verilog/rtl/uart_tx.v

//////////////////////////////////////////////////////////////////////
// File Downloaded from http://www.nandland.com
//////////////////////////////////////////////////////////////////////
// This file contains the UART Transmitter.  This transmitter is able
// to transmit 8 bits of serial data, one start bit, one stop bit,
// and no parity bit.  When transmit is complete o_Tx_done will be
// driven high for one clock cycle.
//
// Set Parameter CLKS_PER_BIT as follows:
// CLKS_PER_BIT = (Frequency of i_Clock)/(Frequency of UART)
// Example: 10 MHz Clock, 115200 baud UART
// (10000000)/(115200) = 87

module uart_tx
  #(parameter CLKS_PER_BIT = 347)
  (
   input       i_Clock,
   input       i_Tx_DV,
   input [7:0] i_Tx_Byte,
   output      o_Tx_Active,
   output reg  o_Tx_Serial,
   output      o_Tx_Done
   );

  parameter s_IDLE         = 3'b000;
  parameter s_TX_START_BIT = 3'b001;
  parameter s_TX_DATA_BITS = 3'b010;
  parameter s_TX_STOP_BIT  = 3'b011;
  parameter s_CLEANUP      = 3'b100;

  reg [2:0]    r_SM_Main     = 0;
  reg [15:0]   r_Clock_Count = 0;
  reg [2:0]    r_Bit_Index   = 0;
  reg [7:0]    r_Tx_Data     = 0;
  reg          r_Tx_Done     = 0;
  reg          r_Tx_Active   = 0;

  always @(posedge i_Clock)
    begin

      case (r_SM_Main)
        s_IDLE :
          begin
            o_Tx_Serial   <= 1'b1;         // Drive Line High for Idle
            r_Tx_Done     <= 1'b0;
            r_Clock_Count <= 0;
            r_Bit_Index   <= 0;

            if (i_Tx_DV == 1'b1)
              begin
                r_Tx_Active <= 1'b1;
                r_Tx_Data   <= i_Tx_Byte;
                r_SM_Main   <= s_TX_START_BIT;
              end
            else
              r_SM_Main <= s_IDLE;
          end // case: s_IDLE


        // Send out Start Bit. Start bit = 0
        s_TX_START_BIT :
          begin
            o_Tx_Serial <= 1'b0;

            // Wait CLKS_PER_BIT-1 clock cycles for start bit to finish
            if (r_Clock_Count < CLKS_PER_BIT-1)
              begin
                r_Clock_Count <= r_Clock_Count + 1;
                r_SM_Main     <= s_TX_START_BIT;
              end
            else
              begin
                r_Clock_Count <= 0;
                r_SM_Main     <= s_TX_DATA_BITS;
              end
          end // case: s_TX_START_BIT


        // Wait CLKS_PER_BIT-1 clock cycles for data bits to finish
        s_TX_DATA_BITS :
          begin
            o_Tx_Serial <= r_Tx_Data[r_Bit_Index];

            if (r_Clock_Count < CLKS_PER_BIT-1)
              begin
                r_Clock_Count <= r_Clock_Count + 1;
                r_SM_Main     <= s_TX_DATA_BITS;
              end
            else
              begin
                r_Clock_Count <= 0;

                // Check if we have sent out all bits
                if (r_Bit_Index < 7)
                  begin
                    r_Bit_Index <= r_Bit_Index + 1;
                    r_SM_Main   <= s_TX_DATA_BITS;
                  end
                else
                  begin
                    r_Bit_Index <= 0;
                    r_SM_Main   <= s_TX_STOP_BIT;
                  end
              end
          end // case: s_TX_DATA_BITS


        // Send out Stop bit.  Stop bit = 1
        s_TX_STOP_BIT :
          begin
            o_Tx_Serial <= 1'b1;

            // Wait CLKS_PER_BIT-1 clock cycles for Stop bit to finish
            if (r_Clock_Count < CLKS_PER_BIT-1)
              begin
                r_Clock_Count <= r_Clock_Count + 1;
                r_SM_Main     <= s_TX_STOP_BIT;
              end
            else
              begin
                r_Tx_Done     <= 1'b1;
                r_Clock_Count <= 0;
                r_SM_Main     <= s_CLEANUP;
                r_Tx_Active   <= 1'b0;
              end
          end // case: s_Tx_STOP_BIT


        // Stay here 1 clock
        s_CLEANUP :
          begin
            r_Tx_Done <= 1'b1;
            r_SM_Main <= s_IDLE;
          end


        default :
          r_SM_Main <= s_IDLE;

      endcase
    end

  assign o_Tx_Active = r_Tx_Active;
  assign o_Tx_Done   = r_Tx_Done;

endmodule