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--**********************************************************************

-- Copyright 2012 by XESS Corp <http://www.xess.com>.

-- This program is free software: you can redistribute it and/or modify

-- it under the terms of the GNU General Public License as published by

-- the Free Software Foundation, either version 3 of the License, or

-- (at your option) any later version.

--

-- This program is distributed in the hope that it will be useful,

-- but WITHOUT ANY WARRANTY; without even the implied warranty of

-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

-- GNU General Public License for more details.

--

-- You should have received a copy of the GNU General Public License

-- along with this program.  If not, see <http://www.gnu.org/licenses/>.

--**********************************************************************

--*********************************************************************

-- SD MEMORY CARD INTERFACE

--

-- Reads/writes a single or multiple blocks of data to/from an SD Flash card.

-- 

-- Based on work by by Steven J. Merrifield, June 2008:

-- http : //stevenmerrifield.com/tools/sd.vhd

-- 

-- Most of what I learned about interfacing to SD/SDHC cards came from here:

-- http://elm-chan.org/docs/mmc/mmc_e.html

--

-- OPERATION

--

--     Set-up:

--         First of all, you have to give the controller a clock signal on the clk_i 

--         input with a higher frequency than the serial clock sent to the SD card 

--         through the sclk_o output. You can set generic parameters for the 

--         controller to tell it the master clock frequency (100 MHz), the SCLK 

--         frequency for initialization (400 KHz), the SCLK frequency for normal 

--         operation (25 MHz), the size of data sectors in the Flash memory (512 bytes),

--         and the type of card (either SD or SDHC). I typically use a 100 MHz 

--         clock if I'm running an SD card with a 25 Mbps serial data stream. 

--       

--     Initialize it:

--         Pulsing the reset_i input high and then bringing it low again will make 

--         the controller initialize the SD card so it will work in SPI mode. 

--         Basically, it sends the card the commands CMD0, CMD8 and then ACMD41 (which

--         is CMD55 followed by CMD41). The busy_o output will be high during the 

--         initialization and will go low once it is done. 

--        

--         After the initialization command sequence, the SD card will send back an R1

--         response byte. If only the IDLE bit of the R1 response is set, then the 

--         controller will repeatedly re-try the ACMD41 command while busy_o remains 

--         high. 

--        

--         If any other bit of the R1 response is set, then an error occurred. The 

--         controller will stall, lower busy_o, and output the R1 response code on the

--         error_o bus. You'll have to pulse reset_i to unfreeze the controller. 

--     

--         If the R1 response is all zeroes (i.e., no errors occurred during the 

--         initialization), then the controller will lower busy_o and wait for a 

--         read or write operation from the host. The controller will only accept new

--         operations when busy_o is low.

--     

--     Write data:

--         To write a data block to the SD card, the address of a block is placed 

--         on the addr_i input bus and the wr_i input is raised. The address and 

--         write strobe can be removed once busy_o goes high to indicate the write 

--         operation is underway. The data to be written to the SD card is passed as 

--         follows: 

--     

--         1. The controller requests a byte of data by raising the hndShk_o output.

--         2. The host applies the next byte to the data_i input bus and raises the 

--            hndShk_i input.

--         3. The controller accepts the byte and lowers the hndShk_o output.

--         4. The host lowers the hndShk_i input.

--     

--         This sequence of steps is repeated until all BLOCK_SIZE_G bytes of the 

--         data block are passed from the host to the controller. Once all the data 

--         is passed, the sector on the SD card will be written and the busy_o output 

--         will be lowered. 

--     

--     Read data:

--         To read a block of data from the SD card, the address of a block is 

--         placed on the addr_i input bus and the rd_i input is raised. The address 

--         and read strobe can be removed once busy_o goes high to indicate the read 

--         operation is underway. The data read from the SD card is passed to the 

--         host as follows: 

--     

--         1. The controller raises the hndShk_o output when the next data byte is available.

--         2. The host reads the byte from the data_o output bus and raises the hndShk_i input.

--         3. The controller lowers the hndShk_o output.

--         4. The host lowers the hndShk_i input.

--     

--         This sequence of steps is repeated until all BLOCK_SIZE_G bytes of the 

--         data block are passed from the controller to the host. Once all the data 

--         is read, the busy_o output will be lowered.

--     

--     Handle errors:

--         If an error is detected during either a read or write operation, then the

--         controller will stall, lower busy_o, and output an error code on the 

--         error_o bus. You'll have to pulse reset_i to unfreeze the controller. That 

--         may seem a bit excessive, but it does guarantee that you can't ignore any 

--         errors that occur.

--

-- TODO:

--

--     * Implement multi-block read and write commands.

--     * Allow host to send/receive SPI commands/data directly to

--       the SD card through the controller.

-- *********************************************************************

library ieee;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

use work.CommonPckg.all;

package SdCardPckg is

  type CardType_t is (SD_CARD_E, SDHC_CARD_E);  -- Define the different types of SD cards.

  component SdCardCtrl is

    generic (

      FREQ_G          : real       := 100.0;  -- Master clock frequency (MHz).

      INIT_SPI_FREQ_G : real       := 0.4;  -- Slow SPI clock freq. during initialization (MHz).

      SPI_FREQ_G      : real       := 25.0;  -- Operational SPI freq. to the SD card (MHz).

      BLOCK_SIZE_G    : natural    := 512;  -- Number of bytes in an SD card block or sector.

      CARD_TYPE_G     : CardType_t := SD_CARD_E  -- Type of SD card connected to this controller.

      );

    port (

      -- Host-side interface signals.

      clk_i      : in  std_logic;       -- Master clock.

      reset_i    : in  std_logic                     := '0';  -- active-high, synchronous  reset.

      rd_i       : in  std_logic                     := '0';  -- active-high read block request.

      wr_i       : in  std_logic                     := '0';  -- active-high write block request.

      continue_i : in  std_logic                     := '0';  -- If true, inc address and continue R/W.

      addr_i     : in  std_logic_vector(31 downto 0) := x"00000000";  -- Block address.

      data_i     : in  std_logic_vector(7 downto 0)  := x"00";  -- Data to write to block.

      data_o     : out std_logic_vector(7 downto 0)  := x"00";  -- Data read from block.

      busy_o     : out std_logic;  -- High when controller is busy performing some operation.

      hndShk_i   : in  std_logic;  -- High when host has data to give or has taken data.

      hndShk_o   : out std_logic;  -- High when controller has taken data or has data to give.

      error_o    : out std_logic_vector(15 downto 0) := (others => '0');

      -- I/O signals to the external SD card.

      cs_bo      : out std_logic                     := '1';  -- Active-low chip-select.

      sclk_o     : out std_logic                     := '0';  -- Serial clock to SD card.

      mosi_o     : out std_logic                     := '1';  -- Serial data output to SD card.

      miso_i     : in  std_logic                     := '0'  -- Serial data input from SD card.

      );

  end component;

end package;

library ieee;

use ieee.math_real.all;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

use work.CommonPckg.all;

use work.SdCardPckg.all;

entity SdCardCtrl is

  generic (

    FREQ_G          : real       := 100.0;     -- Master clock frequency (MHz).

    INIT_SPI_FREQ_G : real       := 0.4;  -- Slow SPI clock freq. during initialization (MHz).

    SPI_FREQ_G      : real       := 25.0;  -- Operational SPI freq. to the SD card (MHz).

    BLOCK_SIZE_G    : natural    := 512;  -- Number of bytes in an SD card block or sector.

    CARD_TYPE_G     : CardType_t := SD_CARD_E  -- Type of SD card connected to this controller.

    );

  port (

    -- Host-side interface signals.

    clk_i      : in  std_logic;         -- Master clock.

    reset_i    : in  std_logic                     := '0';  -- active-high, synchronous  reset.

    rd_i       : in  std_logic                     := '0';  -- active-high read block request.

    wr_i       : in  std_logic                     := '0';  -- active-high write block request.

    continue_i : in  std_logic                     := '0';  -- If true, inc address and continue R/W.

    addr_i     : in  std_logic_vector(31 downto 0) := x"00000000";  -- Block address.

    data_i     : in  std_logic_vector(7 downto 0)  := x"00";  -- Data to write to block.

    data_o     : out std_logic_vector(7 downto 0)  := x"00";  -- Data read from block.

    busy_o     : out std_logic;  -- High when controller is busy performing some operation.

    hndShk_i   : in  std_logic;  -- High when host has data to give or has taken data.

    hndShk_o   : out std_logic;  -- High when controller has taken data or has data to give.

    error_o    : out std_logic_vector(15 downto 0) := (others => '0');

    -- I/O signals to the external SD card.

    cs_bo      : out std_logic                     := '1';  -- Active-low chip-select.

    sclk_o     : out std_logic                     := '0';  -- Serial clock to SD card.

    mosi_o     : out std_logic                     := '1';  -- Serial data output to SD card.

    miso_i      : in  std_logic                     := '0';  -- Serial data input from SD card.

         state_debug_o  : out std_logic_vector(4 downto 0)

    );

end entity;

architecture arch of SdCardCtrl is

  signal sclk_r   : std_logic := '0';  -- Register output drives SD card clock.

  signal hndShk_r : std_logic := '0';  -- Register output drives handshake output to host.

  signal state_debug : unsigned(4 downto 0);

begin

        state_debug_o <= std_logic_vector(state_debug);

  process(clk_i)  -- FSM process for the SD card controller.

    type FsmState_t is (    -- States of the SD card controller FSM.

      START_INIT,  -- Send initialization clock pulses to the deselected SD card.    

      SEND_CMD0,                        -- Put the SD card in the IDLE state.

      CHK_CMD0_RESPONSE,    -- Check card's R1 response to the CMD0.

      SEND_CMD8,   -- This command is needed to initialize SDHC cards.

      GET_CMD8_RESPONSE,                -- Get the R7 response to CMD8.

      SEND_CMD55,                       -- Send CMD55 to the SD card. 

      SEND_CMD41,                       -- Send CMD41 to the SD card.

      CHK_ACMD41_RESPONSE,  -- Check if the SD card has left the IDLE state.     

      WAIT_FOR_HOST_RW,  -- Wait for the host to issue a read or write command.

      RD_BLK,    -- Read a block of data from the SD card.

      WR_BLK,    -- Write a block of data to the SD card.

      WR_WAIT,   -- Wait for SD card to finish writing the data block.

      START_TX,                         -- Start sending command/data.

      TX_BITS,   -- Shift out remaining command/data bits.

      GET_CMD_RESPONSE,  -- Get the R1 response of the SD card to a command.

      RX_BITS,   -- Receive response/data from the SD card.

      DESELECT,  -- De-select the SD card and send some clock pulses (Must enter with sclk at zero.)

      PULSE_SCLK,  -- Issue some clock pulses. (Must enter with sclk at zero.)

      REPORT_ERROR                      -- Report error and stall until reset.

      );

    variable state_v    : FsmState_t := START_INIT;  -- Current state of the FSM.

    variable rtnState_v : FsmState_t;  -- State FSM returns to when FSM subroutine completes.

    -- Timing constants based on the master clock frequency and the SPI SCLK frequencies.

    constant CLKS_PER_INIT_SCLK_C      : real    := FREQ_G / INIT_SPI_FREQ_G;

    constant CLKS_PER_SCLK_C           : real    := FREQ_G / SPI_FREQ_G;

    constant MAX_CLKS_PER_SCLK_C       : real    := realmax(CLKS_PER_INIT_SCLK_C, CLKS_PER_SCLK_C);

    constant MAX_CLKS_PER_SCLK_PHASE_C : natural := integer(round(MAX_CLKS_PER_SCLK_C / 2.0));

    constant INIT_SCLK_PHASE_PERIOD_C  : natural := integer(round(CLKS_PER_INIT_SCLK_C / 2.0));

    constant SCLK_PHASE_PERIOD_C       : natural := integer(round(CLKS_PER_SCLK_C / 2.0));

    constant DELAY_BETWEEN_BLOCK_RW_C  : natural := SCLK_PHASE_PERIOD_C;

    -- Registers for generating slow SPI SCLK from the faster master clock.

    variable clkDivider_v     : natural range 0 to MAX_CLKS_PER_SCLK_PHASE_C;  -- Holds the SCLK period.

    variable sclkPhaseTimer_v : natural range 0 to MAX_CLKS_PER_SCLK_PHASE_C;  -- Counts down to zero, then SCLK toggles.

    constant NUM_INIT_CLKS_C : natural := 160;  -- Number of initialization clocks to SD card.

    variable bitCnt_v        : natural range 0 to NUM_INIT_CLKS_C;  -- Tx/Rx bit counter.

    constant CRC_SZ_C    : natural := 2;  -- Number of CRC bytes for read/write blocks.

    -- When reading blocks of data, get 0xFE + [DATA_BLOCK] + [CRC].

    constant RD_BLK_SZ_C : natural := 1 + BLOCK_SIZE_G + CRC_SZ_C;

    -- When writing blocks of data, send 0xFF + 0xFE + [DATA BLOCK] + [CRC] then receive response byte.

    constant WR_BLK_SZ_C : natural := 1 + 1 + BLOCK_SIZE_G + CRC_SZ_C + 1;

    variable byteCnt_v   : natural range 0 to IntMax(WR_BLK_SZ_C, RD_BLK_SZ_C);  -- Tx/Rx byte counter.

    -- Command bytes for various SD card operations.

    subtype Cmd_t is std_logic_vector(7 downto 0);

    constant CMD0_C          : Cmd_t := std_logic_vector(to_unsigned(16#40# + 0, Cmd_t'length));

    constant CMD8_C          : Cmd_t := std_logic_vector(to_unsigned(16#40# + 8, Cmd_t'length));

    constant CMD55_C         : Cmd_t := std_logic_vector(to_unsigned(16#40# + 55, Cmd_t'length));

    constant CMD41_C         : Cmd_t := std_logic_vector(to_unsigned(16#40# + 41, Cmd_t'length));

    constant READ_BLK_CMD_C  : Cmd_t := std_logic_vector(to_unsigned(16#40# + 17, Cmd_t'length));

    constant WRITE_BLK_CMD_C : Cmd_t := std_logic_vector(to_unsigned(16#40# + 24, Cmd_t'length));

    -- Except for CMD0 and CMD8, SD card ops don't need a CRC, so use a fake one for that slot in the command.

    constant FAKE_CRC_C : std_logic_vector(7 downto 0) := x"FF";

    variable addr_v : unsigned(addr_i'range);  -- Address of current block for R/W operations.

    -- Maximum Tx to SD card consists of command + address + CRC. Data Tx is just a single byte.

    variable tx_v : std_logic_vector(CMD0_C'length + addr_v'length + FAKE_CRC_C'length - 1 downto 0);  -- Data/command to SD card.

    alias txCmd_v is tx_v;              -- Command transmission shift register.

    alias txData_v is tx_v(tx_v'high downto tx_v'high - data_i'length + 1);  -- Data byte transmission shift register.

    variable rx_v               : std_logic_vector(data_i'range);  -- Data/response byte received from SD card.

    -- Various response codes.

    subtype Response_t is std_logic_vector(rx_v'range);

    constant ACTIVE_NO_ERRORS_C : Response_t := "00000000";  -- Normal R1 code after initialization.

    constant IDLE_NO_ERRORS_C   : Response_t := "00000001";  -- Normal R1 code after CMD0.

    constant DATA_ACCEPTED_C    : Response_t := "---00101";  -- SD card accepts data block from host.

    constant DATA_REJ_CRC_C     : Response_t := "---01011";  -- SD card rejects data block from host due to CRC error.

    constant DATA_REJ_WERR_C    : Response_t := "---01101";  -- SD card rejects data block from host due to write error.

    -- Various tokens.

    subtype Token_t is std_logic_vector(rx_v'range);

    constant NO_TOKEN_C         : Token_t    := x"FF";  -- Received before the SD card responds to a block read command.

    constant START_TOKEN_C      : Token_t    := x"FE";  -- Starting byte preceding a data block.

    -- Flags that are set/cleared to affect the operation of the FSM.

    variable getCmdResponse_v : boolean;  -- When true, get R1 response to command sent to SD card.

    variable rtnData_v        : boolean;  -- When true, signal to host when a data byte arrives from SD card.

    variable doDeselect_v     : boolean;  -- When true, de-select SD card after a command is issued.

  begin

    if rising_edge(clk_i) then

      case( state_v ) is

        when START_INIT =>

          state_debug <= to_unsigned(0, 5);

        when SEND_CMD0 =>

          state_debug <= to_unsigned(1, 5);

        when CHK_CMD0_RESPONSE =>

          state_debug <= to_unsigned(2, 5);

        when SEND_CMD8 =>

          state_debug <= to_unsigned(3, 5);

        when GET_CMD8_RESPONSE =>

          state_debug <= to_unsigned(4, 5);

        when SEND_CMD55 =>

          state_debug <= to_unsigned(5, 5);

        when SEND_CMD41 =>

          state_debug <= to_unsigned(6, 5);

        when CHK_ACMD41_RESPONSE =>

          state_debug <= to_unsigned(7, 5);

        when WAIT_FOR_HOST_RW =>

          state_debug <= to_unsigned(8, 5);

        when RD_BLK =>

          state_debug <= to_unsigned(9, 5);

        when WR_BLK =>

          state_debug <= to_unsigned(10, 5);

        when WR_WAIT =>

          state_debug <= to_unsigned(11, 5);

        when START_TX =>

          state_debug <= to_unsigned(12, 5);

        when TX_BITS =>

          state_debug <= to_unsigned(13, 5);

        when GET_CMD_RESPONSE =>

          state_debug <= to_unsigned(14, 5);

        when RX_BITS =>

          state_debug <= to_unsigned(15, 5);

        when DESELECT =>

          state_debug <= to_unsigned(16, 5);

        when PULSE_SCLK =>

          state_debug <= to_unsigned(17, 5);

        when REPORT_ERROR =>

          state_debug <= to_unsigned(18, 5);

      end case ;

      if reset_i = YES then             -- Perform a reset.

        state_v          := START_INIT;  -- Send the FSM to the initialization entry-point.

        sclkPhaseTimer_v := 0;  -- Don't delay the initialization right after reset.

        busy_o           <= YES;  -- Busy while the SD card interface is being initialized.

      elsif sclkPhaseTimer_v /= 0 then

        -- Setting the clock phase timer to a non-zero value delays any further actions

        -- and generates the slower SPI clock from the faster master clock.

        sclkPhaseTimer_v := sclkPhaseTimer_v - 1;

        -- Clock phase timer has reached zero, so check handshaking sync. between host and controller.

        -- Handshaking lets the host control the flow of data to/from the SD card controller.

        -- Handshaking between the SD card controller and the host proceeds as follows:

        --   1: Controller raises its handshake and waits.

        --   2: Host sees controller handshake and raises its handshake in acknowledgement.

        --   3: Controller sees host handshake acknowledgement and lowers its handshake.

        --   4: Host sees controller lower its handshake and removes its handshake.

        --

        -- Handshaking is bypassed when the controller FSM is initializing the SD card.

      elsif state_v /= START_INIT and hndShk_r = '1' and hndShk_i = '0' then

        null;            -- Waiting for the host to acknowledge handshake.

      elsif state_v /= START_INIT and hndShk_r = '1' and hndShk_i = '1' then

        txData_v := data_i;             -- Get any data passed from the host.

        hndShk_r <= '0';  -- The host acknowledged, so lower the controller handshake.

      elsif state_v /= START_INIT and hndShk_r = '0' and hndShk_i = '1' then

        null;            -- Waiting for the host to lower its handshake.

      elsif (state_v = START_INIT) or (hndShk_r = '0' and hndShk_i = '0') then

        -- Both handshakes are low, so the controller operations can proceed.

        busy_o <= YES;  -- Busy by default. Only false when waiting for R/W from host or stalled by error.

        case state_v is

          when START_INIT =>  -- Deselect the SD card and send it a bunch of clock pulses with MOSI high.

            error_o          <= (others => '0');  -- Clear error flags.

            clkDivider_v     := INIT_SCLK_PHASE_PERIOD_C - 1;  -- Use slow SPI clock freq during init.

            sclkPhaseTimer_v := INIT_SCLK_PHASE_PERIOD_C - 1;  -- and set the duration of the next clock phase.

            sclk_r           <= '0';     -- Start with low clock to the SD card.

            hndShk_r         <= '0';     -- Initialize handshake signal.

            addr_v           := (others => '0');  -- Initialize address.

            rtnData_v        := false;  -- No data is returned to host during initialization.

            bitCnt_v         := NUM_INIT_CLKS_C;  -- Generate this many clock pulses.

            state_v          := DESELECT;  -- De-select the SD card and pulse SCLK.

            rtnState_v       := SEND_CMD0;  -- Then go to this state after the clock pulses are done.

          when SEND_CMD0 =>             -- Put the SD card in the IDLE state.

            cs_bo            <= '0';     -- Enable the SD card.

            txCmd_v          := CMD0_C & x"00000000" & x"95";  -- 0x95 is the correct CRC for this command.

            bitCnt_v         := txCmd_v'length;  -- Set bit counter to the size of the command.

            getCmdResponse_v := true;  -- Sending a command that generates a response.

            doDeselect_v     := true;  -- De-select SD card after this command finishes.

            state_v          := START_TX;  -- Go to FSM subroutine to send the command.

            rtnState_v       := CHK_CMD0_RESPONSE;  -- Then check the response to the command.

          when CHK_CMD0_RESPONSE =>  -- Check card's R1 response to the CMD0.

            if rx_v = IDLE_NO_ERRORS_C then

              state_v := SEND_CMD8;  -- Continue init if SD card is in IDLE state with no errors

            else

              state_v := SEND_CMD0;     -- Otherwise, try CMD0 again.

            end if;

          when SEND_CMD8 =>  -- This command is needed to initialize SDHC cards.

            cs_bo            <= '0';     -- Enable the SD card.

            txCmd_v          := CMD8_C & x"000001aa" & x"87";  -- 0x87 is the correct CRC for this command.

            bitCnt_v         := txCmd_v'length;  -- Set bit counter to the size of the command.

            getCmdResponse_v := true;  -- Sending a command that generates a response.

            doDeselect_v     := false;  -- Don't de-select, need to get the R7 response sent from the SD card.

            state_v          := START_TX;  -- Go to FSM subroutine to send the command.

            rtnState_v       := GET_CMD8_RESPONSE;  -- Then go to this state after the command is sent.

          when GET_CMD8_RESPONSE =>     -- Get the R7 response to CMD8.

            cs_bo            <= '0';  -- The SD card should already be enabled, but let's be explicit.

            bitCnt_v         := 31;     -- Four bytes (32 bits) in R7 response.

            getCmdResponse_v := false;  -- Not sending a command that generates a response.

            doDeselect_v     := true;  -- De-select card to end the command after getting the four bytes.

            state_v          := RX_BITS;  -- Go to FSM subroutine to get the R7 response.

            rtnState_v       := SEND_CMD55;  -- Then go here (we don't care what the actual R7 response is).

          when SEND_CMD55 =>  -- Send CMD55 as preamble of ACMD41 initialization command.

            cs_bo            <= '0';     -- Enable the SD card.

            txCmd_v          := CMD55_C & x"00000000" & FAKE_CRC_C;

            bitCnt_v         := txCmd_v'length;  -- Set bit counter to the size of the command.

            getCmdResponse_v := true;  -- Sending a command that generates a response.

            doDeselect_v     := true;  -- De-select SD card after this command finishes.

            state_v          := START_TX;  -- Go to FSM subroutine to send the command.

            rtnState_v       := SEND_CMD41;  -- Then go to this state after the command is sent.

          when SEND_CMD41 =>  -- Send the SD card the initialization command.

            cs_bo            <= '0';     -- Enable the SD card.

            txCmd_v          := CMD41_C & x"40000000" & FAKE_CRC_C;

            bitCnt_v         := txCmd_v'length;  -- Set bit counter to the size of the command.

            getCmdResponse_v := true;  -- Sending a command that generates a response.

            doDeselect_v     := true;  -- De-select SD card after this command finishes.

            state_v          := START_TX;  -- Go to FSM subroutine to send the command.

            rtnState_v       := CHK_ACMD41_RESPONSE;  -- Then check the response to the command.

          when CHK_ACMD41_RESPONSE =>

            -- The CMD55, CMD41 sequence should cause the SD card to leave the IDLE state

            -- and become ready for SPI read/write operations. If still IDLE, then repeat the CMD55, CMD41 sequence.

            -- If one of the R1 error flags is set, then report the error and stall.

            if rx_v = ACTIVE_NO_ERRORS_C then   -- Not IDLE, no errors.

              state_v := WAIT_FOR_HOST_RW;  -- Start processing R/W commands from the host.

            elsif rx_v = IDLE_NO_ERRORS_C then  -- Still IDLE but no errors. 

              state_v := SEND_CMD55;    -- Repeat the CMD55, CMD41 sequence.

            else                        -- Some error occurred.

              state_v := REPORT_ERROR;  -- Report the error and stall.

            end if;

          when WAIT_FOR_HOST_RW =>  -- Wait for the host to read or write a block of data from the SD card.

            clkDivider_v     := SCLK_PHASE_PERIOD_C - 1;  -- Set SPI clock frequency for normal operation.

            getCmdResponse_v := true;  -- Get R1 response to any commands issued to the SD card.

            if rd_i = YES then  -- send READ command and address to the SD card.

              cs_bo <= '0';              -- Enable the SD card.

              if continue_i = YES then  -- Multi-block read. Use stored address.

                if CARD_TYPE_G = SD_CARD_E then  -- SD cards use byte-addressing, 

                  addr_v := addr_v + BLOCK_SIZE_G;  -- so add block-size to get next block address.

                else                    -- SDHC cards use block-addressing,

                  addr_v := addr_v + 1;  -- so just increment current block address.

                end if;

                txCmd_v := READ_BLK_CMD_C & std_logic_vector(addr_v) & FAKE_CRC_C;

              else                      -- Single-block read.

                txCmd_v := READ_BLK_CMD_C & addr_i & FAKE_CRC_C;  -- Use address supplied by host.

                addr_v  := unsigned(addr_i);  -- Store address for multi-block operations.

              end if;

              bitCnt_v   := txCmd_v'length;  -- Set bit counter to the size of the command.

              byteCnt_v  := RD_BLK_SZ_C;

              state_v    := START_TX;  -- Go to FSM subroutine to send the command.

              rtnState_v := RD_BLK;  -- Then go to this state to read the data block.

            elsif wr_i = YES then  -- send WRITE command and address to the SD card.

              cs_bo <= '0';              -- Enable the SD card.

              if continue_i = YES then  -- Multi-block write. Use stored address.

                if CARD_TYPE_G = SD_CARD_E then  -- SD cards use byte-addressing, 

                  addr_v := addr_v + BLOCK_SIZE_G;  -- so add block-size to get next block address.

                else                    -- SDHC cards use block-addressing,

                  addr_v := addr_v + 1;  -- so just increment current block address.

                end if;

                txCmd_v := WRITE_BLK_CMD_C & std_logic_vector(addr_v) & FAKE_CRC_C;

              else                      -- Single-block write.

                txCmd_v := WRITE_BLK_CMD_C & addr_i & FAKE_CRC_C;  -- Use address supplied by host.

                addr_v  := unsigned(addr_i);  -- Store address for multi-block operations.

              end if;

              bitCnt_v   := txCmd_v'length;  -- Set bit counter to the size of the command.

              byteCnt_v  := WR_BLK_SZ_C;    -- Set number of bytes to write.

              state_v    := START_TX;  -- Go to this FSM subroutine to send the command ...

              rtnState_v := WR_BLK;  -- then go to this state to write the data block.

            else              -- Do nothing and wait for command from host.

              cs_bo   <= '1';            -- Deselect the SD card.

              busy_o  <= '0';  -- SD card interface is waiting for R/W from host, so it's not busy.

              state_v := WAIT_FOR_HOST_RW;  -- Keep waiting for command from host.

            end if;

          when RD_BLK =>          -- Read a block of data from the SD card.

            -- Some default values for these...

            rtnData_v  := false;  -- Data is only returned to host in one place.

            bitCnt_v   := rx_v'length - 1;   -- Receiving byte-sized data.

            state_v    := RX_BITS;      -- Call the bit receiver routine.

            rtnState_v := RD_BLK;   -- Return here when done receiving a byte.

            if byteCnt_v = RD_BLK_SZ_C then  -- Initial read to prime the pump.

              byteCnt_v := byteCnt_v - 1;

            elsif byteCnt_v = RD_BLK_SZ_C -1 then  -- Then look for the data block start token.

              if rx_v = NO_TOKEN_C then  -- Receiving 0xFF means the card hasn't responded yet. Keep trying.

                null;

              elsif rx_v = START_TOKEN_C then

                rtnData_v := true;  -- Found the start token, so now start returning data byes to the host.

                byteCnt_v := byteCnt_v - 1;

              else  -- Getting anything else means something strange has happened.

                state_v := REPORT_ERROR;

              end if;

            elsif byteCnt_v >= 3 then  -- Now bytes of data from the SD card are received.

              rtnData_v := true;        -- Return this data to the host.

              byteCnt_v := byteCnt_v - 1;

            elsif byteCnt_v = 2 then  -- Receive the 1st CRC byte at the end of the data block.

              byteCnt_v := byteCnt_v - 1;

            elsif byteCnt_v = 1 then    -- Receive the 2nd

              byteCnt_v := byteCnt_v - 1;

            else    -- Reading is done, so deselect the SD card.

              sclk_r     <= '0';

              bitCnt_v   := 2;

              state_v    := DESELECT;

              rtnState_v := WAIT_FOR_HOST_RW;

            end if;

          when WR_BLK =>             -- Write a block of data to the SD card.

            -- Some default values for these...

            getCmdResponse_v := false;  -- Sending data bytes so there's no command response from SD card.

            bitCnt_v         := txData_v'length;  -- Transmitting byte-sized data.

            state_v          := START_TX;  -- Call the bit transmitter routine.

            rtnState_v       := WR_BLK;  -- Return here when done transmitting a byte.

            if byteCnt_v = WR_BLK_SZ_C then

              txData_v := NO_TOKEN_C;  -- Hold MOSI high for one byte before data block goes out.

            elsif byteCnt_v = WR_BLK_SZ_C - 1 then     -- Send start token.

              txData_v := START_TOKEN_C;   -- Starting token for data block.

            elsif byteCnt_v >= 4 then   -- Now send bytes in the data block.

              hndShk_r <= '1';           -- Signal host to provide data.

            -- The transmit shift register is loaded with data from host in the handshaking section above.

            elsif byteCnt_v = 3 or byteCnt_v = 2 then  -- Send two phony CRC bytes at end of packet.

              txData_v := FAKE_CRC_C;

            elsif byteCnt_v = 1 then

              bitCnt_v   := rx_v'length - 1;

              state_v    := RX_BITS;  -- Get response of SD card to the write operation.

              rtnState_v := WR_WAIT;

            else                        -- Check received response byte.

              if std_match(rx_v, DATA_ACCEPTED_C) then  -- Data block was accepted.

                state_v := WR_WAIT;  -- Wait for the SD card to finish writing the data into Flash.

              else                      -- Data block was rejected.

                error_o(15 downto 8) <= rx_v;

                state_v              := REPORT_ERROR;  -- Report the error.

              end if;

            end if;

            byteCnt_v := byteCnt_v - 1;

          when WR_WAIT =>  -- Wait for SD card to finish writing the data block.

            -- The SD card will pull MISO low while it is busy, and raise it when it is done.

            sclk_r           <= not sclk_r;    -- Toggle the SPI clock...

            sclkPhaseTimer_v := clkDivider_v;  -- and set the duration of the next clock phase.

            if sclk_r = '1' and miso_i = '1' then  -- Data block has been written, so deselect the SD card.

              bitCnt_v   := 2;

              state_v    := DESELECT;

              rtnState_v := WAIT_FOR_HOST_RW;

            end if;

          when START_TX =>

            -- Start sending command/data by lowering SCLK and outputing MSB of command/data

            -- so it has plenty of setup before the rising edge of SCLK.

            sclk_r           <= '0';  -- Lower the SCLK (although it should already be low).

            sclkPhaseTimer_v := clkDivider_v;  -- Set the duration of the low SCLK.

            mosi_o           <= tx_v(tx_v'high);  -- Output MSB of command/data.

            tx_v             := tx_v(tx_v'high-1 downto 0) & ONE;  -- Shift command/data register by one bit.

            bitCnt_v         := bitCnt_v - 1;  -- The first bit has been sent, so decrement bit counter.

            state_v          := TX_BITS;  -- Go here to shift out the rest of the command/data bits.

          when TX_BITS =>  -- Shift out remaining command/data bits and (possibly) get response from SD card.

            sclk_r           <= not sclk_r;    -- Toggle the SPI clock...

            sclkPhaseTimer_v := clkDivider_v;  -- and set the duration of the next clock phase.

            if sclk_r = '1' then

              -- SCLK is going to be flipped from high to low, so output the next command/data bit

              -- so it can setup while SCLK is low.

              if bitCnt_v /= 0 then  -- Keep sending bits until the bit counter hits zero.

                mosi_o   <= tx_v(tx_v'high);

                tx_v     := tx_v(tx_v'high-1 downto 0) & ONE;

                bitCnt_v := bitCnt_v - 1;

              else

                if getCmdResponse_v then

                  state_v  := GET_CMD_RESPONSE;  -- Get a response to the command from the SD card.

                  bitCnt_v := Response_t'length - 1;  -- Length of the expected response.

                else

                  state_v          := rtnState_v;  -- Return to calling state (no need to get a response).

                  sclkPhaseTimer_v := 0;  -- Clear timer so next SPI op can begin ASAP with SCLK low.

                end if;

              end if;

            end if;

          when GET_CMD_RESPONSE =>  -- Get the response of the SD card to a command.

            if sclk_r = '1' and miso_i = '0' then  -- MISO will be held high by SD card until 1st bit of R1 response, which is 0.

              -- Shift in the MSB bit of the response.

              rx_v     := rx_v(rx_v'high-1 downto 0) & miso_i;

              bitCnt_v := bitCnt_v - 1;

              state_v  := RX_BITS;  -- Now receive the reset of the response.

            end if;

            sclk_r           <= not sclk_r;    -- Toggle the SPI clock...

            sclkPhaseTimer_v := clkDivider_v;  -- and set the duration of the next clock phase.

          when RX_BITS =>               -- Receive bits from the SD card.

            if sclk_r = '1' then    -- Bits enter after the rising edge of SCLK.

              rx_v := rx_v(rx_v'high-1 downto 0) & miso_i;

              if bitCnt_v /= 0 then     -- More bits left to receive.

                bitCnt_v := bitCnt_v - 1;

              else                      -- Last bit has been received.

                if rtnData_v then       -- Send the received data to the host.

                  data_o   <= rx_v;     -- Output received data to the host.

                  hndShk_r <= '1';  -- Signal to the host that the data is ready.

                end if;

                if doDeselect_v then

                  bitCnt_v := 1;

                  state_v  := DESELECT;  -- De-select SD card before returning.

                else

                  state_v := rtnState_v;  -- Otherwise, return to calling state without de-selecting.

                end if;

              end if;

            end if;

            sclk_r           <= not sclk_r;    -- Toggle the SPI clock...

            sclkPhaseTimer_v := clkDivider_v;  -- and set the duration of the next clock phase.

          when DESELECT =>  -- De-select the SD card and send some clock pulses (Must enter with sclk at zero.)

            doDeselect_v     := false;  -- Once the de-select is done, clear the flag that caused it.

            cs_bo            <= '1';     -- De-select the SD card.

            mosi_o           <= '1';  -- Keep the data input of the SD card pulled high.

            state_v          := PULSE_SCLK;  -- Pulse the clock so the SD card will see the de-select.

            sclk_r           <= '0';  -- Clock is set low so the next rising edge will see the new CS and MOSI

            sclkPhaseTimer_v := clkDivider_v;  -- Set the duration of the next clock phase.

          when PULSE_SCLK =>  -- Issue some clock pulses. (Must enter with sclk at zero.)

            if sclk_r = '1' then

              if bitCnt_v /= 0 then

                bitCnt_v := bitCnt_v - 1;

              else  -- Return to the calling routine when the pulse counter reaches zero.

                state_v := rtnState_v;

              end if;

            end if;

            sclk_r           <= not sclk_r;    -- Toggle the SPI clock...

            sclkPhaseTimer_v := clkDivider_v;  -- and set the duration of the next clock phase.

          when REPORT_ERROR =>  -- Report the error code and stall here until a reset occurs.

            error_o(rx_v'range) <= rx_v;  -- Output the SD card response as the error code.

            busy_o              <= '0';  -- Not busy.

          when others =>

            state_v := START_INIT;

        end case;

      end if;

    end if;