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`timescale 1ns/100ps

module axi_ad7616_pif (

  // physical interface

  cs_n,
  db_o,
  db_i,
  db_t,
  rd_n,
  wr_n,

  // axi stream master

  m_axis_tdata,
  m_axis_tvalid,
  m_axis_tready,
  m_axis_xfer_req,

  // end of convertion

  end_of_conv,

  // register access

  clk,
  rstn,
  rd_req,
  wr_req,
  wr_data,
  rd_data,
  rd_valid
);

  parameter UP_ADDRESS_WIDTH = 14;

  // IO definitions

  output                          cs_n;
  output  [15:0]                  db_o;
  input   [15:0]                  db_i;
  output                          db_t;
  output                          rd_n;
  output                          wr_n;

  input                           end_of_conv;

  input                           clk;
  input                           rstn;
  input                           rd_req;
  input                           wr_req;
  input   [15:0]                  wr_data;
  output  [15:0]                  rd_data;
  output                          rd_valid;

  output  [31:0]                  m_axis_tdata;
  input                           m_axis_tready;
  output                          m_axis_tvalid;
  input                           m_axis_xfer_req;

  // state registers

  localparam  [ 2:0]    IDLE = 3'h0,
                        CS_LOW = 3'h1,
                        CNTRL0_LOW = 3'h2,
                        CNTRL0_HIGH = 3'h3,
                        CNTRL1_LOW = 3'h4,
                        CNTRL1_HIGH = 3'h5,
                        CS_HIGH = 3'h6;

  // internal registers

  reg     [ 2:0]                  transfer_state = 3'h0;
  reg     [ 2:0]                  transfer_state_next = 3'h0;
  reg     [ 1:0]                  width_counter = 2'h0;

  reg                             wr_req_d = 1'h0;
  reg                             rd_req_d = 1'h0;
  reg                             rd_conv_d = 1'h0;

  reg                             xfer_req_d = 1'h0;

  reg     [15:0]                  data_out_a = 16'h0;
  reg     [15:0]                  data_out_b = 16'h0;
  reg                             rd_db_valid_div2 = 1'h0;
  reg                             rd_valid = 1'h0;

  // internal wires

  wire                            start_transfer;
  wire                            rd_db_valid;

  // FSM state register

  always @(posedge clk) begin
    if (rstn == 1'b0) begin
      transfer_state <= 3'h0;
    end else begin
      transfer_state <= transfer_state_next;
    end
  end

  // counters to control the RD_N and WR_N lines

  assign start_transfer = end_of_conv | rd_req | wr_req;

  always @(posedge clk) begin
    if (rstn == 1'b0) begin
      width_counter <= 2'h0;
    end else begin
      if((transfer_state == CNTRL0_LOW) || (transfer_state == CNTRL0_HIGH) ||
         (transfer_state == CNTRL1_LOW) || (transfer_state == CNTRL1_HIGH))
        width_counter <= width_counter + 1;
      else
        width_counter <= 2'h0;
    end
  end

  always @(negedge clk) begin
    if (transfer_state == IDLE) begin
      wr_req_d <= wr_req;
      rd_req_d <= rd_req;
      rd_conv_d <= end_of_conv;
    end
  end

  // FSM next state logic

  always @(*) begin
    case (transfer_state)
      IDLE : begin
        transfer_state_next <= (start_transfer == 1'b1) ? CS_LOW : IDLE;
      end
      CS_LOW : begin
        transfer_state_next <= CNTRL0_LOW;
      end
      CNTRL0_LOW : begin
        transfer_state_next <= (width_counter != 2'b11) ? CNTRL0_LOW : CNTRL0_HIGH;
      end
      CNTRL0_HIGH : begin
        transfer_state_next <= (width_counter != 2'b11) ? CNTRL0_HIGH :
                               ((wr_req_d == 1'b1) || (rd_req_d == 1'b1)) ? CS_HIGH : CNTRL1_LOW;
      end
      CNTRL1_LOW : begin
        transfer_state_next <= (width_counter != 2'b11) ? CNTRL1_LOW : CNTRL1_HIGH;
      end
      CNTRL1_HIGH : begin
        transfer_state_next <= (width_counter != 2'b11) ? CNTRL1_HIGH : CS_HIGH;
      end
      CS_HIGH : begin
        transfer_state_next <= IDLE;
      end
      default : begin
        transfer_state_next <= IDLE;
      end
    endcase
  end

  // data valid for the register access and m_axis interface

  assign rd_db_valid = ((transfer_state == CS_HIGH) &&
                       ((rd_req_d == 1'b1) || (rd_conv_d == 1'b1))) ? 1'b1 : 1'b0;

  always @(posedge clk) begin
    if (cs_n) begin
      rd_db_valid_div2 <= 1'h0;
    end else begin
      rd_db_valid_div2 <= (rd_db_valid) ? ~rd_db_valid_div2 : rd_db_valid_div2;
    end
  end

  // FSM output logic

  assign db_o = wr_data;

  always @(posedge clk) begin
    data_out_a <= (rd_db_valid) ? db_i : data_out_a;
    data_out_b <= (rd_db_valid) ? data_out_a : data_out_b;
    rd_valid <= rd_db_valid;
  end

  assign rd_data = data_out_a;

  assign cs_n = (transfer_state == IDLE) ? 1'b1 : 1'b0;
  assign db_t = ~wr_req_d;
  assign rd_n = (((transfer_state == CNTRL0_LOW) && ((rd_conv_d == 1'b1) || rd_req_d == 1'b1)) ||
                  (transfer_state == CNTRL1_LOW)) ? 1'b0 : 1'b1;
  assign wr_n = ((transfer_state == CNTRL0_LOW) && (wr_req_d == 1'b1)) ? 1'b0 : 1'b1;

  // Master AXI stream output logic with additional xfer_req signal
  // The first valid data is ALWAYS the first sample of a convertion

  always @(negedge clk) begin
    if (end_of_conv == 1'b1)
      xfer_req_d <= m_axis_xfer_req;
  end

  assign m_axis_tdata = rd_data;
  assign m_axis_tvalid = xfer_req_d & rd_db_valid & rd_db_valid_div2;

endmodule