MMC/rtl/AHBsubordinate/AHBlite_UART.v

module AHBlite_UART(
    input  wire          HCLK,    
    input  wire          HRESETn, 
    input  wire          HSEL,    
    input  wire   [31:0] HADDR,   
    input  wire    [1:0] HTRANS,  
    input  wire    [2:0] HSIZE,   
    input  wire    [3:0] HPROT,   
    input  wire          HWRITE,  
    input  wire   [31:0] HWDATA,  
    input  wire          HREADY,  
    output wire          HREADYOUT, 
    output reg    [31:0] HRDATA,  
    output wire          HRESP,
    input  wire    [7:0] UART_RX,
    input  wire          state,
    output wire          tx_en,
    output wire    [7:0] UART_TX
);

assign HRESP = 1'b0;
assign HREADYOUT = 1'b1;

wire read_en;
assign read_en=HSEL&HTRANS[1]&(~HWRITE)&HREADY;

wire write_en;
assign write_en=HSEL&HTRANS[1]&(HWRITE)&HREADY;

reg [3:0] addr_reg;
always@(posedge HCLK or negedge HRESETn) begin
  if(~HRESETn) addr_reg <= 4'h0;
  else if(read_en || write_en) addr_reg <= HADDR[3:0];
end

reg rd_en_reg;
always@(posedge HCLK or negedge HRESETn) begin
  if(~HRESETn) rd_en_reg <= 1'b0;
  else if(read_en) rd_en_reg <= 1'b1;
  else rd_en_reg <= 1'b0;
end

reg wr_en_reg;
always@(posedge HCLK or negedge HRESETn) begin
  if(~HRESETn) wr_en_reg <= 1'b0;
  else if(write_en) wr_en_reg <= 1'b1;
  else  wr_en_reg <= 1'b0;
end

always@(*) begin
  if(rd_en_reg) begin
    if(addr_reg == 4'h0) HRDATA <= {24'b0,UART_RX};
    else if(addr_reg == 4'h4) HRDATA <= {31'b0,state};
    else HRDATA <= 32'b0;
  end else
    HRDATA <= 32'b0;
end

assign tx_en = wr_en_reg ? 1'b1 : 1'b0;
assign UART_TX = wr_en_reg ? HWDATA[7:0] : 8'b0;


endmodule