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//////////////////////////////////////////////////////////////////////

////                                                              ////

////  uart_regs.v                                                 ////

////                                                              ////

////                                                              ////

////  This file is part of the "UART 16550 compatible" project    ////

////  http://www.opencores.org/cores/uart16550/                   ////

////                                                              ////

////  Documentation related to this project:                      ////

////  - http://www.opencores.org/cores/uart16550/                 ////

////                                                              ////

////  Projects compatibility:                                     ////

////  - WISHBONE                                                  ////

////  RS232 Protocol                                              ////

////  16550D uart (mostly supported)                              ////

////                                                              ////

////  Overview (main Features):                                   ////

////  Registers of the uart 16550 core                            ////

////                                                              ////

////  Known problems (limits):                                    ////

////  Inserts 1 wait state in all WISHBONE transfers              ////

////                                                              ////

////  To Do:                                                      ////

////  Nothing or verification.                                    ////

////                                                              ////

////  Author(s):                                                  ////

////      - gorban@opencores.org                                  ////

////      - Jacob Gorban                                          ////

////      - Igor Mohor (igorm@opencores.org)                      ////

////                                                              ////

////  Created:        2001/05/12                                  ////

////  Last Updated:   (See log for the revision history           ////

////                                                              ////

////                                                              ////

//////////////////////////////////////////////////////////////////////

////                                                              ////

//// Copyright (C) 2000, 2001 Authors                             ////

////                                                              ////

//// This source file may be used and distributed without         ////

//// restriction provided that this copyright statement is not    ////

//// removed from the file and that any derivative work contains  ////

//// the original copyright notice and the associated disclaimer. ////

////                                                              ////

//// This source file is free software; you can redistribute it   ////

//// and/or modify it under the terms of the GNU Lesser General   ////

//// Public License as published by the Free Software Foundation; ////

//// either version 2.1 of the License, or (at your option) any   ////

//// later version.                                               ////

////                                                              ////

//// This source 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 Lesser General Public License for more ////

//// details.                                                     ////

////                                                              ////

//// You should have received a copy of the GNU Lesser General    ////

//// Public License along with this source; if not, download it   ////

//// from http://www.opencores.org/lgpl.shtml                     ////

////                                                              ////

//////////////////////////////////////////////////////////////////////

//

// CVS Revision History

//

// $Log: not supported by cvs2svn $

// Revision 1.41  2004/05/21 11:44:41  tadejm

// Added synchronizer flops for RX input.

//

// Revision 1.40  2003/06/11 16:37:47  gorban

// This fixes errors in some cases when data is being read and put to the FIFO at the same time. Patch is submitted by Scott Furman. Update is very recommended.

//

// Revision 1.39  2002/07/29 21:16:18  gorban

// The uart_defines.v file is included again in sources.

//

// Revision 1.38  2002/07/22 23:02:23  gorban

// Bug Fixes:

//  * Possible loss of sync and bad reception of stop bit on slow baud rates fixed.

//   Problem reported by Kenny.Tung.

//  * Bad (or lack of ) loopback handling fixed. Reported by Cherry Withers.

//

// Improvements:

//  * Made FIFO's as general inferrable memory where possible.

//  So on FPGA they should be inferred as RAM (Distributed RAM on Xilinx).

//  This saves about 1/3 of the Slice count and reduces P&R and synthesis times.

//

//  * Added optional baudrate output (baud_o).

//  This is identical to BAUDOUT* signal on 16550 chip.

//  It outputs 16xbit_clock_rate - the divided clock.

//  It's disabled by default. Define UART_HAS_BAUDRATE_OUTPUT to use.

//

// Revision 1.37  2001/12/27 13:24:09  mohor

// lsr[7] was not showing overrun errors.

//

// Revision 1.36  2001/12/20 13:25:46  mohor

// rx push changed to be only one cycle wide.

//

// Revision 1.35  2001/12/19 08:03:34  mohor

// Warnings cleared.

//

// Revision 1.34  2001/12/19 07:33:54  mohor

// Synplicity was having troubles with the comment.

//

// Revision 1.33  2001/12/17 10:14:43  mohor

// Things related to msr register changed. After THRE IRQ occurs, and one

// character is written to the transmit fifo, the detection of the THRE bit in the

// LSR is delayed for one character time.

//

// Revision 1.32  2001/12/14 13:19:24  mohor

// MSR register fixed.

//

// Revision 1.31  2001/12/14 10:06:58  mohor

// After reset modem status register MSR should be reset.

//

// Revision 1.30  2001/12/13 10:09:13  mohor

// thre irq should be cleared only when being source of interrupt.

//

// Revision 1.29  2001/12/12 09:05:46  mohor

// LSR status bit 0 was not cleared correctly in case of reseting the FCR (rx fifo).

//

// Revision 1.28  2001/12/10 19:52:41  gorban

// Scratch register added

//

// Revision 1.27  2001/12/06 14:51:04  gorban

// Bug in LSR[0] is fixed.

// All WISHBONE signals are now sampled, so another wait-state is introduced on all transfers.

//

// Revision 1.26  2001/12/03 21:44:29  gorban

// Updated specification documentation.

// Added full 32-bit data bus interface, now as default.

// Address is 5-bit wide in 32-bit data bus mode.

// Added wb_sel_i input to the core. It's used in the 32-bit mode.

// Added debug interface with two 32-bit read-only registers in 32-bit mode.

// Bits 5 and 6 of LSR are now only cleared on TX FIFO write.

// My small test bench is modified to work with 32-bit mode.

//

// Revision 1.25  2001/11/28 19:36:39  gorban

// Fixed: timeout and break didn't pay attention to current data format when counting time

//

// Revision 1.24  2001/11/26 21:38:54  gorban

// Lots of fixes:

// Break condition wasn't handled correctly at all.

// LSR bits could lose their values.

// LSR value after reset was wrong.

// Timing of THRE interrupt signal corrected.

// LSR bit 0 timing corrected.

//

// Revision 1.23  2001/11/12 21:57:29  gorban

// fixed more typo bugs

//

// Revision 1.22  2001/11/12 15:02:28  mohor

// lsr1r error fixed.

//

// Revision 1.21  2001/11/12 14:57:27  mohor

// ti_int_pnd error fixed.

//

// Revision 1.20  2001/11/12 14:50:27  mohor

// ti_int_d error fixed.

//

// Revision 1.19  2001/11/10 12:43:21  gorban

// Logic Synthesis bugs fixed. Some other minor changes

//

// Revision 1.18  2001/11/08 14:54:23  mohor

// Comments in Slovene language deleted, few small fixes for better work of

// old tools. IRQs need to be fix.

//

// Revision 1.17  2001/11/07 17:51:52  gorban

// Heavily rewritten interrupt and LSR subsystems.

// Many bugs hopefully squashed.

//

// Revision 1.16  2001/11/02 09:55:16  mohor

// no message

//

// Revision 1.15  2001/10/31 15:19:22  gorban

// Fixes to break and timeout conditions

//

// Revision 1.14  2001/10/29 17:00:46  gorban

// fixed parity sending and tx_fifo resets over- and underrun

//

// Revision 1.13  2001/10/20 09:58:40  gorban

// Small synopsis fixes

//

// Revision 1.12  2001/10/19 16:21:40  gorban

// Changes data_out to be synchronous again as it should have been.

//

// Revision 1.11  2001/10/18 20:35:45  gorban

// small fix

//

// Revision 1.10  2001/08/24 21:01:12  mohor

// Things connected to parity changed.

// Clock devider changed.

//

// Revision 1.9  2001/08/23 16:05:05  mohor

// Stop bit bug fixed.

// Parity bug fixed.

// WISHBONE read cycle bug fixed,

// OE indicator (Overrun Error) bug fixed.

// PE indicator (Parity Error) bug fixed.

// Register read bug fixed.

//

// Revision 1.10  2001/06/23 11:21:48  gorban

// DL made 16-bit long. Fixed transmission/reception bugs.

//

// Revision 1.9  2001/05/31 20:08:01  gorban

// FIFO changes and other corrections.

//

// Revision 1.8  2001/05/29 20:05:04  gorban

// Fixed some bugs and synthesis problems.

//

// Revision 1.7  2001/05/27 17:37:49  gorban

// Fixed many bugs. Updated spec. Changed FIFO files structure. See CHANGES.txt file.

//

// Revision 1.6  2001/05/21 19:12:02  gorban

// Corrected some Linter messages.

//

// Revision 1.5  2001/05/17 18:34:18  gorban

// First 'stable' release. Should be sythesizable now. Also added new header.

//

// Revision 1.0  2001-05-17 21:27:11+02  jacob

// Initial revision

//

//

// synopsys translate_off

`include "timescale.v"

// synopsys translate_on

`include "uart_defines.v"

`define UART_DL1 7:0

`define UART_DL2 15:8

module uart_regs (clk,

                  wb_rst_i, wb_addr_i, wb_dat_i, wb_dat_o, wb_we_i, wb_re_i,

                  // additional signals

                  modem_inputs,

                  stx_pad_o, srx_pad_i,

`ifdef DATA_BUS_WIDTH_8

`else

                  // debug interface signals    enabled

                  ier, iir, fcr, mcr, lcr, msr, lsr, rf_count, tf_count, tstate, rstate,

`endif

                  rts_pad_o, dtr_pad_o, int_o

`ifdef UART_HAS_BAUDRATE_OUTPUT

                  , baud_o

`endif

                  );

   input clk;

   input wb_rst_i;

   input [`UART_ADDR_WIDTH-1:0] wb_addr_i;

   input [7:0]                   wb_dat_i;

   output [7:0]          wb_dat_o;

   input                        wb_we_i;

   input                        wb_re_i;

   output                       stx_pad_o;

   input                        srx_pad_i;

   input [3:0]                   modem_inputs;

   output                       rts_pad_o;

   output                       dtr_pad_o;

   output                       int_o;

`ifdef UART_HAS_BAUDRATE_OUTPUT

   output                       baud_o;

`endif

`ifdef DATA_BUS_WIDTH_8

`else

   // if 32-bit databus and debug interface are enabled

   output [3:0]          ier;

   output [3:0]          iir;

   output [1:0]          fcr;  /// bits 7 and 6 of fcr. Other bits are ignored

   output [4:0]          mcr;

   output [7:0]          lcr;

   output [7:0]          msr;

   output [7:0]          lsr;

   output [`UART_FIFO_COUNTER_W-1:0] rf_count;

   output [`UART_FIFO_COUNTER_W-1:0] tf_count;

   output [2:0]               tstate;

   output [3:0]               rstate;

`endif

   wire [3:0]                         modem_inputs;

   reg                               enable;

`ifdef UART_HAS_BAUDRATE_OUTPUT

   assign baud_o = enable; // baud_o is actually the enable signal

`endif

   wire                              stx_pad_o;         // received from transmitter module

   wire                              srx_pad_i;

   wire                              srx_pad;

   reg [7:0]                          wb_dat_o;

   wire [`UART_ADDR_WIDTH-1:0]        wb_addr_i;

   wire [7:0]                         wb_dat_i;

   reg [3:0]                          ier;

   reg [3:0]                          iir;

   reg [1:0]                          fcr;  /// bits 7 and 6 of fcr. Other bits are ignored

   reg [4:0]                          mcr;

   reg [7:0]                          lcr;

   reg [7:0]                          msr;

   reg [15:0]                         dl;  // 32-bit divisor latch

   reg [7:0]                          scratch; // UART scratch register

   reg                               start_dlc; // activate dlc on writing to UART_DL1

   reg                               lsr_mask_d; // delay for lsr_mask condition

   reg                               msi_reset; // reset MSR 4 lower bits indicator

   //reg                                                                                threi_clear; // THRE interrupt clear flag

   reg [15:0]                         dlc;  // 32-bit divisor latch counter

   reg                               int_o;

   reg [3:0]                          trigger_level; // trigger level of the receiver FIFO

   reg                               rx_reset;

   reg                               tx_reset;

   wire                              dlab;                         // divisor latch access bit

   wire                              cts_pad_i, dsr_pad_i, ri_pad_i, dcd_pad_i; // modem status bits

   wire                              loopback;             // loopback bit (MCR bit 4)

   wire                              cts, dsr, ri, dcd;    // effective signals

   wire                              cts_c, dsr_c, ri_c, dcd_c; // Complement effective signals (considering loopback)

   wire                              rts_pad_o, dtr_pad_o;                 // modem control outputs

   // LSR bits wires and regs

   wire [7:0]                         lsr;

   wire                              lsr0, lsr1, lsr2, lsr3, lsr4, lsr5, lsr6, lsr7;

   reg                               lsr0r, lsr1r, lsr2r, lsr3r, lsr4r, lsr5r, lsr6r, lsr7r;

   wire                              lsr_mask; // lsr_mask

   //

   // ASSINGS

   //

   assign                                                                       lsr[7:0] = { lsr7r, lsr6r, lsr5r, lsr4r, lsr3r, lsr2r, lsr1r, lsr0r };

   assign                                                                       {cts_pad_i, dsr_pad_i, ri_pad_i, dcd_pad_i} = modem_inputs;

   assign                                                                       {cts, dsr, ri, dcd} = ~{cts_pad_i,dsr_pad_i,ri_pad_i,dcd_pad_i};

   assign                  {cts_c, dsr_c, ri_c, dcd_c} = loopback ? {mcr[`UART_MC_RTS],mcr[`UART_MC_DTR],mcr[`UART_MC_OUT1],mcr[`UART_MC_OUT2]}

   : {cts_pad_i,dsr_pad_i,ri_pad_i,dcd_pad_i};

   assign                                                                       dlab = lcr[`UART_LC_DL];

   assign                                                                       loopback = mcr[4];

   // assign modem outputs

   assign                                                                       rts_pad_o = mcr[`UART_MC_RTS];

   assign                                                                       dtr_pad_o = mcr[`UART_MC_DTR];

   // Interrupt signals

   wire                              rls_int;  // receiver line status interrupt

   wire                              rda_int;  // receiver data available interrupt

   wire                              ti_int;   // timeout indicator interrupt

   wire                              thre_int; // transmitter holding register empty interrupt

   wire                              ms_int;   // modem status interrupt

   // FIFO signals

   reg                               tf_push;

   reg                               rf_pop;

   wire [`UART_FIFO_REC_WIDTH-1:0]   rf_data_out;

   wire                              rf_error_bit; // an error (parity or framing) is inside the fifo

   wire                              rf_overrun;

   wire                              rf_push_pulse;

   wire [`UART_FIFO_COUNTER_W-1:0]   rf_count;

   wire [`UART_FIFO_COUNTER_W-1:0]   tf_count;

   wire [2:0]                         tstate;

   wire [3:0]                         rstate;

   wire [9:0]                         counter_t;

   wire                              thre_set_en; // THRE status is delayed one character time when a character is written to fifo.

   reg [7:0]                          block_cnt;   // While counter counts, THRE status is blocked (delayed one character cycle)

   reg [7:0]                          block_value; // One character length minus stop bit

   // Transmitter Instance

   wire                              serial_out;

   uart_transmitter transmitter(clk, wb_rst_i, lcr, tf_push, wb_dat_i, enable, serial_out, tstate, tf_count, tx_reset, lsr_mask);

   // Synchronizing and sampling serial RX input

   uart_sync_flops    i_uart_sync_flops

     (

      .rst_i           (wb_rst_i),

      .clk_i           (clk),

      .stage1_rst_i    (1'b0),

      .stage1_clk_en_i (1'b1),

      .async_dat_i     (srx_pad_i),

      .sync_dat_o      (srx_pad)

      );

   defparam i_uart_sync_flops.width      = 1;

   defparam i_uart_sync_flops.init_value = 1'b1;

   // handle loopback

   wire                              serial_in = loopback ? serial_out : srx_pad;

   assign stx_pad_o = loopback ? 1'b1 : serial_out;

   // Receiver Instance

   uart_receiver receiver(clk, wb_rst_i, lcr, rf_pop, serial_in, enable,

                          counter_t, rf_count, rf_data_out, rf_error_bit, rf_overrun, rx_reset, lsr_mask, rstate, rf_push_pulse);

   // Asynchronous reading here because the outputs are sampled in uart_wb.v file 

   always @(dl or dlab or ier or iir or scratch

            or lcr or lsr or msr or rf_data_out or wb_addr_i or wb_re_i)   // asynchrounous reading

     begin

        case (wb_addr_i)

          `UART_REG_RB   : wb_dat_o = dlab ? dl[`UART_DL1] : rf_data_out[10:3];

          `UART_REG_IE  : wb_dat_o = dlab ? dl[`UART_DL2] : {4'd0,ier};

          `UART_REG_II  : wb_dat_o = {4'b1100,iir};

          `UART_REG_LC  : wb_dat_o = lcr;

          `UART_REG_LS  : wb_dat_o = lsr;

          `UART_REG_MS  : wb_dat_o = msr;

          `UART_REG_SR  : wb_dat_o = scratch;

          default:  wb_dat_o = 8'b0; // ??

        endcase // case(wb_addr_i)

     end // always @ (dl or dlab or ier or iir or scratch...

   // rf_pop signal handling

   always @(posedge clk or posedge wb_rst_i)

     begin

        if (wb_rst_i)

          rf_pop <=  0;

        else

          if (rf_pop)   // restore the signal to 0 after one clock cycle

            rf_pop <=  0;

          else

            if (wb_re_i && wb_addr_i == `UART_REG_RB && !dlab)

              rf_pop <=  1; // advance read pointer

     end

   wire         lsr_mask_condition;

   wire         iir_read;

   wire         msr_read;

   wire         fifo_read;

   wire         fifo_write;

   assign lsr_mask_condition = (wb_re_i && wb_addr_i == `UART_REG_LS && !dlab);

   assign iir_read = (wb_re_i && wb_addr_i == `UART_REG_II && !dlab);

   assign msr_read = (wb_re_i && wb_addr_i == `UART_REG_MS && !dlab);

   assign fifo_read = (wb_re_i && wb_addr_i == `UART_REG_RB && !dlab);

   assign fifo_write = (wb_we_i && wb_addr_i == `UART_REG_TR && !dlab);

   // lsr_mask_d delayed signal handling

   always @(posedge clk or posedge wb_rst_i)

     begin

        if (wb_rst_i)

          lsr_mask_d <=  0;

        else // reset bits in the Line Status Register

          lsr_mask_d <=  lsr_mask_condition;

     end

   // lsr_mask is rise detected

   assign lsr_mask = lsr_mask_condition && ~lsr_mask_d;

   // msi_reset signal handling

   always @(posedge clk or posedge wb_rst_i)

     begin

        if (wb_rst_i)

          msi_reset <=  1;

        else

          if (msi_reset)

            msi_reset <=  0;

          else

            if (msr_read)

              msi_reset <=  1; // reset bits in Modem Status Register

     end

   //

   //   WRITES AND RESETS   //

   //

   // Line Control Register

   always @(posedge clk or posedge wb_rst_i)

     if (wb_rst_i)

       lcr <=  8'b00000011; // 8n1 setting

     else

       if (wb_we_i && wb_addr_i==`UART_REG_LC)

         lcr <=  wb_dat_i;

   // Interrupt Enable Register or UART_DL2

   always @(posedge clk or posedge wb_rst_i)

     if (wb_rst_i)

       begin

          ier <=  4'b0000; // no interrupts after reset

`ifdef PRESCALER_PRESET_HARD

          dl[`UART_DL2] <=  `PRESCALER_HIGH_PRESET;

`else

          dl[`UART_DL2] <=  8'b0;

`endif

       end

     else

       if (wb_we_i && wb_addr_i==`UART_REG_IE)

         if (dlab)

           begin

              dl[`UART_DL2] <=

`ifdef PRESCALER_PRESET_HARD

                dl[`UART_DL2];

`else

              wb_dat_i;

`endif

           end

         else

           ier <=  wb_dat_i[3:0]; // ier uses only 4 lsb

   // FIFO Control Register and rx_reset, tx_reset signals

   always @(posedge clk or posedge wb_rst_i)

     if (wb_rst_i) begin

        fcr <=  2'b11;

        rx_reset <=  0;

        tx_reset <=  0;

     end else

       if (wb_we_i && wb_addr_i==`UART_REG_FC) begin

          fcr <=  wb_dat_i[7:6];

          rx_reset <=  wb_dat_i[1];

          tx_reset <=  wb_dat_i[2];

       end else begin

          rx_reset <=  0;

          tx_reset <=  0;

       end

   // Modem Control Register

   always @(posedge clk or posedge wb_rst_i)

     if (wb_rst_i)

       mcr <=  5'b0;

     else

       if (wb_we_i && wb_addr_i==`UART_REG_MC)

         mcr <=  wb_dat_i[4:0];

   // Scratch register

   // Line Control Register

   always @(posedge clk or posedge wb_rst_i)

     if (wb_rst_i)

       scratch <=  0; // 8n1 setting

     else

       if (wb_we_i && wb_addr_i==`UART_REG_SR)

         scratch <=  wb_dat_i;

   // TX_FIFO or UART_DL1

   always @(posedge clk or posedge wb_rst_i)

     if (wb_rst_i)

       begin

`ifdef PRESCALER_PRESET_HARD

          dl[`UART_DL1]  <=  `PRESCALER_LOW_PRESET;

`else

          dl[`UART_DL1]  <=  8'b0;

`endif

          tf_push   <=  1'b0;

          start_dlc <=  1'b0;

       end

     else

       if (wb_we_i && wb_addr_i==`UART_REG_TR)

         if (dlab)

           begin

`ifdef PRESCALER_PRESET_HARD

              dl[`UART_DL1] <= dl[`UART_DL1];

`else

              dl[`UART_DL1] <=  wb_dat_i;

`endif

              start_dlc <=  1'b1; // enable DL counter

              tf_push <=  1'b0;

           end

         else

           begin

              tf_push   <=  1'b1;

              start_dlc <=  1'b0;

           end // else: !if(dlab)

       else

         begin

            start_dlc <=  1'b0;

            tf_push   <=  1'b0;

         end // else: !if(dlab)

   // Receiver FIFO trigger level selection logic (asynchronous mux)

   always @(fcr)

     case (fcr[`UART_FC_TL])

       2'b00 : trigger_level = 1;

       2'b01 : trigger_level = 4;

       2'b10 : trigger_level = 8;

       2'b11 : trigger_level = 14;

     endcase // case(fcr[`UART_FC_TL])

   //

   //  STATUS REGISTERS  //

   //

   // Modem Status Register

   reg [3:0] delayed_modem_signals;

   always @(posedge clk or posedge wb_rst_i)

     begin

        if (wb_rst_i)

          begin

             msr <=  0;

             delayed_modem_signals[3:0] <=  0;

          end

        else begin

           msr[`UART_MS_DDCD:`UART_MS_DCTS] <=  msi_reset ? 4'b0 :

                                                msr[`UART_MS_DDCD:`UART_MS_DCTS] | ({dcd, ri, dsr, cts} ^ delayed_modem_signals[3:0]);

           msr[`UART_MS_CDCD:`UART_MS_CCTS] <=  {dcd_c, ri_c, dsr_c, cts_c};

           delayed_modem_signals[3:0] <=  {dcd, ri, dsr, cts};

        end

     end

   // Line Status Register

   // activation conditions

   assign lsr0 = (rf_count==0 && rf_push_pulse);  // data in receiver fifo available set condition

   assign lsr1 = rf_overrun;     // Receiver overrun error

   assign lsr2 = rf_data_out[1]; // parity error bit

   assign lsr3 = rf_data_out[0]; // framing error bit

   assign lsr4 = rf_data_out[2]; // break error in the character

   assign lsr5 = (tf_count==5'b0 && thre_set_en);  // transmitter fifo is empty

   assign lsr6 = (tf_count==5'b0 && thre_set_en && (tstate == /*`S_IDLE */ 0)); // transmitter empty

   assign lsr7 = rf_error_bit | rf_overrun;

   // lsr bit0 (receiver data available)

   reg   lsr0_d;

   always @(posedge clk or posedge wb_rst_i)

     if (wb_rst_i) lsr0_d <=  0;

     else lsr0_d <=  lsr0;

   always @(posedge clk or posedge wb_rst_i)

     if (wb_rst_i) lsr0r <=  0;

     else lsr0r <=  (rf_count==1 && rf_pop && !rf_push_pulse || rx_reset) ? 0 : // deassert condition

                    lsr0r || (lsr0 && ~lsr0_d); // set on rise of lsr0 and keep asserted until deasserted 

   // lsr bit 1 (receiver overrun)

   reg   lsr1_d; // delayed

   always @(posedge clk or posedge wb_rst_i)

     if (wb_rst_i) lsr1_d <=  0;

     else lsr1_d <=  lsr1;

   always @(posedge clk or posedge wb_rst_i)

     if (wb_rst_i) lsr1r <=  0;

     else       lsr1r <=        lsr_mask ? 0 : lsr1r || (lsr1 && ~lsr1_d); // set on rise

   // lsr bit 2 (parity error)

   reg   lsr2_d; // delayed

   always @(posedge clk or posedge wb_rst_i)

     if (wb_rst_i) lsr2_d <=  0;

     else lsr2_d <=  lsr2;

   always @(posedge clk or posedge wb_rst_i)

     if (wb_rst_i) lsr2r <=  0;

     else lsr2r <=  lsr_mask ? 0 : lsr2r || (lsr2 && ~lsr2_d); // set on rise

   // lsr bit 3 (framing error)

   reg   lsr3_d; // delayed

   always @(posedge clk or posedge wb_rst_i)

     if (wb_rst_i) lsr3_d <=  0;

     else lsr3_d <=  lsr3;

   always @(posedge clk or posedge wb_rst_i)

     if (wb_rst_i) lsr3r <=  0;

     else lsr3r <=  lsr_mask ? 0 : lsr3r || (lsr3 && ~lsr3_d); // set on rise

   // lsr bit 4 (break indicator)