// IEEE 1149.1 - digital
// IEEE 1149.4 - analog / mixed-signal
// IEEE 1149.5 - system level
// IEEE 1149.6 - ac-coupled, differential nets
// IEEE 1532   - boundary scan (superset of IEEE 1149.1)

// References:
// http://www.interfacebus.com/Design_Connector_JTAG_Bus.html

// Designing to IEEE 1149.1-2013

module jtag_tap(
    input wire jtag_tck,
    input wire jtag_tms,  // tie high to disable module
    input wire jtag_tdi,
    output reg jtag_tdo,
    input wire jtag_trst, // [optional] async active low reset
);
// NOTE: do not connect jtag_trst to any system logic (only tie to test logic)
// For instance a reset input to the IC must not change the JTAG TAP state

// TODO: TMP controller (IEEE 1149.1-2013 6.2.1)

// State values according to IEEE 1149.1-2013
localparam  STATE_EXIT_2_DR         = 4'h0,
            STATE_EXIT_1_DR         = 4'h1,
            STATE_SHIFT_DR          = 4'h2,
            STATE_PAUSE_DR          = 4'h3,
            STATE_SELECT_IR_SCAN    = 4'h4,
            STATE_UPDATE_DR         = 4'h5,
            STATE_CAPTURE_DR        = 4'h6,
            STATE_SELECT_DR_SCAN    = 4'h7,
            STATE_EXIT_2_IR         = 4'h8,
            STATE_EXIT_1_IR         = 4'h9,
            STATE_SHIFT_IR          = 4'hA,
            STATE_PAUSE_IR          = 4'hB,
            STATE_RUN_TEST_IDLE     = 4'hC,
            STATE_UPDATE_IR         = 4'hD,
            STATE_CAPTURE_IR        = 4'hD,
            STATE_TEST_LOGIC_RESET  = 4'hF;

localparam  INSTR_BYPASS            = 8'hFF, // required always // may also be other codes: recommend 0
            INSTR_SAMPLE            = 8'h, // required always
            INSTR_PRELOAD           = 8'h, // required always
            INSTR_EXTEST            = 8'h, // required always
            INSTR_IDCODE            = 8'h, // required if IDCODE register is included
            INSTR_USERCODE          = 8'h, // required if IDCODE register is included in a user-programmable component that does not allow programming via this test logic
            // INSTR_CLAMP             = 8'hxx,
            // INSTR_HIGHZ             = 8'hxx,
            INSTR_IC_RESET          = 8'h,   // required if reset selection register is included
            // INSTR_CLAMP_HOLD        = 8'hxx, // required if TMP controller & TMP status register are included
            // INSTR_CLAMP_RELEASE     = 8'hxx, // required if TMP controller & TMP status register are included
            // INSTR_TMP_STATUS        = 8'hxx, // required if TMP controller & TMP status register are included
            // recommended if there is programmable I/O
            // INSTR_INIT_SETUP        = 8'h, // required if initialization data register is included
            // INSTR_INIT_SETUP_CLAMP  = 8'h, // required if initialization data register is included
            // INSTR_INIT_RUN          = 8'h, // required if initialization status register is included


// required registers



reg [3:0] state = STATE_TEST_LOGIC_RESET;
reg [7:0] reg_instruction_shift, reg_instruction;
reg [7:0] reg_data;

always @(posedge jtag_tck or posedge jtag_trst) begin : state_transitions
    if (jtag_trst == 1'b0) begin
        state <= STATE_TEST_LOGIC_RESET;
    end else begin
        case (state) begin
            STATE_TEST_LOGIC_RESET: state <= jtag_tms ? STATE_TEST_LOGIC_RESET  : STATE_RUN_TEST_IDLE;
            STATE_RUN_TEST_IDLE:    state <= jtag_tms ? STATE_SELECT_DR_SCAN    : STATE_RUN_TEST_IDLE;
            STATE_SELECT_DR_SCAN:   state <= jtag_tms ? STATE_SELECT_IR_SCAN    : STATE_CAPTURE_DR;
            STATE_CAPTURE_DR:       state <= jtag_tms ? STATE_EXIT_1_DR         : STATE_SHIFT_DR;
            STATE_SHIFT_DR:         state <= jtag_tms ? STATE_EXIT_1_DR         : STATE_SHIFT_DR;
            STATE_EXIT_1_DR:        state <= jtag_tms ? STATE_UPDATE_DR         : STATE_PAUSE_DR;
            STATE_PAUSE_DR:         state <= jtag_tms ? STATE_EXIT_2_DR         : STATE_PAUSE_DR;
            STATE_EXIT_2_DR:        state <= jtag_tms ? STATE_UPDATE_DR         : STATE_SHIFT_DR;
            STATE_UPDATE_DR:        state <= jtag_tms ? STATE_SELECT_DR_SCAN    : STATE_RUN_TEST_IDLE;
            STATE_SELECT_IR_SCAN:   state <= jtag_tms ? STATE_TEST_LOGIC_RESET  : STATE_CAPTURE_IR;
            STATE_CAPTURE_IR:       state <= jtag_tms ? STATE_EXIT_1_IR         : STATE_SHIFT_IR;
            STATE_SHIFT_IR:         state <= jtag_tms ? STATE_EXIT_1_IR         : STATE_SHIFT_IR;
            STATE_EXIT_1_IR:        state <= jtag_tms ? STATE_UPDATE_IR         : STATE_PAUSE_IR;
            STATE_PAUSE_IR:         state <= jtag_tms ? STATE_EXIT_2_IR         : STATE_PAUSE_IR;
            STATE_EXIT_2_IR:        state <= jtag_tms ? STATE_UPDATE_IR         : STATE_SHIFT_IR;
            STATE_UPDATE_IR:        state <= jtag_tms ? STATE_SELECT_DR_SCAN    : STATE_RUN_TEST_IDLE;
        end
    end
end

always @(posedge jtag_tck or posedge jtag_trst) begin : actions_posedge
    if (jtag_trst == 1'b0 || state == STATE_TEST_LOGIC_RESET) begin
            reg_instruction_shift <= 8'hxx;
            reg_instruction       <= INSTR_IDCODE;
    end else begin
        case (state)
            STATE_TEST_LOGIC_RESET: begin
                reg_instruction_shift <= 8'hxx;
                reg_instruction       <= INSTR_IDCODE;
            end
            STATE_CAPTURE_IR: begin
                // TODO: check MSB/LSB first
                reg_instruction_shift[1:0] <= 2'b01;
                reg_instruction_shift[7:2] <= 6'h00; // these bits may be chosen to be any fixed value
                // reg_instruction <= reg_instruction;
            end
            STATE_SHIFT_IR: begin
                // TODO: check MSB/LSB first
                reg_instruction_shift[7] <= jtag_tdi;
                reg_instruction_shift[6:0] <= reg_instruction_shift[7:1];
                // reg_instruction <= reg_instruction;
            end
            STATE_EXIT_1_IR,
            STATE_EXIT_2_IR,
            STATE_PAUSE_IR: begin
                // reg_instruction_shift <= reg_instruction_shift;
                // reg_instruction <= reg_instruction;
            end
            STATE_UPDATE_IR: begin
                // reg_instruction_shift <= reg_instruction_shift;
                reg_instruction <= reg_instruction_shift;
                // TODO: do this on negedge? see pg 70
            end
            default: begin
                reg_instruction_shift <= 8'hxx;
                // reg_instruction <= reg_instruction;
            end
        endcase
        // case (state) begin
        //     STATE_TEST_LOGIC_RESET: state <= jtag_tms ? STATE_TEST_LOGIC_RESET  : STATE_RUN_TEST_IDLE;
        //     STATE_RUN_TEST_IDLE:    state <= jtag_tms ? STATE_SELECT_DR_SCAN    : STATE_RUN_TEST_IDLE;
        //     STATE_SELECT_DR_SCAN:   state <= jtag_tms ? STATE_SELECT_IR_SCAN    : STATE_CAPTURE_DR;
        //     STATE_CAPTURE_DR:       state <= jtag_tms ? STATE_EXIT_1_DR         : STATE_SHIFT_DR;
        //     STATE_SHIFT_DR:         state <= jtag_tms ? STATE_EXIT_1_DR         : STATE_SHIFT_DR;
        //     STATE_EXIT_1_DR:        state <= jtag_tms ? STATE_UPDATE_DR         : STATE_PAUSE_DR;
        //     STATE_PAUSE_DR:         state <= jtag_tms ? STATE_EXIT_2_DR         : STATE_PAUSE_DR;
        //     STATE_EXIT_2_DR:        state <= jtag_tms ? STATE_UPDATE_DR         : STATE_SHIFT_DR;
        //     STATE_UPDATE_DR:        state <= jtag_tms ? STATE_SELECT_DR_SCAN    : STATE_RUN_TEST_IDLE;
        //     STATE_SELECT_IR_SCAN:   state <= jtag_tms ? STATE_TEST_LOGIC_RESET  : STATE_CAPTURE_IR;
        //     STATE_CAPTURE_IR:       state <= jtag_tms ? STATE_EXIT_1_IR         : STATE_SHIFT_IR;
        //     STATE_SHIFT_IR:         state <= jtag_tms ? STATE_EXIT_1_IR         : STATE_SHIFT_IR;
        //     STATE_EXIT_1_IR:        state <= jtag_tms ? STATE_UPDATE_IR         : STATE_PAUSE_IR;
        //     STATE_PAUSE_IR:         state <= jtag_tms ? STATE_EXIT_2_IR         : STATE_PAUSE_IR;
        //     STATE_EXIT_2_IR:        state <= jtag_tms ? STATE_UPDATE_IR         : STATE_SHIFT_IR;
        //     STATE_UPDATE_IR:        state <= jtag_tms ? STATE_SELECT_DR_SCAN    : STATE_RUN_TEST_IDLE;
        // end
    end
end

always @(negedge jtag_tck or posedge jtag_trst) begin : actions_negedge
    if (jtag_trst == 1'b0 || state == STATE_TEST_LOGIC_RESET) begin
    end else begin
        // case (state) begin
        //     STATE_TEST_LOGIC_RESET: state <= jtag_tms ? STATE_TEST_LOGIC_RESET  : STATE_RUN_TEST_IDLE;
        //     STATE_RUN_TEST_IDLE:    state <= jtag_tms ? STATE_SELECT_DR_SCAN    : STATE_RUN_TEST_IDLE;
        //     STATE_SELECT_DR_SCAN:   state <= jtag_tms ? STATE_SELECT_IR_SCAN    : STATE_CAPTURE_DR;
        //     STATE_CAPTURE_DR:       state <= jtag_tms ? STATE_EXIT_1_DR         : STATE_SHIFT_DR;
        //     STATE_SHIFT_DR:         state <= jtag_tms ? STATE_EXIT_1_DR         : STATE_SHIFT_DR;
        //     STATE_EXIT_1_DR:        state <= jtag_tms ? STATE_UPDATE_DR         : STATE_PAUSE_DR;
        //     STATE_PAUSE_DR:         state <= jtag_tms ? STATE_EXIT_2_DR         : STATE_PAUSE_DR;
        //     STATE_EXIT_2_DR:        state <= jtag_tms ? STATE_UPDATE_DR         : STATE_SHIFT_DR;
        //     STATE_UPDATE_DR:        state <= jtag_tms ? STATE_SELECT_DR_SCAN    : STATE_RUN_TEST_IDLE;
        //     STATE_SELECT_IR_SCAN:   state <= jtag_tms ? STATE_TEST_LOGIC_RESET  : STATE_CAPTURE_IR;
        //     STATE_CAPTURE_IR:       state <= jtag_tms ? STATE_EXIT_1_IR         : STATE_SHIFT_IR;
        //     STATE_SHIFT_IR:         state <= jtag_tms ? STATE_EXIT_1_IR         : STATE_SHIFT_IR;
        //     STATE_EXIT_1_IR:        state <= jtag_tms ? STATE_UPDATE_IR         : STATE_PAUSE_IR;
        //     STATE_PAUSE_IR:         state <= jtag_tms ? STATE_EXIT_2_IR         : STATE_PAUSE_IR;
        //     STATE_EXIT_2_IR:        state <= jtag_tms ? STATE_UPDATE_IR         : STATE_SHIFT_IR;
        //     STATE_UPDATE_IR:        state <= jtag_tms ? STATE_SELECT_DR_SCAN    : STATE_RUN_TEST_IDLE;
        // end
    end
end



endmodule