source: XOpenSparcT1/trunk/OC-UART/uart_regs.v @ 6

//////////////////////////////////////////////////////////////////////
////                                                              ////
////  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):                                                  ////
////      - [email protected]                                  ////
////      - Jacob Gorban                                          ////
////      - Igor Mohor ([email protected])                      ////
////                                                              ////
////  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 [`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] : 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 <= #1 0; 
        else
        if (rf_pop)     // restore the signal to 0 after one clock cycle
                rf_pop <= #1 0;
        else
        if (wb_re_i && wb_addr_i == `UART_REG_RB && !dlab)
                rf_pop <= #1 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 <= #1 0;
        else // reset bits in the Line Status Register
                lsr_mask_d <= #1 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 1;
        else
        if (msi_reset)
                msi_reset <= #1 0;
        else
        if (msr_read)
                msi_reset <= #1 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 <= #1 8'b00000011; // 8n1 setting
        else
        if (wb_we_i && wb_addr_i==`UART_REG_LC)
                lcr <= #1 wb_dat_i;

// Interrupt Enable Register or UART_DL2
always @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i)
        begin
                ier <= #1 4'b0000; // no interrupts after reset
                dl[`UART_DL2] <= #1 8'b0;
        end
        else
        if (wb_we_i && wb_addr_i==`UART_REG_IE)
                if (dlab)
                begin
                        dl[`UART_DL2] <= #1 wb_dat_i;
                end
                else
                        ier <= #1 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 <= #1 2'b11; 
                rx_reset <= #1 0;
                tx_reset <= #1 0;
        end else
        if (wb_we_i && wb_addr_i==`UART_REG_FC) begin
                fcr <= #1 wb_dat_i[7:6];
                rx_reset <= #1 wb_dat_i[1];
                tx_reset <= #1 wb_dat_i[2];
        end else begin
                rx_reset <= #1 0;
                tx_reset <= #1 0;
        end

// Modem Control Register
always @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i)
                mcr <= #1 5'b0; 
        else
        if (wb_we_i && wb_addr_i==`UART_REG_MC)
                        mcr <= #1 wb_dat_i[4:0];

// Scratch register
// Line Control Register
always @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i)
                scratch <= #1 0; // 8n1 setting
        else
        if (wb_we_i && wb_addr_i==`UART_REG_SR)
                scratch <= #1 wb_dat_i;

// TX_FIFO or UART_DL1
always @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i)
        begin
                dl[`UART_DL1]  <= #1 8'b0;
                tf_push   <= #1 1'b0;
                start_dlc <= #1 1'b0;
        end
        else
        if (wb_we_i && wb_addr_i==`UART_REG_TR)
                if (dlab)
                begin
                        dl[`UART_DL1] <= #1 wb_dat_i;
                        start_dlc <= #1 1'b1; // enable DL counter
                        tf_push <= #1 1'b0;
                end
                else
                begin
                        tf_push   <= #1 1'b1;
                        start_dlc <= #1 1'b0;
                end // else: !if(dlab)
        else
        begin
                start_dlc <= #1 1'b0;
                tf_push   <= #1 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 <= #1 0;
                delayed_modem_signals[3:0] <= #1 0;
          end
        else begin
                msr[`UART_MS_DDCD:`UART_MS_DCTS] <= #1 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] <= #1 {dcd_c, ri_c, dsr_c, cts_c};
                delayed_modem_signals[3:0] <= #1 {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 <= #1 0;
        else lsr0_d <= #1 lsr0;

always @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) lsr0r <= #1 0;
        else lsr0r <= #1 (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 <= #1 0;
        else lsr1_d <= #1 lsr1;

always @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) lsr1r <= #1 0;
        else    lsr1r <= #1     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 <= #1 0;
        else lsr2_d <= #1 lsr2;

always @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) lsr2r <= #1 0;
        else lsr2r <= #1 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 <= #1 0;
        else lsr3_d <= #1 lsr3;

always @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) lsr3r <= #1 0;
        else lsr3r <= #1 lsr_mask ? 0 : lsr3r || (lsr3 && ~lsr3_d); // set on rise

// lsr bit 4 (break indicator)
reg lsr4_d; // delayed

always @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) lsr4_d <= #1 0;
        else lsr4_d <= #1 lsr4;

always @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) lsr4r <= #1 0;
        else lsr4r <= #1 lsr_mask ? 0 : lsr4r || (lsr4 && ~lsr4_d);

// lsr bit 5 (transmitter fifo is empty)
reg lsr5_d;

always @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) lsr5_d <= #1 1;
        else lsr5_d <= #1 lsr5;

always @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) lsr5r <= #1 1;
        else lsr5r <= #1 (fifo_write) ? 0 :  lsr5r || (lsr5 && ~lsr5_d);

// lsr bit 6 (transmitter empty indicator)
reg lsr6_d;

always @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) lsr6_d <= #1 1;
        else lsr6_d <= #1 lsr6;

always @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) lsr6r <= #1 1;
        else lsr6r <= #1 (fifo_write) ? 0 : lsr6r || (lsr6 && ~lsr6_d);

// lsr bit 7 (error in fifo)
reg lsr7_d;

always @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) lsr7_d <= #1 0;
        else lsr7_d <= #1 lsr7;

always @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) lsr7r <= #1 0;
        else lsr7r <= #1 lsr_mask ? 0 : lsr7r || (lsr7 && ~lsr7_d);

// Frequency divider
always @(posedge clk or posedge wb_rst_i) 
begin
        if (wb_rst_i)
                dlc <= #1 0;
        else
                if (start_dlc | ~ (|dlc))
                        dlc <= #1 dl - 1;               // preset counter
                else
                        dlc <= #1 dlc - 1;              // decrement counter
end

// Enable signal generation logic
always @(posedge clk or posedge wb_rst_i)
begin
        if (wb_rst_i)
                enable <= #1 1'b0;
        else
                if (|dl & ~(|dlc))     // dl>0 & dlc==0
                        enable <= #1 1'b1;
                else
                        enable <= #1 1'b0;
end

// Delaying THRE status for one character cycle after a character is written to an empty fifo.
always @(lcr)
  case (lcr[3:0])
    4'b0000                             : block_value =  95; // 6 bits
    4'b0100                             : block_value = 103; // 6.5 bits
    4'b0001, 4'b1000                    : block_value = 111; // 7 bits
    4'b1100                             : block_value = 119; // 7.5 bits
    4'b0010, 4'b0101, 4'b1001           : block_value = 127; // 8 bits
    4'b0011, 4'b0110, 4'b1010, 4'b1101  : block_value = 143; // 9 bits
    4'b0111, 4'b1011, 4'b1110           : block_value = 159; // 10 bits
    4'b1111                             : block_value = 175; // 11 bits
  endcase // case(lcr[3:0])

// Counting time of one character minus stop bit
always @(posedge clk or posedge wb_rst_i)
begin
  if (wb_rst_i)
    block_cnt <= #1 8'd0;
  else
  if(lsr5r & fifo_write)  // THRE bit set & write to fifo occured
    block_cnt <= #1 block_value;
  else
  if (enable & block_cnt != 8'b0)  // only work on enable times
    block_cnt <= #1 block_cnt - 1;  // decrement break counter
end // always of break condition detection

// Generating THRE status enable signal
assign thre_set_en = ~(|block_cnt);


//
//      INTERRUPT LOGIC
//

assign rls_int  = ier[`UART_IE_RLS] && (lsr[`UART_LS_OE] || lsr[`UART_LS_PE] || lsr[`UART_LS_FE] || lsr[`UART_LS_BI]);
assign rda_int  = ier[`UART_IE_RDA] && (rf_count >= {1'b0,trigger_level});
assign thre_int = ier[`UART_IE_THRE] && lsr[`UART_LS_TFE];
assign ms_int   = ier[`UART_IE_MS] && (| msr[3:0]);
assign ti_int   = ier[`UART_IE_RDA] && (counter_t == 10'b0) && (|rf_count);

reg      rls_int_d;
reg      thre_int_d;
reg      ms_int_d;
reg      ti_int_d;
reg      rda_int_d;

// delay lines
always  @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) rls_int_d <= #1 0;
        else rls_int_d <= #1 rls_int;

always  @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) rda_int_d <= #1 0;
        else rda_int_d <= #1 rda_int;

always  @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) thre_int_d <= #1 0;
        else thre_int_d <= #1 thre_int;

always  @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) ms_int_d <= #1 0;
        else ms_int_d <= #1 ms_int;

always  @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) ti_int_d <= #1 0;
        else ti_int_d <= #1 ti_int;

// rise detection signals

wire     rls_int_rise;
wire     thre_int_rise;
wire     ms_int_rise;
wire     ti_int_rise;
wire     rda_int_rise;

assign rda_int_rise    = rda_int & ~rda_int_d;
assign rls_int_rise       = rls_int & ~rls_int_d;
assign thre_int_rise   = thre_int & ~thre_int_d;
assign ms_int_rise        = ms_int & ~ms_int_d;
assign ti_int_rise        = ti_int & ~ti_int_d;

// interrupt pending flags
reg     rls_int_pnd;
reg     rda_int_pnd;
reg     thre_int_pnd;
reg     ms_int_pnd;
reg     ti_int_pnd;

// interrupt pending flags assignments
always  @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) rls_int_pnd <= #1 0; 
        else 
                rls_int_pnd <= #1 lsr_mask ? 0 :                                                // reset condition
                                                        rls_int_rise ? 1 :                                              // latch condition
                                                        rls_int_pnd && ier[`UART_IE_RLS];       // default operation: remove if masked

always  @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) rda_int_pnd <= #1 0; 
        else 
                rda_int_pnd <= #1 ((rf_count == {1'b0,trigger_level}) && fifo_read) ? 0 :       // reset condition
                                                        rda_int_rise ? 1 :                                              // latch condition
                                                        rda_int_pnd && ier[`UART_IE_RDA];       // default operation: remove if masked

always  @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) thre_int_pnd <= #1 0; 
        else 
                thre_int_pnd <= #1 fifo_write || (iir_read & ~iir[`UART_II_IP] & iir[`UART_II_II] == `UART_II_THRE)? 0 : 
                                                        thre_int_rise ? 1 :
                                                        thre_int_pnd && ier[`UART_IE_THRE];

always  @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) ms_int_pnd <= #1 0; 
        else 
                ms_int_pnd <= #1 msr_read ? 0 : 
                                                        ms_int_rise ? 1 :
                                                        ms_int_pnd && ier[`UART_IE_MS];

always  @(posedge clk or posedge wb_rst_i)
        if (wb_rst_i) ti_int_pnd <= #1 0; 
        else 
                ti_int_pnd <= #1 fifo_read ? 0 : 
                                                        ti_int_rise ? 1 :
                                                        ti_int_pnd && ier[`UART_IE_RDA];
// end of pending flags

// INT_O logic
always @(posedge clk or posedge wb_rst_i)
begin
        if (wb_rst_i)   
                int_o <= #1 1'b0;
        else
                int_o <= #1 
                                        rls_int_pnd             ?       ~lsr_mask                                       :
                                        rda_int_pnd             ? 1                                                             :
                                        ti_int_pnd              ? ~fifo_read                                    :
                                        thre_int_pnd    ? !(fifo_write & iir_read) :
                                        ms_int_pnd              ? ~msr_read                                             :
                                        0;      // if no interrupt are pending
end


// Interrupt Identification register
always @(posedge clk or posedge wb_rst_i)
begin
        if (wb_rst_i)
                iir <= #1 1;
        else
        if (rls_int_pnd)  // interrupt is pending
        begin
                iir[`UART_II_II] <= #1 `UART_II_RLS;    // set identification register to correct value
                iir[`UART_II_IP] <= #1 1'b0;            // and clear the IIR bit 0 (interrupt pending)
        end else // the sequence of conditions determines priority of interrupt identification
        if (rda_int)
        begin
                iir[`UART_II_II] <= #1 `UART_II_RDA;
                iir[`UART_II_IP] <= #1 1'b0;
        end
        else if (ti_int_pnd)
        begin
                iir[`UART_II_II] <= #1 `UART_II_TI;
                iir[`UART_II_IP] <= #1 1'b0;
        end
        else if (thre_int_pnd)
        begin
                iir[`UART_II_II] <= #1 `UART_II_THRE;
                iir[`UART_II_IP] <= #1 1'b0;
        end
        else if (ms_int_pnd)
        begin
                iir[`UART_II_II] <= #1 `UART_II_MS;
                iir[`UART_II_IP] <= #1 1'b0;
        end else        // no interrupt is pending
        begin
                iir[`UART_II_II] <= #1 0;
                iir[`UART_II_IP] <= #1 1'b1;
        end
end

endmodule