/*
    usb_uart_bridge_ep

    This is the endpoint to uart translator.  Two things to highlight:  the directions
    IN and OUT are set with respect to the HOST, and also in USB, the HOST runs all
    endpoint interactions.

    The out endpoint interface.  This is the out w.r.t. the host, hence in to
    us.  There are request grant, data available and data get signals, stall and
    acked signals.  And the data itself.

    The in endpoint interface.  This is the in w.r.t. the host, hence out to us.
    This interface also has a req and grant.  There's a put signal and a free
    signal.  Stall and acked.  And the data.

    To get data in and out there are two pipeline interfaces - one in and one out.

    OUT (or into this device)

    Roughly, the USB innards signal that a packet has arrived by raising out_ep_data_available.
    The data multiplexor has to be switched, so the interface is requested.  This is
    combinatorial logic so clever req and grant stuff can happen in the same line.

         assign out_ep_req = ( out_ep_req_reg || out_ep_data_avail );

    With the interface granted, the data is free to get.  Every cycle that the out_ep_data_get
    signal is high, the input address is advanced.  Inside the USB innards, when the 
    read address pointer equals the address of the write pointer (when all the data is 
    retreived, the out_ep_data_available flag is lowered and we withdraw our request for
    the interface and go back to idle.

    Interestingly, if you stop taking data... you lose your buffer.  So don't.

    IN (or out of this device back to the host)

    The IN EP works by providing a buffer and waiting for the local logic to fill it, 
    or to say that it's done.  When this happens the interface switches to a new state where
    it waits for a token from the host.  When it get the token, it sends the data.  When that
    is acknoledged, the buffer is released and returned ready to be filled again.

    in_ep_data_free signals that there's a buffer waiting.  And that signal goes low when 
    the buffer is full and not available.

    In the case where a buffer is not full - just sitting around with some data in it, a decision
    has to be made at some point just to send.  This is handled by a timeout mechanism, which 
    asserts in_ep_data_done and lets the buffer be sent.

    In the case where the buffer fills to the top, in_ep_data_free goes low by itself.

*/

module usb_uart_bridge_ep (
  input clk,
  input reset,

  ////////////////////
  // out endpoint interface
  ////////////////////
  output out_ep_req,               // request the data interface for the out endpoint
  input out_ep_grant,              // data interface granted
  input out_ep_data_avail,         // flagging data available to get from the host - stays up until the cycle upon which it is empty
  input out_ep_setup,              // [setup packet sent? - not used here]
  output out_ep_data_get,          // request to get the data
  input [7:0] out_ep_data,         // data from the host
  output out_ep_stall,             // an output enabling the device to stop inputs (not used)
  input out_ep_acked,              // indicating that the outgoing data was acked

  ////////////////////
  // in endpoint interface
  ////////////////////
  output in_ep_req,                // request the data interface for the in endpoint
  input in_ep_grant,               // data interface granted
  input in_ep_data_free,           // end point is ready for data - (specifically there is a buffer and it has space)
                                   // after going low it takes a while to get another back, but it does this automatically
  output in_ep_data_put,           // forces end point to read our data
  output [7:0] in_ep_data,         // data back to the host
  output in_ep_data_done,          // signalling that we're done sending data
  output in_ep_stall,              // an output enabling the device to stop outputs (not used)
  input in_ep_acked,               // indicating that the outgoing data was acked

  // uart pipeline in
  input [7:0] uart_in_data,
  input       uart_in_valid,
  output      uart_in_ready,

  // uart pipeline out
  output [7:0] uart_out_data,
  output       uart_out_valid,
  input        uart_out_ready,

  output [3:0] debug
);

  // Timeout counter width.
  localparam TimeoutWidth = 3;

  // We don't stall
  assign out_ep_stall = 1'b0;
  assign in_ep_stall = 1'b0;

  // Registers for the out pipeline (out of the module)
  reg [7:0] uart_out_data_reg;
  reg [7:0] uart_out_data_overflow_reg;
  reg       uart_out_valid_reg;

  // registers for the out end point (out of the host)
  reg       out_ep_req_reg;
  reg       out_ep_data_get_reg;

  // out pipeline / out endpoint state machine state (6 states -> 3 bits)
  reg [1:0] pipeline_out_state;

  localparam PipelineOutState_Idle         = 0;
  localparam PipelineOutState_WaitData     = 1;
  localparam PipelineOutState_PushData     = 2;
  localparam PipelineOutState_WaitPipeline = 3;

  // connect the pipeline registers to the outgoing ports
  assign uart_out_data = uart_out_data_reg;
  assign uart_out_valid = uart_out_valid_reg;

  // automatically make the bus request from the data_available
  // latch it with out_ep_req_reg
  assign out_ep_req = ( out_ep_req_reg || out_ep_data_avail );

  wire out_granted_data_available;

  assign out_granted_data_available = out_ep_req && out_ep_grant;

  assign out_ep_data_get = ( uart_out_ready || ~uart_out_valid_reg ) && out_ep_data_get_reg;

  reg [7:0] out_stall_data;
  reg       out_stall_valid;

  // do HOST OUT, DEVICE IN, PIPELINE OUT (!)
  always @(posedge clk) begin
      if ( reset ) begin
          pipeline_out_state <= PipelineOutState_Idle;
          uart_out_data_reg <= 0;
          uart_out_valid_reg <= 0;
          out_ep_req_reg <= 0;
          out_ep_data_get_reg <= 0;
          out_stall_data <= 0;
          out_stall_valid <= 0;
      end else begin
          case( pipeline_out_state )
              PipelineOutState_Idle: begin
                  // Waiting for the data_available signal indicating that a data packet has arrived
                  if ( out_granted_data_available ) begin
                      // indicate that we want the data
                      out_ep_data_get_reg <= 1;
                      // although the bus has been requested automatically, we latch the request so we control it
                      out_ep_req_reg <= 1;
                      // now wait for the data to set up
                      pipeline_out_state <= PipelineOutState_WaitData;
                      uart_out_valid_reg <= 0;
                      out_stall_data <= 0;
                      out_stall_valid <= 0;
                  end
              end
              PipelineOutState_WaitData: begin
                  // it takes one cycle for the juices to start flowing
                  // we got here when we were starting or if the outgoing pipe stalled
                  if ( uart_out_ready || ~uart_out_valid_reg ) begin
                      //if we were stalled, we can send the byte we caught while we were stalled
                      // the actual stalled byte now having been VALID & READY'ed
                      if ( out_stall_valid ) begin
                        uart_out_data_reg <= out_stall_data;
                        uart_out_valid_reg <= 1;
                        out_stall_data <= 0;
                        out_stall_valid <= 0;
                        if ( out_ep_data_avail )
                          pipeline_out_state <= PipelineOutState_PushData;
                        else begin
                          pipeline_out_state <= PipelineOutState_WaitPipeline;
                        end
                      end else begin
                          pipeline_out_state <= PipelineOutState_PushData;
                      end
                  end
              end
              PipelineOutState_PushData: begin
                  // can grab a character if either the out was accepted or the out reg is empty
                  if ( uart_out_ready || ~uart_out_valid_reg ) begin
                    // now we really have got some data and a place to shove it
                    uart_out_data_reg <= out_ep_data;
                    uart_out_valid_reg <= 1;
                    if ( ~out_ep_data_avail ) begin
                        // stop streaming, now just going to wait until the character is accepted
                        out_ep_data_get_reg <= 0;
                        pipeline_out_state <= PipelineOutState_WaitPipeline;
                    end
                  end else begin
                    // We're in push data so there is a character, but our pipeline has stalled
                    // need to save the character and wait.
                    out_stall_data <= out_ep_data;
                    out_stall_valid <= 1;
                    pipeline_out_state <= PipelineOutState_WaitData;                     
                    if ( ~out_ep_data_avail )
                        out_ep_data_get_reg <= 0;
                  end
              end
              PipelineOutState_WaitPipeline: begin
                  // unhand the bus (don't want to block potential incoming) - be careful, this works instantly!
                  out_ep_req_reg <= 0;
                  if ( uart_out_ready ) begin
                      uart_out_valid_reg <= 0;
                      uart_out_data_reg <= 0;
                      pipeline_out_state <= PipelineOutState_Idle;
                  end

              end

          endcase
      end
  end

  // in endpoint control registers
  reg       in_ep_req_reg;
  reg       in_ep_data_done_reg;

  // in pipeline / in endpoint state machine state (4 states -> 2 bits)
  reg [1:0] pipeline_in_state;

  localparam PipelineInState_Idle      = 0;
  localparam PipelineInState_WaitData  = 1;
  localparam PipelineInState_CycleData = 2;
  localparam PipelineInState_WaitEP    = 3;

  // connect the pipeline register to the outgoing port
  assign uart_in_ready = ( pipeline_in_state == PipelineInState_CycleData ) && in_ep_data_free;

  // uart_in_valid and a buffer being ready is the request for the bus. 
  // It is granted automatically if available, and latched on by the SM.
  // Note once requested, uart_in_valid may go on and off as data is available.
  // When requested, connect the end point registers to the outgoing ports
  assign in_ep_req = ( uart_in_valid && in_ep_data_free) || in_ep_req_reg;

  // Confirmation that the bus was granted
  wire in_granted_in_valid = in_ep_grant && uart_in_valid;

  // Here are the things we use to get data sent
  // ... put this word
  assign in_ep_data_put = ( pipeline_in_state == PipelineInState_CycleData ) && uart_in_valid && in_ep_data_free;
  // ... we're done putting - send the buffer
  assign in_ep_data_done = in_ep_data_done_reg;
  // ... the actual data, direct from the pipeline to the usb in buffer
  assign in_ep_data = uart_in_data;

  // If we have a half filled buffer, send it after a while by using a timer
  // 4 bits of counter, we'll just count up until bit 3 is high... 8 clock cycles seems more than enough to wait
  // to send the packet
  reg [TimeoutWidth:0] in_ep_timeout;

  // do PIPELINE IN, FPGA/Device OUT, Host IN
  always @(posedge clk) begin
      if ( reset ) begin
          pipeline_in_state <= PipelineInState_Idle;
          in_ep_req_reg <= 0;
          in_ep_data_done_reg <= 0;
      end else begin
          case( pipeline_in_state )
              PipelineInState_Idle: begin
                  in_ep_data_done_reg <= 0;
                  if ( in_granted_in_valid && in_ep_data_free ) begin
                      // got the bus, there is free space, now do the data
                      // confirm request bus - this will hold the request up until we're done with it
                      in_ep_req_reg <= 1;
                      pipeline_in_state <= PipelineInState_CycleData;
                  end
              end
              PipelineInState_CycleData: begin
                  if  (uart_in_valid ) begin
                      if ( ~in_ep_data_free ) begin
                        // back to idle
                        pipeline_in_state <= PipelineInState_Idle;
                        // release the bus
                        in_ep_req_reg <= 0;
                      end
                  end else begin
                      // No valid character.  Let's just pause for a second to see if any more are forthcoming.
                      // clear the timeout counter
                      in_ep_timeout <= 0;
                      pipeline_in_state <= PipelineInState_WaitData;
                 end
              end
              PipelineInState_WaitData: begin
                  in_ep_timeout <= in_ep_timeout + 1;
                  if ( uart_in_valid ) begin
                      pipeline_in_state <= PipelineInState_CycleData;                
                  end else begin
                        // check for a timeout
                        if ( in_ep_timeout[ TimeoutWidth ] ) begin
                            in_ep_data_done_reg <= 1;
                            pipeline_in_state <= PipelineInState_WaitEP;
                        end
                  end
              end

              PipelineInState_WaitEP: begin
                  // maybe done is ignored if putting?
                  in_ep_data_done_reg <= 0;

                  // back to idle
                  pipeline_in_state <= PipelineInState_Idle;
                  // release the bus
                  in_ep_req_reg <= 0;
              end
          endcase
      end
  end

  assign debug = { in_ep_data_free, in_ep_data_done, pipeline_in_state[ 1 ], pipeline_in_state[ 0 ] };  

endmodule