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

-- uartcomponent.vhd

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

-- Author:  Dan Pederson

--          Copyright 2004 Digilent, Inc.

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

-- Description: This file defines a UART which transfers data to and 

--                              from serial and parallel information.  It requires two 

--                              major processes:  receiving and transferring.  The 

--                              receiving portion reads serially transmitted data, and

--                              converts it into parallel data, while the transferring

--                              portion reads parallel data, and transmits it as serial 

--                              data.  There are three error signals provided with this 

--                              UART.  They are frame error, parity error, and overwrite 

--                              error signals.  This UART is configured to use an ODD 

--                              parity bit at a baud rate of 9600. 

--              

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

-- Revision History:

--      07/15/04 (DanP) Created 

--              05/24/05 (DanP) Updated commenting style

--              06/06/05 (DanP) Synchronized state machines to fix timing bug           

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

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.STD_LOGIC_ARITH.ALL;

use IEEE.STD_LOGIC_UNSIGNED.ALL;

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

--

--Title:        UARTcomponent entity

--

--Inputs:       7       :       RXD

--                                      CLK

--                                      DBIN

--                                      RDA

--                                      RD

--                                      WR

--                                      RST

--

--Outputs:      7       :       TXD

--                                      DBOUT

--                                      RDA

--                                      TBE

--                                      PE

--                                      FE

--                                      OE                                              

--

--Description:  This describes the UART component entity.  The inputs are

--                              the Pegasus 50 MHz clock, a reset button, The RXD from 

--                              the serial cable, an 8-bit data bus from the parallel 

--                              port, and Read Data Available (RDA)and Transfer Buffer 

--                              Empty(TBE) handshaking signals.  The outputs are the TXD 

--                              signal for the serial port, an 8-bit data bus for the

--                              parallel port, RDA and TBE handshaking signals, and three

--                              error signals for parity, frame, and overwrite errors.

--

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

entity UARTcomponent is

        Generic (

                --@48MHz

--              BAUD_DIVIDE_G : integer := 26;  --115200 baud

--              BAUD_RATE_G   : integer := 417

                --@26.6MHz

                BAUD_DIVIDE_G : integer := 14;  --115200 baud

                BAUD_RATE_G   : integer := 231

        );

        Port (

                TXD     : out   std_logic       := '1';                                 -- Transmitted serial data output

                RXD     : in    std_logic;                                                      -- Received serial data input

                CLK     : in    std_logic;                                                      -- Clock signal

                DBIN    : in    std_logic_vector (7 downto 0);           -- Input parallel data to be transmitted

                DBOUT   : out   std_logic_vector (7 downto 0);           -- Recevived parallel data output

                RDA             : inout  std_logic;                                                     -- Read Data Available

                TBE             : out   std_logic       := '1';                                 -- Transfer Buffer Emty

                RD              : in    std_logic;                                                      -- Read Strobe

                WR              : in    std_logic;                                                      -- Write Strobe

                PE              : out   std_logic;                                                      -- Parity error         

                FE              : out   std_logic;                                                      -- Frame error

                OE              : out   std_logic;                                                      -- Overwrite error

                RST             : in    std_logic       := '0');                         -- Reset signal

end UARTcomponent;

architecture Behavioral of UARTcomponent is

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

-- Local Type and Signal Declarations

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

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

--Title:        Local Type Declarations

--

--Description:  There are two state machines used in this entity.  The

--                              rstate is used to synchronize the receiving portion of 

--                              the UART, and the tstate is used to synchronize the 

--                              sending portion of the UART.

--

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

        type rstate is (

                strIdle,

                strEightDelay,

                strGetData,

                strWaitFor0,

                strWaitFor1,

                strCheckStop

        );

        type tstate is (

                sttIdle,

                sttTransfer,

                sttShift,

                sttDelay,

                sttWaitWrite

                );

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

--

--Title:  Local Signal Declarations

--

--Description:  The constants and signals used by this entity are 

--                              described below:

--

--                              -baudRate       :       This is the Baud Rate constant used to 

--                                                              synchronize the Pegasus 50 MHz clock with a 

--                                                              baud rate of 9600.  To get this number, divide 

--                                                              50MHz by 9600.

--                              -baudDivide     :       This is the Baud Rate divider used to safely

--                                                              read data transmitted at a baud rate of 9600.

--                                                              It is simply the above described baudRate

--                                                              constant divided by 16.

--

--                              -rdReg          :       this is the receive holding register

--                              -rdSReg         :       this is the receive shift register

--                              -tfReg          :       this is the transfer holding register

--                              -tfSReg         :       this is the transfer shifting register 

--                              -clkDiv         :       counter used to get rClk

--                              -ctr            :       used for delay times

--                              -tfCtr          :       used to delay in the transfer process

--                              -dataCtr        :       counts the number of read data bits

--                              -parError       :       parity error bit

--                              -frameError     :       frame error bit

--                              -CE                     :       clock enable bit for the writing latch

--                              -ctRst          :       reset for the ctr

--                              -load           :       load signal used to load the transfer shift

--                                                              register

--                              -shift          :       shift signal used to unload the transfer

--                                                              shift register

--                              -par            :       represents the parity in the transfer

--                                                              holding register

--                              -tClkRST        :       reset for the tfCtr     

--                              -rShift         :       shift signal used to load the receive shift

--                                                              register

--                              -dataRST        :       reset for the dataCtr

--                              -dataIncr       :       signal to increment the dataCtr

--                              -tfIncr         :       signal to increment the tfCtr

--                              -tDelayCtr      :       counter used to delay the transfer state 

--                                                              machine.

--                              -tDelayRst      :       reset signal for the tDelayCtr counter.

--                               

--                              The following signals are used by the two state machines

--                              for state control:

--                              -Receive State Machine  :       strCur, strNext

--                              -Transfer State Machine :       sttCur, sttNext

--      

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

--  @26.7MHz

--      constant baudRate       :       std_logic_vector(12 downto 0) := "1 0100 0101 1000";  

--      constant baudRate       :       std_logic_vector(12 downto 0) := conv_std_logic_vector(1406,13); -- 19200

--      constant baudRate       :       std_logic_vector(12 downto 0) := conv_std_logic_vector(703,13);  -- 38400

--      constant baudRate       :       std_logic_vector(12 downto 0) := conv_std_logic_vector(469,13);  -- 57600

--   constant baudRate  :       std_logic_vector(12 downto 0) := conv_std_logic_vector(417,13);  --115200

--  @26.7MHz

--      constant baudDivide     :       std_logic_vector(8 downto 0)    := conv_std_logic_vector(1,9);  -- Used for simulation

--      constant baudDivide     :       std_logic_vector(8 downto 0)    := conv_std_logic_vector(88,9); -- Used for  19 200 baud

--      constant baudDivide     :       std_logic_vector(8 downto 0)    := conv_std_logic_vector(44,9); -- Used for  38 400 baud

--      constant baudDivide     :       std_logic_vector(8 downto 0)    := conv_std_logic_vector(29,9); -- Used for  57 600 baud

--      constant baudDivide     :       std_logic_vector(8 downto 0)    := conv_std_logic_vector(26,9); -- Used for 115 200 baud

        constant baudRate       :       std_logic_vector(12 downto 0)   := conv_std_logic_vector(BAUD_RATE_G,13);  --115200

        constant baudDivide     :       std_logic_vector(8 downto 0)     := conv_std_logic_vector(BAUD_DIVIDE_G-1,9); -- Used for 115 200 baud

        signal rdReg            :       std_logic_vector(7 downto 0)     := "00000000";

        signal rdSReg           :       std_logic_vector(9 downto 0)     := "1111111111";

        signal tfReg            :       std_logic_vector(7 downto 0);

        signal tfSReg           :       std_logic_vector(10 downto 0)    := "11111111111";

        signal clkDiv           :       std_logic_vector(9 downto 0)             := "0000000000";

        signal ctr                      :       std_logic_vector(3 downto 0)             := "0000";

        signal tfCtr            :       std_logic_vector(3 downto 0)             := "0000";

        signal dataCtr          :       std_logic_vector(3 downto 0)             := "0000";

        signal parError         :       std_logic;

        signal frameError       :       std_logic;

        signal CE                       :       std_logic;

        signal ctRst            :       std_logic       := '0';

        signal load                     :       std_logic       := '0';

        signal shift            :       std_logic       := '0';

        signal par                      :       std_logic;

   signal tClkRST               :       std_logic       := '0';

        signal rShift           :       std_logic       := '0';

        signal dataRST          :       std_logic       := '0';

        signal dataIncr         :       std_logic       := '0';

        signal tfIncr           :       std_logic       := '0';

        signal tDelayCtr        :       std_logic_vector (12 downto 0);

        signal tDelayRst        :       std_logic := '0';

        signal strCur           :  rstate       := strIdle;

        signal strNext          :  rstate;

        signal sttCur           :  tstate       := sttIdle;

        signal sttNext          :  tstate;

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

-- Module Implementation

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

begin

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

--

--Title:  Initial signal definitions

--

--Description:  The following lines of code define 4 internal and 1

--                              external signal.  The most significant bit of the rdSReg

--                              signifies the frame error bit, so frameError is tied to

--                              that signal.  The parError is high if there is a parity

--                              error, so it is set equal to the inverse of rdSReg(8) 

--                              XOR-ed with the data bits.  In this manner, it can 

--                              determine if the parity bit found in rdSReg(8) matches 

--                              the data bits.  The parallel information output is equal

--                              to rdReg, so DBOUT is set equal to rdReg.  Likewise, the 

--                              input parallel information is equal to DBIN, so tfReg is 

--                              set equal to DBIN.  Because the tfSReg is used to shift

--                              out transmitted data, the TXD port is set equal to the

--                              first bit of tfsReg.  Finally, the par signal represents 

--                              the parity of the data, so par is set to the inverse of 

--                              the data bits XOR-ed together.  This UART can be changed 

--                              to use EVEN parity if the "not" is omitted from the par 

--                              definition.

--

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

        frameError <= not rdSReg(9);

        parError <= not ( rdSReg(8) xor (((rdSReg(0) xor rdSReg(1)) xor

                (rdSReg(2) xor rdSReg(3))) xor ((rdSReg(4) xor rdSReg(5)) xor

                (rdSReg(6) xor rdSReg(7)))) );

        DBOUT <= rdReg;

        tfReg <= DBIN;

        TXD <= tfsReg(0);

        par <=  not ( ((tfReg(0) xor tfReg(1)) xor (tfReg(2) xor tfReg(3))) xor

                ((tfReg(4) xor tfReg(5)) xor (tfReg(6) xor tfReg(7))) );

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

--

--Title: Clock Divide counter 

--

--Description:  This process defines clkDiv as a signal that increments

--                              with the clock up until it is either reset by ctRst, or

--                              equals baudDivide.  This signal is used to define a 

--                              counter called ctr that increments at the rate of the 

--                              divided baud rate.

--

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

        process (CLK, clkDiv)

                begin

                        if (CLK = '1' and CLK'event) then

                                if (clkDiv = baudDivide or ctRst = '1') then

                                        clkDiv <= "0000000000";

                                else

                                        clkDiv <= clkDiv +1;

                                end if;

                        end if;

                end process;

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

--

--Title: Transfer delay counter 

--

--Description:  This process defines tDelayCtr as a counter that runs

--                              until it equals baudRate, or until it is reset by 

--                              tDelayRst.  This counter is used to measure delay times

--                              when sending data out on the TXD signal.  When the 

--                              counter is equal to baudRate, or is reset, it is set 

--                              equal to 0.

--

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

        process (CLK, tDelayCtr)

                begin

                        if (CLK = '1' and CLK'event) then

                                if (tDelayCtr = baudRate or tDelayRst = '1') then

                                        tDelayCtr <= "0000000000000";

                                else

                                        tDelayCtr <= tDelayCtr+1;

                                end if;

                        end if;

                end process;

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

--

--Title: ctr set up 

--

--Description:  This process sets up ctr, which uses clkDiv to count

--                              increase at a rate needed to properly receive data in

--                              from RXD.  If ctRst is strobed, the counter is reset.  If

--                              clkDiv is equal to baudDivide, then ctr is incremented

--                              once.  This signal is used by the receiving state machine

--                              to measure delay times between RXD reads.               

--

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

        process (CLK)

                begin

                        if CLK = '1' and CLK'Event then

                                if ctRst = '1' then

                                        ctr <= "0000";

                                elsif clkDiv = baudDivide then

                                        ctr <= ctr + 1;

                                else

                                        ctr <= ctr;

                                end if;

                        end if;

                end process;

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

--

--Title: transfer counter 

--

--Description:  This process makes tfCtr increment whenever the tfIncr

--                              signal is strobed high.  If the tClkRst signal is strobed

--                              high, the tfCtr is reset to "0000."  This counter is used

--                              to keep track of how many data bits have been 

--                              transmitted.

--

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

        process (CLK, tClkRST)

                begin

                        if (CLK = '1' and CLK'event) then

                                if tClkRST = '1' then

                                        tfCtr <= "0000";

                                elsif tfIncr = '1' then

                                        tfCtr <= tfCtr +1;

                                end if;

                        end if;

                end process;

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

--

--Title: Error and RDA flag controller 

--

--Description:  This process controls the error flags FE, OE, and PE, as

--                              well as the Read Data Available (RDA) flag.  When CE goes

--                              high, it means that data has been read into the rdSReg.

--                              This process then analyzes the read data for errors, sets

--                              rdReg equal to the eight data bits in rdSReg, and flags

--                              RDA to indicate that new data is present in rdReg.  FE 

--                              and PE are simply equal to the frameError and parError 

--                              signals.  OE is flagged high if RDA is already high when 

--                              CE is strobed.  This means that unread data was still in 

--                              the rdReg when it was written over with the new data.   

--

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

        process (CLK, RST, RD, CE)

                begin

                        if RD = '1' or RST = '1' then

                                FE <= '0';

                                OE <= '0';

                                RDA <= '0';

                                PE <= '0';

                        elsif CLK = '1' and CLK'event then

                                if CE = '1' then

                                        FE <= frameError;

                                        PE <= parError;

                                        rdReg(7 downto 0) <= rdSReg (7 downto 0);

                                        if RDA = '1' then

                                                OE <= '1';

                                        else

                                                OE <= '0';

                                                RDA <= '1';

                                        end if;

                                end if;

                        end if;

                end process;

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

--

--Title: Receiving shift register 

--

--Description:  This process controls the receiving shift register 

--                              (rdSReg).  Whenever rShift is high, implying that data 

--                              needs to be shifted in, rdSReg is shifts in RXD to the 

--                              most significant bit, while shifting its existing data 

--                              right.

--

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

        process (CLK, rShift)

                begin

                        if CLK = '1' and CLK'Event then

                                if rShift = '1' then

                                        rdSReg <= (RXD & rdSReg(9 downto 1));

                                end if;

                        end if;

                end process;

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

--

--Title: Incoming Data counter 

--

--Description:  This process controls the dataCtr to keep track of 

--                              shifted values into the rdSReg.  The dataCtr signal is 

--                              incremented once every time dataIncr is strobed high.

--                              

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

        process (CLK, dataRST)

                begin

                        if (CLK = '1' and CLK'event) then

                                if dataRST = '1' then

                                        dataCtr <= "0000";

                                elsif dataIncr = '1' then

                                        dataCtr <= dataCtr +1;

                                end if;

                        end if;

                end process;

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

--

--Title: Receiving State Machine controller 

--

--Description:  This process takes care of the Receiving state machine

--                              movement.  It causes the next state to be evaluated on

--                              each rising edge of CLK.  If the RST signal is strobed,

--                              the state is changed to the default starting state,

--                              which is strIdle

--

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

        process (CLK, RST)

                begin

                        if CLK = '1' and CLK'Event then

                                if RST = '1' then -- najj

                                        strCur <= strIdle;

                                else

                                        strCur <= strNext;

                                end if;

                        end if;

                end process;

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

--

--Title: Receiving State Machine  

--

--Description:  This process contains all of the next state logic for the

--                              Receiving state machine.

--

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

        process (strCur, ctr, RXD, dataCtr)

                begin

                        case strCur is

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

--

--Title: strIdle state 

--

--Description:  This state is the idle and startup default stage for the

--                              Receiving state machine.  The machine stays in this state

--                              until the RXD signal goes low.  When this occurs, the

--                              ctRst signal is strobed to reset ctr for the next state,

--                              which is strEightDelay.  

--

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

                                when strIdle =>

                                        dataIncr <= '0';

                                        rShift <= '0';

                                        dataRst <= '1';

                                        CE <= '0';

                                        ctRst <= '1';

                                        if RXD = '0' then

                                                strNext <= strEightDelay;

                                        else

                                                strNext <= strIdle;

                                        end if;

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

--

--Title: strEightDelay state 

--

--Description:  This state simply delays the state machine for eight clock

--                              cycles.  This is needed so that the incoming RXD data 

--                              signal is read in the middle of each data emission.  This

--                              ensures an accurate RXD signal reading.  ctr counts from 

--                              0 to 8 to keep track of rClk cycles.  When it equals 8 

--                              (1000) the next state, strWaitFor0, is loaded.  During 

--                              this state, the dataRst signal is strobed high to reset 

--                              the shift-in data counter (dataCtr).

--

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

                                when strEightDelay =>

                                        dataIncr <= '0';

                                        rShift <= '0';

                                        dataRst <= '1';

                                        CE <= '0';

                                        ctRst <= '0';

                                        if ctr(3 downto 0) = "1000" then

                                                strNext <= strWaitFor0;

                                        else

                                                strNext <= strEightDelay;

                                        end if;

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

--

--Title: strGetData state 

--

--Description:  In this state, the dataIncr and rShift signals are 

--                              strobed high for one clock cycle.  By doing this, the 

--                              rdSReg shift register shifts in RXD once, while the 

--                              dataCtr is incremented by one.  This state simply 

--                              captures the incoming data on RXD into the rdSReg shift 

--                              register.  The next state loaded is strWaitFor0, which 

--                              starts the two delay states needed between data shifts. 

--

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

                                when strGetData =>

                                        CE <= '0';

                                        dataRst <= '0';

                                        ctRst <= '0';

                                        dataIncr <= '1';

                                        rShift <= '1';

                                        strNext <= strWaitFor0;

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

--

--Title: strWaitFor0 state 

--

--Description:  This state is a delay state, which delays the receive

--                              state machine if not all of the incoming serial data has

--                              not been shifted in yet.  If dataCtr does not equal 10

--                              (1010), the state is stayed in until the fourth bit of

--                              ctr is equal to 1.  When this happens, half of the delay

--                              has been achieved, and the second delay state is loaded, 

--                              which is strWaitFor1.  If dataCtr does equal 10 (1010),

--                              all of the needed data has been acquired, so the 

--                              strCheckStop state is loaded to check for errors and 

--                              reset the receive state machine.

--

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

                                when strWaitFor0 =>

                                        CE <= '0';

                                        dataRst <= '0';

                                        ctRst <= '0';

                                        dataIncr <= '0';

                                        rShift <= '0';

                                        if dataCtr = "1010" then

                                                strNext <= strCheckStop;

                                        elsif ctr(3) = '0' then

                                                strNext <= strWaitFor1;

                                        else

                                                strNext <= strWaitFor0;

                                        end if;

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

--

--Title: strEightDelay state 

--

--Description:  This state is much like strWaitFor0, except it waits for

--                              the fourth bit of ctr to equal 1.  Once this occurs, the

--                              strGetData state is loaded in order to shift in the next

--                              data bit from RXD.  Because strWaitFor0 is the only state

--                              that calls this state, no other signals need to be 

--                              checked.

--

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

                                when strWaitFor1 =>

                                        CE <= '0';

                                        dataRst <= '0';

                                        ctRst <= '0';

                                        dataIncr <=     '0';

                                        rShift <= '0';

                                        if ctr(3) = '0' then

                                                strNext <= strWaitFor1;

                                        else

                                                strNext <= strGetData;

                                        end if;

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

--

--Title: strCheckStop state 

--

--Description:  This state allows the newly acquired data to be checked

--                              for errors.  The CE flag is strobed to start the

--                              previously defined error checking process.  This state is

--                              passed straight through to the strIdle state.

--

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

                                when strCheckStop =>

                                        dataIncr <= '0';

                                        rShift <= '0';

                                        dataRst <= '0';

                                        ctRst <= '0';

                                        CE <= '1';

                                        strNext <= strIdle;

                        end case;

                end process;

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

--

--Title: Transfer shift register controller 

--

--Description:  This process uses the load, shift, and clk signals to 

--                              control the transfer shift register (tfSReg).  Once load

--                              is equal to '1', the tfSReg gets a '1', the parity bit,

--                              the data bits found in tfReg, and a '0'.  Under this

--                              format, the shift register can be used to shift out the

--                              appropriate signal to serially transfer the data.  The

--                              data is shifted out of the tfSReg whenever shift = '1'. 

--

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

        process (load, shift, CLK, tfSReg)

                begin

                        if CLK = '1' and CLK'Event then

                                if load = '1' then

                                        tfSReg (10 downto 0) <= ('1' & par & tfReg(7 downto 0) &'0');

                                elsif shift = '1' then

                                        tfSReg (10 downto 0) <= ('1' & tfSReg(10 downto 1));

                                end if;

                        end if;

                end process;

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

--

--Title: Transfer State Machine controller 

--

--Description:  This process takes care of the Transfer state machine

--                              movement.  It causes the next state to be evaluated on

--                              each rising edge of CLK.  If the RST signal is strobed,

--                              the state is changed to the default starting state, which

--                              is sttIdle.

--

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

        process (CLK, RST)

                begin

                        if (CLK = '1' and CLK'Event) then

                                if RST = '1' then

                                        sttCur <= sttIdle;

                                else

                                        sttCur <= sttNext;

                                end if;

                        end if;

                end process;

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

--

--Title: Transfer State Machine 

--

--Description:  This process controls the next state logic in the 

--                              transfer state machine.  The transfer state machine 

--                              controls the shift and load signals that are used to load 

--                              and transmit the parallel data in a serial form.  It also 

--                              controls the Transmit Buffer Empty (TBE) signal that 

--                              indicates if the transmit buffer (tfSReg) is in use or 

--                              not.

--

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

        process (sttCur, tfCtr, WR, tDelayCtr)

                begin

                        case sttCur is

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

--

--Title: sttIdle state 

--

--Description:  This state is the idle and startup default stage for the 

--                              transfer state machine.  The state is stayed in until

--                              the WR signal goes high.  Once it goes high, the 

--                              sttTransfer state is loaded.  The load and shift signals

--                              are held low in the sttIdle state, while the TBE signal

--                              is held high to indicate that the transmit buffer is not

--                              currently in use.  Once the idle state is left, the TBE

--                              signal is held low to indicate that the transfer state

--                              machine is using the transmit buffer.

--

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

                                when sttIdle =>

                                        TBE <= '1';

                                        tClkRST <= '0';

                                        tfIncr <= '0';

                                        shift <= '0';

                                        load <= '0';

                                        tDelayRst <= '1';

                                        if WR = '0' then

                                                sttNext <= sttIdle;

                                        else

                                                sttNext <= sttTransfer;

                                        end if;

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

--

--Title: sttTransfer state

--

--Description:  This state sets the load, tClkRST, and tDelayRst signals 

--                              high, while setting the TBE signal low.  The load signal 

--                              is set high to load the transfer shift register with the 

--                              appropriate data, while the tClkRST and tDelayRst signals 

--                              are strobed to reset the tfCtr and tDelayCtr.  The next

--                              state loaded is the sttDelay state.

--                      

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

                                when sttTransfer =>

                                        TBE <= '0';

                                        shift <= '0';

                                        load <= '1';

                                        tClkRST <= '1';

                                        tfIncr <= '0';

                                        tDelayRst <= '1';

                                        sttNext <= sttDelay;

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

--

--Title: sttShift state 

--

--Description:  This state strobes the shift and tfIncr signals high, and

--                              checks the tfCtr to see if enough data has been 

--                              transmitted.  By strobing the shift and tfIncr signals

--                              high, the tfSReg is shifted, and the tfCtr is incremented

--                              once.  If tfCtr does not equal 9 (1001), then not all of

--                              the bits have been transmitted, so the next state loaded 

--                              is the sttDelay state.  If tfCtr does equal 9, the final 

--                              state, sttWaitWrite, is loaded.

--

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

                                when sttShift =>

                                        TBE <= '0';

                                        shift <= '1';

                                        load <= '0';

                                        tfIncr <= '1';

                                        tClkRST <= '0';

                                        tDelayRst <= '0';

                                        if tfCtr = "1010" then