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

// This sourcecode is released under BSD license.

// Please see http://www.opensource.org/licenses/bsd-license.php for details!

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

//

// Copyright (c) 2010, Stefan Fischer <Ste.Fis@OpenCores.org>

// All rights reserved.

//

// Redistribution and use in source and binary forms, with or without 

// modification, are permitted provided that the following conditions are met:

//

//  * Redistributions of source code must retain the above copyright notice, 

//    this list of conditions and the following disclaimer.

//  * Redistributions in binary form must reproduce the above copyright notice,

//    this list of conditions and the following disclaimer in the documentation

//    and/or other materials provided with the distribution. 

//  * Neither the name of the author nor the names of his contributors may be 

//    used to endorse or promote products derived from this software without 

//    specific prior written permission.

//

// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" 

// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 

// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 

// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE 

// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 

// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 

// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 

// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 

// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 

// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 

// POSSIBILITY OF SUCH DAMAGE.

//

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

// filename: wbs_uart.v

// description: synthesizable wishbone slave uart sio module using Xilinx (R)

//              macros and adding some functionality like a configurable 

//              baud rate and buffer level checking 

// todo4user: add other uart functionality as needed, i. e. interrupt logic or

//            modem control signals

// version: 0.0.0

// changelog: - 0.0.0, initial release

//            - ...

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

module wbs_uart (

  rst,

  clk,

  wbs_cyc_i,

  wbs_stb_i,

  wbs_we_i,

  wbs_adr_i,

  wbs_dat_m2s_i,

  wbs_dat_s2m_o,

  wbs_ack_o,

  uart_rx_si_i,

  uart_tx_so_o

);

  input rst;

  wire  rst;

  input clk;

  wire  clk;

  input wbs_cyc_i;

  wire  wbs_cyc_i;

  input wbs_stb_i;

  wire  wbs_stb_i;

  input wbs_we_i;

  wire  wbs_we_i;

  input[7:0] wbs_adr_i;

  wire [7:0] wbs_adr_i;

  input[7:0] wbs_dat_m2s_i;

  wire [7:0] wbs_dat_m2s_i;

  output[7:0] wbs_dat_s2m_o;

  reg   [7:0] wbs_dat_s2m_o;

  output wbs_ack_o;

  reg    wbs_ack_o;

  input uart_rx_si_i;

  wire  uart_rx_si_i;

  output uart_tx_so_o;

  wire   uart_tx_so_o;

  wire wb_reg_we;

  parameter ADDR_MSB = 1;

  parameter[7:0] UART_RXTX_ADDR = 8'h00;

  parameter[7:0] UART_SR_ADDR = 8'h01;

  parameter UART_SR_RX_F_FLAG = 0;

  parameter UART_SR_RX_HF_FLAG = 1;

  parameter UART_SR_RX_DP_FLAG = 2;

  parameter UART_SR_TX_F_FLAG = 4;

  parameter UART_SR_TX_HF_FLAG = 5;

  parameter[7:0] UART_BAUD_LO_ADDR = 8'h02;

  parameter[7:0] UART_BAUD_HI_ADDR = 8'h03;

  reg[15:0] baud_count;

  reg[15:0] baud_limit;

  reg en_16_x_baud;

  reg rx_read_buffer;

  wire rx_buffer_full;

  wire rx_buffer_half_full;

  wire rx_buffer_data_present;

  wire[7:0] rx_data_out;

  reg tx_write_buffer;

  wire tx_buffer_full;

  wire tx_buffer_half_full;

  // internal register write enable signal

  assign wb_reg_we = wbs_cyc_i && wbs_stb_i && wbs_we_i;

  always@(posedge clk) begin

    // baud rate configuration:

    // baud_limit = round( system clock frequency / (16 * baud rate) ) - 1

    // i. e. 9600 baud at 50 MHz system clock =>

    // baud_limit = round( 50.0E6 / (16 * 9600) ) - 1 = 325 = 0x0145

    // baud timer

    if (baud_count == baud_limit) begin

      baud_count <= 16'h0000;

      en_16_x_baud <= 1'b1;

    end else begin

      baud_count <= baud_count + 1;

      en_16_x_baud <= 1'b0;

    end

    rx_read_buffer <= 1'b0;

    tx_write_buffer <= 1'b0;

    wbs_dat_s2m_o <= 8'h00;

    // registered wishbone slave handshake (default)

    wbs_ack_o <= wbs_cyc_i && wbs_stb_i && (! wbs_ack_o);

    case(wbs_adr_i[ADDR_MSB:0])

      // receive/transmit buffer access

      UART_RXTX_ADDR[ADDR_MSB:0]: begin

        if (wbs_cyc_i && wbs_stb_i)

          // overwriting wishbone slave handshake for blocking transactions 

          // to rx/tx fifos by using buffer status flags

          if (wbs_we_i) begin

            tx_write_buffer <= (! tx_buffer_full) && (! wbs_ack_o);

            wbs_ack_o <= (! tx_buffer_full) && (! wbs_ack_o);

          end else begin

            rx_read_buffer <= rx_buffer_data_present && (! wbs_ack_o);

            wbs_ack_o <= rx_buffer_data_present && (! wbs_ack_o);

          end

        wbs_dat_s2m_o <= rx_data_out;

      end

      // status register access

      UART_SR_ADDR[ADDR_MSB:0]: begin

        wbs_dat_s2m_o[UART_SR_RX_F_FLAG] <= rx_buffer_full;

        wbs_dat_s2m_o[UART_SR_RX_HF_FLAG] <= rx_buffer_half_full;

        wbs_dat_s2m_o[UART_SR_RX_DP_FLAG] <= rx_buffer_data_present;

        wbs_dat_s2m_o[UART_SR_TX_F_FLAG] <= tx_buffer_full;

        wbs_dat_s2m_o[UART_SR_TX_HF_FLAG] <= tx_buffer_half_full;

      end

      // baud rate register access / low byte

      UART_BAUD_LO_ADDR[ADDR_MSB:0]: begin

        if (wb_reg_we) begin

          baud_limit[7:0] <= wbs_dat_m2s_i;

          baud_count <= 16'h0000;

        end

        wbs_dat_s2m_o <= baud_limit[7:0];

      end

      // baud rate register access / high byte

      UART_BAUD_HI_ADDR[ADDR_MSB:0]: begin

        if (wb_reg_we)  begin

          baud_limit[15:8] <= wbs_dat_m2s_i;

          baud_count <= 16'h0000;

        end

        wbs_dat_s2m_o <= baud_limit[15:8];

      end

      default: ;

    endcase

    if (rst)

      wbs_ack_o <= 1'b0;

  end

  // Xilinx (R) uart macro instances

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

  uart_rx inst_uart_rx (

    .serial_in(uart_rx_si_i),

    .data_out(rx_data_out),

    .read_buffer(rx_read_buffer),

    .reset_buffer(rst),

    .en_16_x_baud(en_16_x_baud),

    .buffer_data_present(rx_buffer_data_present),

    .buffer_full(rx_buffer_full),

    .buffer_half_full(rx_buffer_half_full),

    .clk(clk)

  );

  uart_tx inst_uart_tx (

    .data_in(wbs_dat_m2s_i),

    .write_buffer(tx_write_buffer),

    .reset_buffer(rst),

    .en_16_x_baud(en_16_x_baud),

    .serial_out(uart_tx_so_o),

    .buffer_full(tx_buffer_full),

    .buffer_half_full(tx_buffer_half_full),

    .clk(clk)

  );

endmodule