Subversion Repositories spacewire_light

--

--  SpaceWire Transmitter

--

--  This entity translates outgoing characters and tokens into

--  data-strobe signalling.

--

--  The output stage is driven by a separate transmission clock "txclk" which

--  will typically be faster than the system clock. The actual transmission

--  rate is determined by dividing the transmission clock by an integer factor.

--

--  The code is tuned for implementation on Xilinx Spartan-3.

--

--  Concept

--  -------

--

--  Logic in the system clock domain generates a stream of tokens to be

--  transmitted. These tokens are encoded as instances of the token_type

--  record. Tokens are queued in a two-slot FIFO buffer (r.token0 and r.token1)

--  with a 1-bit pointer (r.tokmux) pointing to the head of the queue.

--  When a token is pushed into the buffer, a flag register is flipped

--  (r.sysflip0 and r.sysflip1) to indicate to the txclk domain that the

--  buffer slot has been refilled.

--

--  The txclk domain pulls tokens from the FIFO buffer, flipping flag

--  registers (rtx.txflip0 and rtx.txflip1) to indicate to the system clock

--  domain that a token has been pulled. When the system clock domain detects

--  that a token has been consumed, it refills the buffer slot with a new

--  token (assuming that there are tokens waiting to be transmitted).

--  Whenever the FIFO buffer is empty, the txclk domain sends NULLs instead.

--  This can happen either when there are no tokens to send, or when the

--  system clock domain is late to refill the buffer.

--

--  Details

--  -------

--

--  Logic in the system clock domain accepts transmission requests through

--  the external interface of the entity. Pending requests are translated

--  into a stream of tokens. The tokens are pushed to the txclk domain through

--  the FIFO buffer as described above.

--

--  The data path through the txclk domain is divided into stages A through F

--  in a half-hearted attempt to keep things simple. Stage A is just a

--  synchronizer for the buffer status flags from the system clock domain.

--

--  Stage B takes a token from the FIFO buffer and updates a buffer status

--  flag to indicate that the buffer slot needs to be refilled. If the FIFO

--  is empty, a NULL is inserted. Stage B is triggered one clock after

--  stage E switches to a new token. If the previous token was ESC, stage B

--  skips a turn because stage C will already know what to do.

--

--  Stage C takes a token from stage B and translates it into a bit pattern.

--  Time codes and NULL tokens are broken into two separate tokens starting

--  with ESC. Stage C is triggered one clock after the shift buffer in

--  stage E drops to 3 tokens.

--

--  Stage D completes the task of translating tokens to bit patterns and

--  distinguishes between 10-bit and 4-bit tokens. It is not explicitly

--  triggered but simply follows stage C.

--

--  Stage E is the bit shift register. It shifts when "txclken" is high.

--  A one-hot counter keeps track of the number of bits remaining in

--  the register. When the register falls empty, it loads a new 10-bit or

--  4-bit pattern as prepared by stage D. Stage E also computes parity.

--

--  Stage F performs data strobe encoding. When the transmitter is disabled,

--  the outputs of stage F fall to zero in a controlled way.

-- 

--  To generate the transmission bit clock, the txclk is divided by an

--  integer factor (divcnt+1) using an 8-bit down counter. The implementation

--  of this counter has become quite complicated in order to meet timing goals.

--  The counter consists of 4 blocks of two bits each (txclkcnt), with a

--  carry-save concept used between blocks (txclkcy). Detection of terminal

--  count (txclkdone) has a pipeline delay of two cycles. Therefore a separate

--  concept is used if the initial count is less than 2 (txdivnorm). This is

--  all glued together in the final assignment to txclken.

--

--  The initial count for txclk division (divcnt) comes from the system clock

--  domain and thus needs to be synchronized for use in the txclk domain.

--  To facilitate this, the system clock domain latches the value of divcnt

--  once every 6 sysclk cycles and sets a flag to indicate when the latched

--  value can safely be used by the txclk domain.

--

--  A tricky aspect of the design is the initial state of the txclk logic.

--  When the transmitter is enabled (txen goes high), the txclk logic starts

--  with the first ESC pattern already set up in stage D, and stage C ready

--  to produce the FCT part of the first NULL.

--

--  The following guidelines are used to get good timing for the txclk domain:

--   * The new value of a register depends on at most 4 inputs (single LUT),

--     or in a few cases on 5 inputs (two LUTs and F5MUX).

--   * Synchronous resets may be used, but only if the reset signal comes

--     directly from a register (no logic in set/reset path);

--   * Clock enables may be used, but only if the enable signal comes directly

--     from a register (no logic in clock enable path).

--

--  Synchronization issues

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

--

--  There is a two-slot FIFO buffer between the system and txclk domains.

--  After the txclk domain pulls a token from the buffer, the system clock

--  domain should ideally refill the buffer before the txclk domain again

--  tries to pull from the same buffer slot. If the refill occurs late,

--  the txclk domain needs to insert a NULL token which is inefficient

--  use of bandwidth.

--

--  Assuming the transmission consists of a stream of data characters,

--  10 bits per character, there are exactly 2*10 bit periods between

--  successive reads from the same buffer slot by the txclk logic.

--

--  The time needed for the system clock logic to refill a buffer slot =

--     1 txclk period   (update of rtx.txflipN)

--   + 1 txclk period   (routing delay between domains)

--   + 2 sysclk periods (synchronizer for txflipN)

--   + 1 sysclk period  (refill buffer slot and update r.sysflipN)

--   + 1 txclk period   (routing delay between domains)

--   + 2 txclk periods  (synchronizer for sysflipN)

--   = 5 txclk periods + 3 sysclk periods

--

--  If for example txclk is 4 times as fast as sysclk, this amounts to

--   5 txclk + 3 sysclk = 5 + 3*4 txclk = 17 txclk

--  is less than 20 bit periods even at maximum transmission rate, so

--  no problem there.

--

--  This is different when the data stream includes 4-bit tokens.

--  See the datasheet for an analysis of that case.

--

--  Implementation guidelines

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

--

--  To minimize clock skew, IOB flip-flops should be used to drive

--  spw_do and spw_so.

--

--  "txclk" must be at least as fast as the system clock;

--  "txclk" does not need to be phase-related to the system clock;

--  it is allowed for "txclk" to be equal to "clk".

--

--  The following timing constraints are needed:

--   * PERIOD constraint on the system clock;

--   * PERIOD constraint on "txclk";

--   * FROM-TO constraint from "txclk" to the system clock, equal to

--     one "txclk" period;

--   * FROM-TO constraint from the system clock to "txclk", equal to

--     one "txclk" period.

--

library ieee;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

use work.spwpkg.all;

entity spwxmit_fast is

    port (

        -- System clock.

        clk:        in  std_logic;

        -- Transmit clock.

        txclk:      in  std_logic;

        -- Synchronous reset (active-high)

        -- Used asynchronously by fast clock domain (must be glitch-free).

        rst:        in  std_logic;

        -- Scaling factor minus 1, used to scale the system clock into the

        -- transmission bit rate. The system clock is divided by

        -- (unsigned(divcnt) + 1). Changing this signal will immediately

        -- change the transmission rate.

        divcnt:     in  std_logic_vector(7 downto 0);

        -- Input signals from spwlink.

        xmiti:      in  spw_xmit_in_type;

        -- Output signals to spwlink.

        xmito:      out spw_xmit_out_type;

        -- Data Out signal to SpaceWire bus.

        spw_do:     out std_logic;

        -- Strobe Out signal to SpaceWire bus.

        spw_so:     out std_logic

    );

    -- Turn off FSM extraction to avoid synchronization problems.

    attribute FSM_EXTRACT: string;

    attribute FSM_EXTRACT of spwxmit_fast: entity is "NO";

    -- Turn off register duplication to avoid synchronization problems.

    attribute REGISTER_DUPLICATION: string;

    attribute REGISTER_DUPLICATION of spwxmit_fast: entity is "FALSE";

end entity spwxmit_fast;

architecture spwxmit_fast_arch of spwxmit_fast is

    -- Convert boolean to std_logic.

    type bool_to_logic_type is array(boolean) of std_ulogic;

    constant bool_to_logic: bool_to_logic_type := (false => '0', true => '1');

    -- Data records passed between clock domains.

    type token_type is record

        tick:       std_ulogic;                     -- send time code

        fct:        std_ulogic;                     -- send FCT

        flag:       std_ulogic;                     -- send EOP or EEP

        char:       std_logic_vector(7 downto 0);   -- character or time code

    end record;

    -- Registers in txclk domain

    type txregs_type is record

        -- sync to system clock domain

        txflip0:    std_ulogic;

        txflip1:    std_ulogic;

        -- stage A

        a_sysflip0: std_logic_vector(1 downto 0);

        a_sysflip1: std_logic_vector(1 downto 0);

        -- stage B

        b_update:   std_ulogic;

        b_mux:      std_ulogic;

        b_txflip:   std_ulogic;

        b_valid:    std_ulogic;

        b_token:    token_type;

        -- stage C

        c_update:   std_ulogic;

        c_esc:      std_ulogic;

        c_fct:      std_ulogic;

        c_bits:     std_logic_vector(8 downto 0);

        -- stage D

        d_bits:     std_logic_vector(8 downto 0);

        d_cnt4:     std_ulogic;

        d_cnt10:    std_ulogic;

        -- stage E

        e_valid:    std_ulogic;

        e_shift:    std_logic_vector(9 downto 0);

        e_count:    std_logic_vector(9 downto 0);

        e_parity:   std_ulogic;

        -- stage F

        f_spwdo:    std_ulogic;

        f_spwso:    std_ulogic;

        -- tx clock enable logic

        txclken:    std_ulogic;

        txclkpre:   std_ulogic;

        txclkcnt:   std_logic_vector(7 downto 0);

        txclkcy:    std_logic_vector(2 downto 0);

        txclkdone:  std_logic_vector(1 downto 0);

        txclkdiv:   std_logic_vector(7 downto 0);

        txdivnorm:  std_ulogic;

        txdivsafe:  std_logic_vector(1 downto 0);

        -- tx enable logic

        txensync:   std_logic_vector(1 downto 0);

    end record;

    -- Registers in system clock domain

    type regs_type is record

        -- sync status to txclk domain

        txenreg:    std_ulogic;

        txdivreg:   std_logic_vector(7 downto 0);

        txdivnorm:  std_ulogic;

        txdivtmp:   std_logic_vector(1 downto 0);

        txdivsafe:  std_ulogic;

        -- data stream to txclk domain

        sysflip0:   std_ulogic;

        sysflip1:   std_ulogic;

        token0:     token_type;

        token1:     token_type;

        tokmux:     std_ulogic;

        -- sync feedback from txclk domain

        txflip0:    std_logic_vector(1 downto 0);

        txflip1:    std_logic_vector(1 downto 0);

        -- transmitter management

        pend_fct:   std_ulogic;                     -- '1' if an outgoing FCT is pending

        pend_char:  std_ulogic;                     -- '1' if an outgoing N-Char is pending

        pend_data:  std_logic_vector(8 downto 0);   -- control flag and data bits of pending char

        pend_tick:  std_ulogic;                     -- '1' if an outgoing time tick is pending

        pend_time:  std_logic_vector(7 downto 0);   -- data bits of pending time tick

        allow_fct:  std_ulogic;                     -- '1' when allowed to send FCTs

        allow_char: std_ulogic;                     -- '1' when allowed to send data and time

        sent_fct:   std_ulogic;                     -- '1' when at least one FCT token was sent

    end record;

    -- Initial state of system clock domain

    constant regs_reset: regs_type := (

        txenreg     => '0',

        txdivreg    => (others => '0'),

        txdivnorm   => '0',

        txdivtmp    => "00",

        txdivsafe   => '0',

        sysflip0    => '0',

        sysflip1    => '0',

        token0      => ( tick => '0', fct => '0', flag => '0', char => (others => '0') ),

        token1      => ( tick => '0', fct => '0', flag => '0', char => (others => '0') ),

        tokmux      => '0',

        txflip0     => "00",

        txflip1     => "00",

        pend_fct    => '0',

        pend_char   => '0',

        pend_data   => (others => '0'),

        pend_tick   => '0',

        pend_time   => (others => '0'),

        allow_fct   => '0',

        allow_char  => '0',

        sent_fct    => '0' );

    -- Registers

    signal rtx:     txregs_type;

    signal rtxin:   txregs_type;

    signal r:       regs_type := regs_reset;

    signal rin:     regs_type;

    -- Reset synchronizer for txclk domain

    signal s_tx_rst_sync: std_logic_vector(1 downto 0) := "11";

    signal s_tx_reset:    std_ulogic := '1';

    -- Output flip-flops

    signal s_spwdo: std_logic;

    signal s_spwso: std_logic;

    -- Force use of IOB flip-flops

    attribute IOB: string;

    attribute IOB of s_spwdo: signal is "TRUE";

    attribute IOB of s_spwso: signal is "TRUE";

begin

    -- Drive SpaceWire output signals

    spw_do      <= s_spwdo;

    spw_so      <= s_spwso;

    -- Combinatorial process

    process (r, rtx, rst, divcnt, xmiti, s_tx_reset) is

        variable v:         regs_type;

        variable vtx:       txregs_type;

        variable v_needtoken: std_ulogic;

        variable v_havetoken: std_ulogic;

        variable v_token:     token_type;

    begin

        v           := r;

        vtx         := rtx;

        v_needtoken := '0';

        v_havetoken := '0';

        v_token     := ( tick => '0', fct => '0', flag => '0', char => (others => '0') );

        -- ---- FAST CLOCK DOMAIN ----

        -- Stage A: Synchronize token buffer counts from system clock domain.

        vtx.a_sysflip0(0) := r.sysflip0;

        vtx.a_sysflip0(1) := rtx.a_sysflip0(0);

        vtx.a_sysflip1(0) := r.sysflip1;

        vtx.a_sysflip1(1) := rtx.a_sysflip1(0);

        -- Stage B: Multiplex tokens from system clock domain.

        -- Update stage B three bit periods after updating stage C

        -- (i.e. in time for the next update of stage C).

        -- Do not update stage B if the last token from stage C was ESC;

        -- stage C already knows what token to put after the ESC.

        vtx.b_update := rtx.txclken and rtx.e_count(0) and (not rtx.c_esc);

        if rtx.b_mux = '0' then

            vtx.b_txflip := rtx.txflip0;

        else

            vtx.b_txflip := rtx.txflip1;

        end if;

        if rtx.b_update = '1' then

            if rtx.b_mux = '0' then

                -- get token from slot 0

                vtx.b_valid := rtx.a_sysflip0(1) xor rtx.b_txflip;

                vtx.b_token := r.token0;

                -- update mux flag if we got a valid token

                vtx.b_mux   := rtx.a_sysflip0(1) xor rtx.b_txflip;

                vtx.txflip0 := rtx.a_sysflip0(1);

                vtx.txflip1 := rtx.txflip1;

            else

                -- get token from slot 1

                vtx.b_valid := rtx.a_sysflip1(1) xor rtx.b_txflip;

                vtx.b_token := r.token1;

                -- update mux flag if we got a valid token

                vtx.b_mux   := not (rtx.a_sysflip1(1) xor rtx.b_txflip);

                vtx.txflip0 := rtx.txflip0;

                vtx.txflip1 := rtx.a_sysflip1(1);

            end if;

        end if;

        -- Stage C: Prepare to transmit EOP, EEP or a data character.

        vtx.c_update := rtx.txclken and rtx.e_count(3);

        if rtx.c_update = '1' then

            -- NULL is broken into two tokens: ESC + FCT

            -- time codes are broken into two tokens: ESC + char

            vtx.c_esc   := (rtx.b_token.tick or (not rtx.b_valid)) and

                           (not rtx.c_esc);

            vtx.c_fct   := rtx.b_token.fct or (not rtx.b_valid);

            if rtx.b_token.flag = '1' then

                if rtx.b_token.char(0) = '0' then

                    -- prepare to send EOP

                    vtx.c_bits  := "000000101"; -- EOP = P101

                else

                    -- prepare to send EEP

                    vtx.c_bits  := "000000011"; -- EEP = P110

                end if;

            else

                -- prepare to send data char

                vtx.c_bits  := rtx.b_token.char & '0';

            end if;

        end if;

        -- Stage D: Prepare to transmit FCT, ESC, or the stuff from stage C.

        if rtx.c_esc = '1' then

            -- prepare to send ESC

            vtx.d_bits  := "000000111";     -- ESC = P111

            vtx.d_cnt4  := '1';             -- 3 bits + implicit parity bit

            vtx.d_cnt10 := '0';

        elsif rtx.c_fct = '1' then

            -- prepare to send FCT

            vtx.d_bits  := "000000001";     -- FCT = P100

            vtx.d_cnt4  := '1';             -- 3 bits + implicit parity bit

            vtx.d_cnt10 := '0';

        else

            -- send the stuff from stage C.

            vtx.d_bits  := rtx.c_bits;

            vtx.d_cnt4  := rtx.c_bits(0);

            vtx.d_cnt10 := not rtx.c_bits(0);

        end if;

        -- Stage E: Shift register.

        if rtx.txclken = '1' then

            if rtx.e_count(0) = '1' then

                -- reload shift register; output parity bit

                vtx.e_valid  := '1';

                vtx.e_shift(vtx.e_shift'high downto 1) := rtx.d_bits;

                vtx.e_shift(0) := not (rtx.e_parity xor rtx.d_bits(0));

                vtx.e_count  := rtx.d_cnt10 & "00000" & rtx.d_cnt4 & "000";

                vtx.e_parity := rtx.d_bits(0);

            else

                -- shift bits to output; update parity bit

                vtx.e_shift  := '0' & rtx.e_shift(rtx.e_shift'high downto 1);

                vtx.e_count  := '0' & rtx.e_count(rtx.e_count'high downto 1);

                vtx.e_parity := rtx.e_parity xor rtx.e_shift(1);

            end if;

        end if;

        -- Stage F: Data/strobe encoding.

        if rtx.txclken = '1' then

            if rtx.e_valid = '1' then

                -- output next data/strobe bits

                vtx.f_spwdo := rtx.e_shift(0);

                vtx.f_spwso := not (rtx.e_shift(0) xor rtx.f_spwdo xor rtx.f_spwso);

            else

                -- gentle reset of spacewire signals

                vtx.f_spwdo := rtx.f_spwdo and rtx.f_spwso;

                vtx.f_spwso := '0';

            end if;

        end if;

        -- Generate tx clock enable

        -- An 8-bit counter decrements on every clock. A txclken pulse is

        -- produced 2 cycles after the counter reaches value 2. Counter reload

        -- values of 0 and 1 are handled as special cases.

        -- count down in blocks of two bits

        vtx.txclkcnt(1 downto 0) := std_logic_vector(unsigned(rtx.txclkcnt(1 downto 0)) - 1);

        vtx.txclkcnt(3 downto 2) := std_logic_vector(unsigned(rtx.txclkcnt(3 downto 2)) - unsigned(rtx.txclkcy(0 downto 0)));

        vtx.txclkcnt(5 downto 4) := std_logic_vector(unsigned(rtx.txclkcnt(5 downto 4)) - unsigned(rtx.txclkcy(1 downto 1)));

        vtx.txclkcnt(7 downto 6) := std_logic_vector(unsigned(rtx.txclkcnt(7 downto 6)) - unsigned(rtx.txclkcy(2 downto 2)));

        -- propagate carry in blocks of two bits

        vtx.txclkcy(0) := bool_to_logic(rtx.txclkcnt(1 downto 0) = "00");

        vtx.txclkcy(1) := rtx.txclkcy(0) and bool_to_logic(rtx.txclkcnt(3 downto 2) = "00");

        vtx.txclkcy(2) := rtx.txclkcy(1) and bool_to_logic(rtx.txclkcnt(5 downto 4) = "00");

        -- detect value 2 in counter

        vtx.txclkdone(0) := bool_to_logic(rtx.txclkcnt(3 downto 0) = "0010");

        vtx.txclkdone(1) := bool_to_logic(rtx.txclkcnt(7 downto 4) = "0000");

        -- trigger txclken

        vtx.txclken  := (rtx.txclkdone(0) and rtx.txclkdone(1)) or rtx.txclkpre;

        vtx.txclkpre := (not rtx.txdivnorm) and ((not rtx.txclkpre) or (not rtx.txclkdiv(0)));

        -- reload counter

        if rtx.txclken = '1' then

            vtx.txclkcnt  := rtx.txclkdiv;

            vtx.txclkcy   := "000";

            vtx.txclkdone := "00";

        end if;

        -- Synchronize txclkdiv

        vtx.txdivsafe(0) := r.txdivsafe;

        vtx.txdivsafe(1) := rtx.txdivsafe(0);

        if rtx.txdivsafe(1) = '1' then

            vtx.txclkdiv  := r.txdivreg;

            vtx.txdivnorm := r.txdivnorm;

        end if;

        -- Synchronize txen signal.

        vtx.txensync(0) := r.txenreg;

        vtx.txensync(1) := rtx.txensync(0);

        -- Transmitter disabled.

        if rtx.txensync(1) = '0' then

            vtx.txflip0   := '0';

            vtx.txflip1   := '0';

            vtx.b_update  := '0';

            vtx.b_mux     := '0';

            vtx.b_valid   := '0';

            vtx.c_update  := '0';

            vtx.c_esc     := '1';           -- need to send 2nd part of NULL

            vtx.c_fct     := '1';

            vtx.d_bits    := "000000111";   -- ESC = P111

            vtx.d_cnt4    := '1';           -- 3 bits + implicit parity bit

            vtx.d_cnt10   := '0';

            vtx.e_valid   := '0';

            vtx.e_parity  := '0';

            vtx.e_count   := (0 => '1', others => '0');

        end if;

        -- Reset.

        if s_tx_reset = '1' then

            vtx.f_spwdo   := '0';

            vtx.f_spwso   := '0';

            vtx.txensync  := "00";

            vtx.txclken   := '0';

            vtx.txclkpre  := '1';

            vtx.txclkcnt  := (others => '0');

            vtx.txclkdiv  := (others => '0');

            vtx.txdivnorm := '0';

        end if;

        -- ---- SYSTEM CLOCK DOMAIN ----

        -- Hold divcnt and txen for use by txclk domain.

        v.txdivtmp  := std_logic_vector(unsigned(r.txdivtmp) - 1);

        if r.txdivtmp = "00" then

            if r.txdivsafe = '0' then

                -- Latch the current value of divcnt and txen.

                v.txdivsafe := '1';

                v.txdivtmp  := "01";

                v.txdivreg  := divcnt;

                if unsigned(divcnt(divcnt'high downto 1)) = 0 then

                    v.txdivnorm := '0';

                else

                    v.txdivnorm := '1';

                end if;

                v.txenreg   := xmiti.txen;

            else

                -- Drop the txdivsafe flag but keep latched values.

                v.txdivsafe := '0';

            end if;

        end if;

        -- Pass falling edge of txen signal as soon as possible.

        if xmiti.txen = '0' then

            v.txenreg   := '0';

        end if;

        -- Synchronize feedback from txclk domain.

        v.txflip0(0) := rtx.txflip0;

        v.txflip0(1) := r.txflip0(0);

        v.txflip1(0) := rtx.txflip1;

        v.txflip1(1) := r.txflip1(0);

        -- Store requests for FCT transmission.

        if xmiti.fct_in = '1' and r.allow_fct = '1' then

            v.pend_fct  := '1';

        end if;

        if xmiti.txen = '0' then

            -- Transmitter disabled; reset state.

            v.sysflip0    := '0';

            v.sysflip1    := '0';

            v.tokmux      := '0';

            v.pend_fct    := '0';

            v.pend_char   := '0';

            v.pend_tick   := '0';

            v.allow_fct   := '0';

            v.allow_char  := '0';

            v.sent_fct    := '0';

        else

            -- Determine if a new token is needed.

            if r.tokmux = '0' then

                if r.sysflip0 = r.txflip0(1) then

                    v_needtoken := '1';

                end if;

            else

                if r.sysflip1 = r.txflip1(1) then

                    v_needtoken := '1';

                end if;

            end if;

            -- Prepare new token.

            if r.allow_char = '1' and r.pend_tick = '1' then

                -- prepare to send time code

                v_token.tick  := '1';

                v_token.fct   := '0';

                v_token.flag  := '0';

                v_token.char  := r.pend_time;

                v_havetoken   := '1';

                if v_needtoken = '1' then

                    v.pend_tick := '0';

                end if;

            elsif r.allow_fct = '1' and (xmiti.fct_in = '1' or r.pend_fct = '1') then

                -- prepare to send FCT

                v_token.tick  := '0';

                v_token.fct   := '1';

                v_token.flag  := '0';

                v_havetoken   := '1';

                if v_needtoken = '1' then

                    v.pend_fct  := '0';

                    v.sent_fct  := '1';

                end if;

            elsif r.allow_char = '1' and r.pend_char = '1' then

                -- prepare to send N-Char

                v_token.tick  := '0';

                v_token.fct   := '0';

                v_token.flag  := r.pend_data(8);

                v_token.char  := r.pend_data(7 downto 0);

                v_havetoken   := '1';

                if v_needtoken = '1' then

                    v.pend_char := '0';

                end if;

            end if;

            -- Put new token in slot.

            if v_havetoken = '1' then

                if r.tokmux = '0' then

                    if r.sysflip0 = r.txflip0(1) then

                        v.sysflip0  := not r.sysflip0;

                        v.token0    := v_token;

                        v.tokmux    := '1';

                    end if;

                else

                    if r.sysflip1 = r.txflip1(1) then

                        v.sysflip1  := not r.sysflip1;

                        v.token1    := v_token;

                        v.tokmux    := '0';

                    end if;

                end if;

            end if;

            -- Determine whether we are allowed to send FCTs and characters

            v.allow_fct  := not xmiti.stnull;

            v.allow_char := (not xmiti.stnull) and (not xmiti.stfct) and r.sent_fct;

            -- Store request for data transmission.

            if xmiti.txwrite = '1' and r.allow_char = '1' and r.pend_char = '0' then

                v.pend_char  := '1';

                v.pend_data  := xmiti.txflag & xmiti.txdata;

            end if;

            -- Store requests for time tick transmission.

            if xmiti.tick_in = '1' then

                v.pend_tick := '1';

                v.pend_time := xmiti.ctrl_in & xmiti.time_in;

            end if;

        end if;

        -- Synchronous reset of system clock domain.

        if rst = '1' then

            v := regs_reset;

        end if;

        -- Drive outputs.

        -- Note: the outputs are combinatorially dependent on certain inputs.

        -- Set fctack high if (FCT requested) and (FCTs allowed) AND

        -- (no FCT pending)

        xmito.fctack <= xmiti.fct_in and xmiti.txen and r.allow_fct and

                        (not r.pend_fct);

        -- Set txrdy high if (character requested) AND (characters allowed) AND

        -- (no character pending)

        xmito.txack <= xmiti.txwrite and xmiti.txen and r.allow_char and

                       (not r.pend_char);