//------------------------------------------------------------------------------
// uart_tx.sv
// Konstantin Pavlov, pavlovconst@gmail.com
//------------------------------------------------------------------------------

// INFO ------------------------------------------------------------------------
//  Straightforward yet simple UART transmitter implementation
//    for FPGA written in Verilog
//
//  One stop bit setting is hardcoded
//  Features continuous data output at BAUD levels up to CLK_HZ / 2
//  Features early asynchronous 'busy' set and reset to gain time to prepare new data
//  If multiple UartTX instances should be inferred - make tx_sample_cntr logic
//    that is common for all TX instances for effective chip area usage


/* --- INSTANTIATION TEMPLATE BEGIN ---

uart_tx #(
  .CLK_HZ( 200_000_000 ),  // in Hertz
  .BAUD( 9600 )            // max. BAUD is CLK_HZ / 2
) tx1 (
  .clk(  ),
  .nrst( 1'b1 ),
  //.tx_do_sample(  ),
  .tx_data(  ),
  .tx_start(  ),
  .tx_busy(  ),
  .txd(  )
);

--- INSTANTIATION TEMPLATE END ---*/


module uart_tx #( parameter
  CLK_HZ = 200_000_000,
  BAUD = 9600,
  bit [15:0] BAUD_DIVISOR = CLK_HZ / BAUD
)(
  input clk,
  input nrst,
  //input tx_do_sample,

  input [7:0] tx_data,
  input tx_start,                 // write strobe
  output logic tx_busy = 1'b0,
  output logic txd = 1'b1
);

logic [9:0] tx_shifter = '0;
logic [15:0] tx_sample_cntr = '0;
always_ff @ (posedge clk) begin
  if( (~nrst) || (tx_sample_cntr[15:0] == '0) ) begin
    tx_sample_cntr[15:0] <= (BAUD_DIVISOR-1'b1);
  end else begin
    tx_sample_cntr[15:0] <= tx_sample_cntr[15:0] - 1'b1;
  end
end

logic tx_do_sample;
assign tx_do_sample = (tx_sample_cntr[15:0] == '0);


always_ff @ (posedge clk) begin
  if( ~nrst ) begin
    tx_busy <= 1'b0;
    tx_shifter[9:0] <= '0;
    txd <= 1'b1;
  end else begin
    if( ~tx_busy ) begin
      // asynchronous data load and 'busy' set
      if( tx_start ) begin
        tx_shifter[9:0] <= { 1'b1,tx_data[7:0],1'b0 };
        tx_busy <= 1'b1;
      end
    end else begin

      if( tx_do_sample ) begin    // next bit
        // txd MUST change only on tx_do_sample although data may be loaded earlier
        { tx_shifter[9:0],txd } <= { tx_shifter[9:0],txd } >> 1;
        // early asynchronous 'busy' reset
        if( ~|tx_shifter[9:1] ) begin
          // txd still holds data, but shifter is ready to get new info
          tx_busy <= 1'b0;
        end
      end // tx_do_sample

    end // ~tx_busy
  end // ~nrst
end

endmodule