS=false) then

                                        -- activate a new row if the current write is outside the active row or bank

              if doActivate = YES then

                                        -- activate new row only if all previous activations, writes, reads are done

                if (activateInProgress = NO) and (wrInProgress = NO) and (rdInProgress = NO) then

                  cmd_x                   <= PCHG_CMD;  -- initiate precharge of the SDRAM

                  sAddr_x(CMDBIT_POS)     <= ONE_BANK;  -- precharge this bank

                  timer_x                 <= RP_CYCLES;  -- set timer for this operation

                  activeFlag_x(bankIndex) <= NO;  -- rows in this bank are inactive after a precharge operation

                  state_x                 <= ACTIVATE;  -- activate the new row after the precharge is done

                end if;

                                        -- write to the currently active row if no previous read operations are in progress

              elsif rdInProgress = NO then

                cmd_x                     <= WRITE_CMD;  -- initiate the write operation

                sDataDir_x                <= OUTPUT;  -- turn on drivers to send data to SDRAM

                                        -- set timer so precharge doesn't occur too soon after write operation

                wrTimer_x                 <= WR_CYCLES;

                                        -- insert a flag into the 1-bit pipeline shift register that will exit on the

                                        -- next cycle.  The write into SDRAM is not actually done by that time, but

                                        -- this doesn't matter to the host

                wrPipeline_x(0)           <= WRITE;

                opBegun_x                 <= YES;  -- tell the host the requested operation has begun

              end if;

            end if;