jtag verilog

JTAG (Joint Test Action Group) 是一种用于测试和编程电子设备的标准接口。在 Verilog 中，可以使用 JTAG 接口来进行芯片级别的测试和调试。以下是一个简单的 JTAG 模块的 Verilog 代码示例：

module jtag (
    input jtag_clk,
    input jtag_rst_n,
    input jtag_tms,
    input jtag_tdi,
    output jtag_tdo
);

    // JTAG state machine
    reg [2:0] jtag_state;

    always @(posedge jtag_clk or negedge jtag_rst_n) begin
        if (~jtag_rst_n) begin
            jtag_state <= 3'b000; // Test-Logic-Reset state
        end else begin
            case (jtag_state)
                3'b000: begin // Test-Logic-Reset state
                    jtag_state <= jtag_tms ? 3'b001 : 3'b000;
                end
                3'b001: begin // Run-Test-Idle state
                    jtag_state <= jtag_tms ? 3'b010 : 3'b001;
                end
                3'b010: begin // Select-DR-Scan state
                    jtag_state <= jtag_tms ? 3'b011 : 3'b010;
                end
                3'b011: begin // Capture-DR state
                    jtag_state <= jtag_tms ? 3'b100 : 3'b011;
                end
                3'b100: begin // Shift-DR state
                    jtag_state <= jtag_tms ? 3'b101 : 3'b100;
                end
                3'b101: begin // Exit1-DR state
                    jtag_state <= jtag_tms ? 3'b110 : 3'b101;
                end
                3'b110: begin // Pause-DR state
                    jtag_state <= jtag_tms ? 3'b111 : 3'b110;
                end
                3'b111: begin // Exit2-DR state
                    jtag_state <= jtag_tms ? 3'b001 : 3'b111;
                end
            endcase
        end
    end

    // JTAG shift register
    reg [7:0] jtag_shift;

    always @(posedge jtag_clk or negedge jtag_rst_n) begin
        if (~jtag_rst_n) begin
            jtag_shift <= 8'h00;
        end else begin
            case (jtag_state)
                3'b100: begin // Shift-DR state
                    jtag_shift <= {jtag_shift[6:0], jtag_tdi};
                end
                3'b101: begin // Exit1-DR state
                    jtag_shift <= {jtag_shift[6:0], jtag_tdi};
                end
                3'b111: begin // Exit2-DR state
                    jtag_shift <= {jtag_shift[6:0], jtag_tdi};
                end
            endcase
        end
    end

    // JTAG output
    assign jtag_tdo = jtag_shift[7];

endmodule

该模块包含 JTAG 时钟（`jtag_clk`）、复位信号（`jtag_rst_n`）、状态机输入信号（`jtag_tms`）、数据输入信号（`jtag_tdi`）和数据输出信号（`jtag_tdo`）。JTAG 状态机根据输入信号 `jtag_tms` 的值进行状态转移，并将数据输入 `jtag_tdi` 传输到一个 8 位移位寄存器中。最后，数据输出 `jtag_tdo` 从移位寄存器的 MSB 输出。