module exp (
   clk,
   reset,

   dds_out,
   
   expdin,
   expdout,
   expaddr,
   expread,
   expwrite
);

input		clk;
input		reset;

output [7:0]	dds_out;	     // DDS output

output [7:0]	expdin;		  // TO the PIC core.
input [7:0]	expdout;	     // FROM the PIC core.
input [6:0]	expaddr;	     // Address
input		expread;	 // Asserted (high) when PIC is reading FROM us.
input		expwrite;	// Asserted (high) when PIC is writing TO us.

reg [7:0]	expdin;
reg [7:0]	dds_out;

reg [7:0]	ddsstep;
reg [0:0]	ctl;

reg [9:0]	accum;
reg [7:0]	sinout;
reg [7:0]	sin_rom [0:1023];


initial begin
   $display ("Reading in SIN data for example DDS in EXP.V from sindata.hex");
   $readmemh ("sindata.hex", sin_rom);
end

always @(posedge clk) begin
   if (reset) begin
      accum <= 0;
   end
   else begin
      accum <= accum + ddsstep;
   end
end

always @(posedge clk) begin
   if (reset) begin
      sinout <= 0;
   end
   else begin
      sinout <= sin_rom[accum];
   end
end

always @(posedge clk) begin
   if (reset) begin
      dds_out <= 0;
   end
   else begin
      if (ctl[0]) begin
         dds_out <= sinout;
      end
      else begin
         dds_out <= 0;
      end
   end
end

// Drive the expdin bus back to the PIC.
always @(expread or expaddr) begin
   if (expread) begin
      case (expaddr)
         7'h7F:    expdin <= ddsstep;
         default:  expdin <= 0;
      endcase
   end
   else begin
      expdin <= 0;
   end
end

// 
always @(posedge clk) begin
   if (reset) begin
      ctl      <= 0;
      ddsstep  <= 0;
   end
   else begin
      if (expwrite) begin
         case (expaddr) 
            7'h7E: ctl     <= expdout[7:0];
            7'h7F: ddsstep <= expdout[7:0];
         endcase
      end
   end
end
endmodule