OpenFPGA/yosys/tests/asicworld/code_hdl_models_uart.v

//-----------------------------------------------------
// Design Name : uart 
// File Name   : uart.v
// Function    : Simple UART
// Coder       : Deepak Kumar Tala
//-----------------------------------------------------
module uart (
reset          ,
txclk          ,
ld_tx_data     ,
tx_data        ,
tx_enable      ,
tx_out         ,
tx_empty       ,
rxclk          ,
uld_rx_data    ,
rx_data        ,
rx_enable      ,
rx_in          ,
rx_empty
);
// Port declarations
input        reset          ;
input        txclk          ;
input        ld_tx_data     ;
input  [7:0] tx_data        ;
input        tx_enable      ;
output       tx_out         ;
output       tx_empty       ;
input        rxclk          ;
input        uld_rx_data    ;
output [7:0] rx_data        ;
input        rx_enable      ;
input        rx_in          ;
output       rx_empty       ;

// Internal Variables 
reg [7:0]    tx_reg         ;
reg          tx_empty       ;
reg          tx_over_run    ;
reg [3:0]    tx_cnt         ;
reg          tx_out         ;
reg [7:0]    rx_reg         ;
reg [7:0]    rx_data        ;
reg [3:0]    rx_sample_cnt  ;
reg [3:0]    rx_cnt         ;  
reg          rx_frame_err   ;
reg          rx_over_run    ;
reg          rx_empty       ;
reg          rx_d1          ;
reg          rx_d2          ;
reg          rx_busy        ;

// UART RX Logic
always @ (posedge rxclk or posedge reset)
if (reset) begin
  rx_reg        <= 0; 
  rx_data       <= 0;
  rx_sample_cnt <= 0;
  rx_cnt        <= 0;
  rx_frame_err  <= 0;
  rx_over_run   <= 0;
  rx_empty      <= 1;
  rx_d1         <= 1;
  rx_d2         <= 1;
  rx_busy       <= 0;
end else begin
  // Synchronize the asynch signal
  rx_d1 <= rx_in;
  rx_d2 <= rx_d1;
  // Uload the rx data
  if (uld_rx_data) begin
    rx_data  <= rx_reg;
    rx_empty <= 1;
  end
  // Receive data only when rx is enabled
  if (rx_enable) begin
    // Check if just received start of frame
    if (!rx_busy && !rx_d2) begin
      rx_busy       <= 1;
      rx_sample_cnt <= 1;
      rx_cnt        <= 0;
    end
    // Start of frame detected, Proceed with rest of data
    if (rx_busy) begin
       rx_sample_cnt <= rx_sample_cnt + 1;
       // Logic to sample at middle of data
       if (rx_sample_cnt == 7) begin
          if ((rx_d2 == 1) && (rx_cnt == 0)) begin
            rx_busy <= 0;
          end else begin
            rx_cnt <= rx_cnt + 1; 
            // Start storing the rx data
            if (rx_cnt > 0 && rx_cnt < 9) begin
              rx_reg[rx_cnt - 1] <= rx_d2;
            end
            if (rx_cnt == 9) begin
               rx_busy <= 0;
               // Check if End of frame received correctly
               if (rx_d2 == 0) begin
                 rx_frame_err <= 1;
               end else begin
                 rx_empty     <= 0;
                 rx_frame_err <= 0;
                 // Check if last rx data was not unloaded,
                 rx_over_run  <= (rx_empty) ? 0 : 1;
               end
            end
          end
       end 
    end 
  end
  if (!rx_enable) begin
    rx_busy <= 0;
  end
end

// UART TX Logic
always @ (posedge txclk or posedge reset)
if (reset) begin
  tx_reg        <= 0;
  tx_empty      <= 1;
  tx_over_run   <= 0;
  tx_out        <= 1;
  tx_cnt        <= 0;
end else begin
   if (ld_tx_data) begin
      if (!tx_empty) begin
        tx_over_run <= 0;
      end else begin
        tx_reg   <= tx_data;
        tx_empty <= 0;
      end
   end
   if (tx_enable && !tx_empty) begin
     tx_cnt <= tx_cnt + 1;
     if (tx_cnt == 0) begin
       tx_out <= 0;
     end
     if (tx_cnt > 0 && tx_cnt < 9) begin
        tx_out <= tx_reg[tx_cnt -1];
     end
     if (tx_cnt == 9) begin
       tx_out <= 1;
       tx_cnt <= 0;
       tx_empty <= 1;
     end
   end
   if (!tx_enable) begin
     tx_cnt <= 0;
   end
end

endmodule