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

////                                                              ////

////  OMS 8051 cores UART Interface Module                        ////

////                                                              ////

////  This file is part of the OMS 8051 cores project             ////

////  http://www.opencores.org/cores/oms8051mini/                 ////

////                                                              ////

////  Description                                                 ////

////  OMS 8051 definitions.                                       ////

////                                                              ////

////  To Do:                                                      ////

////    nothing                                                   ////

////                                                              ////

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

////      - Dinesh Annayya, dinesha@opencores.org                 ////

////                                                              ////

////  Revision : Nov 26, 2016                                      //// 

////                                                              ////

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

////                                                              ////

//// Copyright (C) 2000 Authors and OPENCORES.ORG                 ////

////                                                              ////

//// 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                     ////

////                                                              ////

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

// UART rx state machine

module uart_rxfsm (

             reset_n        ,

             baud_clk_16x   ,

             cfg_rx_enable  ,

             cfg_stop_bit   ,

             cfg_pri_mod    ,

             error_ind      ,

       // FIFO control signal

             fifo_aval      ,

             fifo_wr        ,

             fifo_data      ,

          // Line Interface

             si

          );

input             reset_n        ; // active low reset signal

input             baud_clk_16x   ; // baud clock-16x

input             cfg_rx_enable  ; // transmit interface enable

input             cfg_stop_bit   ; // stop bit 

                                   // 0 --> 1 stop, 1 --> 2 Stop

input   [1:0]     cfg_pri_mod    ;// Priority Mode

                                   // 2'b00 --> None

                                   // 2'b10 --> Even priority

                                   // 2'b11 --> Odd priority

output [1:0]      error_ind     ; // 2'b00 --> Normal

                                  // 2'b01 --> framing error

                                  // 2'b10 --> parity error

                                  // 2'b11 --> fifo full

//--------------------------------------

//   FIFO control signal

//--------------------------------------

input             fifo_aval      ; // fifo empty

output            fifo_wr        ; // fifo write, assumed no back to back write

output  [7:0]     fifo_data      ; // fifo write data

// Line Interface

input             si             ;  // rxd pin

reg     [7:0]    fifo_data       ; // fifo write data

reg              fifo_wr         ; // fifo write 

reg    [1:0]     error_ind       ;

reg    [2:0]     cnt             ;

reg    [3:0]     offset          ; // free-running counter from 0 - 15

reg    [3:0]     rxpos           ; // stable rx position

reg    [2:0]     rxstate         ;

parameter idle_st      = 3'b000;

parameter xfr_start    = 3'b001;

parameter xfr_data_st  = 3'b010;

parameter xfr_pri_st   = 3'b011;

parameter xfr_stop_st1 = 3'b100;

parameter xfr_stop_st2 = 3'b101;

always @(negedge reset_n or posedge baud_clk_16x) begin

   if(reset_n == 0) begin

      rxstate   <= 3'b0;

      offset    <= 4'b0;

      rxpos     <= 4'b0;

      cnt       <= 3'b0;

      error_ind <= 2'b0;

      fifo_wr   <= 1'b0;

      fifo_data <= 8'h0;

   end

   else begin

      offset     <= offset + 1;

      case(rxstate)

       idle_st   : begin

            if(!si) begin // Start indication

               if(fifo_aval && cfg_rx_enable) begin

                 rxstate   <=   xfr_start;

                 cnt       <=   0;

                 rxpos     <=   offset + 8; // Assign center rxoffset

                 error_ind <= 2'b00;

               end

               else begin

                  error_ind <= 2'b11; // fifo full error indication

               end

            end else begin

               error_ind <= 2'b00; // Reset Error

            end

         end

      xfr_start : begin

            // Make Sure that minimum 8 cycle low is detected

            if(cnt < 7 && si) begin // Start indication

               rxstate <=   idle_st;

            end

            else if(cnt == 7 && !si) begin // Start indication

                rxstate <=   xfr_data_st;

                cnt     <=   0;

            end else begin

              cnt  <= cnt +1;

            end

         end

      xfr_data_st : begin

             if(rxpos == offset) begin

                fifo_data[cnt] <= si;

                cnt            <= cnt+1;

                if(cnt == 7) begin

                   fifo_wr <= 1;

                   if(cfg_pri_mod == 2'b00)  // No Priority

                       rxstate <=   xfr_stop_st1;

                   else rxstate <= xfr_pri_st;

                end

             end

          end

       xfr_pri_st   : begin

            fifo_wr <= 0;

            if(rxpos == offset) begin

               if(cfg_pri_mod == 2'b10)  // even priority

                  if( si != ^fifo_data) error_ind <= 2'b10;

               else  // Odd Priority

                  if( si != ~(^fifo_data)) error_ind <= 2'b10;

               rxstate <=   xfr_stop_st1;

            end

         end

       xfr_stop_st1  : begin

          fifo_wr <= 0;

          if(rxpos == offset) begin

             if(si) begin

               if(cfg_stop_bit) // Two Stop bit

                  rxstate <=   xfr_stop_st2;

               else

                  rxstate <=   idle_st;

             end else begin // Framing error

                error_ind <= 2'b01;

                rxstate   <=   idle_st;

             end

          end

       end

       xfr_stop_st2  : begin

          if(rxpos == offset) begin

             if(si) begin

                rxstate <=   idle_st;

             end else begin // Framing error

                error_ind <= 2'b01;

                rxstate   <=   idle_st;

             end

          end

       end

    endcase

   end

end

endmodule