前面发了个发送模块，这次补齐，完整。
串口计数器，波特率适配
uart_clk.v

module uart_clk(
    input wire clk,
    input wire rst_n,
    input wire tx_clk_en,
    input wire rx_clk_en,
    input wire[1:0] baud_sel,
    output wire tx_clk,
    output wire rx_clk
);
localparam OSC = 50_000_000;
reg[16:0] bauds[0:3] ;

reg[16:0] tx_cnt;
reg[16:0] rx_cnt;

initial begin
    bauds[0] <= OSC/19200 - 1;
    bauds[1] <= OSC/38400 - 1;
    bauds[2] <= OSC/57600 - 1;
    bauds[3] <= OSC/115200 - 1;
end

always @(posedge clk or negedge rst_n ) begin
    if (!rst_n )
        tx_cnt <= 0;
    else if (!tx_clk_en || tx_cnt === bauds[baud_sel])
        tx_cnt <= 0;
    else
        tx_cnt <= tx_cnt + 1;
end

assign tx_clk = (tx_cnt  === bauds[baud_sel]) ? 1'b1 : 1'b0;

always @(posedge clk or negedge rst_n) begin
    if (!rst_n)
        rx_cnt <= 0;
    else if (!rx_clk_en || rx_cnt === bauds[baud_sel])
        rx_cnt <= 0;
    else
        rx_cnt <= rx_cnt + 1;
end

assign  rx_clk = (rx_cnt === (bauds[baud_sel] >>1)) ? 1'b1 : 1'b0;
    
endmodule

发送模块
tx.v

module tx (
    input wire clk,
    input wire rst_n,
    input wire tx_en,
    input wire tx_clk,
    input wire[7:0] tx_data,
    output reg tx_clk_en,
    output reg txd,
    output reg busy,
    output reg done
);

reg [1:0] stage;
reg [2:0] curbit;
reg [7:0] tx_rdata;

always @(posedge clk or negedge rst_n) begin
    if (!rst_n) begin
        busy <= 1'b0;
    end else if (tx_en && !busy) begin
        busy <= 1'b1;
        tx_rdata <= tx_data;
    end else if (done)
        busy <= 1'b0;
end

always @(busy) begin
    if(busy)
        tx_clk_en <= 1'b1;
    else 
        tx_clk_en <= 1'b0;
end

always @(posedge clk or negedge rst_n) begin
    if (!rst_n) begin
        txd <= 1'b1;
        done <= 1'b0;
        stage <= 2'd0;
    end else if (busy) begin
        if (tx_clk) begin
            if(stage === 2'd0) begin
                txd <= 1'b0;
                done <= 1'b0;
                curbit <= 3'd0;
                stage <= 2'b1;
            end else if (stage === 2'b1) begin
                txd <= tx_rdata[curbit];
                if(curbit == 3'd7)
                    stage <= 2'd2;
                else
                    curbit <= curbit + 1;
            end else if (stage == 2'd2) begin
                txd <= 1'b1;
                stage <= stage + 1;
            end else if (stage == 2'd3) begin
                txd <= 1'b1;
                done <= 1'b1;
                stage <= 2'd0;
            end
        end
    end else begin
        txd <= 1'b1;
        done <= 1'b0;
        stage <= 2'd0;
    end
end

endmodule

接收模块

rx.v

module rx (
    input wire clk,
    input wire rst_n,
    input wire rx_en,
    input wire rx_clk,
    input wire rxd,
    output reg rx_clk_en,
    output reg[7:0] rx_data,
    output reg busy,
    output reg done
);

reg[7:0] tmp;
reg[3:0] tmpres;

reg [ 1:0] stage;
reg [2:0] curbit;
reg [7:0] rx_rdata;

wire rxd_negedge;
always @(posedge clk or negedge rst_n) begin
    if (!rst_n)
        tmp <= 8'b0;
    else
        tmp <= (tmp << 1) | rxd;
    tmpres <= {tmp[7:6], tmp[1:0]};
end
assign rxd_negedge = ( tmpres === 4'b1100) ? 1'b1 : 1'b0;

always @(posedge clk or negedge rst_n) begin
    if (!rst_n) begin
        busy <= 1'b0;
    end else if (rx_en && rxd_negedge && !busy) begin
        busy <= 1'b1;
    end else if (done)
        busy <= 1'b0;
end

always @(busy) begin
    if (busy)
        rx_clk_en <= 1'b1;
    else
        rx_clk_en <= 1'b0;
end

always @(posedge clk or negedge rst_n) begin
    if (!rst_n) begin
        done <= 1'b0;
        stage <= 2'd0;
        rx_rdata <= 8'd0;
    end else if (busy) begin
        if (rx_clk) begin
            if (stage === 2'd0) begin
                rx_rdata <= 8'd0;
                done <= 1'b0;
                curbit <= 3'd0;
                stage <= 2'b1;
            end else if (stage === 2'b1) begin
                rx_rdata[curbit] <= rxd;
                if (curbit == 3'd7)
                    stage <= 2'd2;
                else
                    curbit <= curbit + 1;
            end else if (stage === 2'd2) begin
                rx_data <= rx_rdata;
                done <= 1'b1;
                stage <= 2'd0;
            end else
                stage <= 2'd0;
        end
    end else begin
        rx_rdata <= 8'd0;
        done <= 1'b0;
        stage <= 2'd0;
    end
end

endmodule

组合
uart.v

module uart(
    input wire clk,
    input wire rst_n,
    input wire rx_en,
    input wire rxd,
    input wire tx_en,
    output wire[7:0] rx_data,
    input wire[7:0] tx_data,
    input  wire[1:0] baud_sel,
    output wire txd,
    output wire rbusy,
    output wire rdone,
    output wire tbusy,
    output wire tdone
);


wire rx_clk,rx_clk_en;
wire tx_clk, tx_clk_en;

uart_clk uclk1(clk,rst_n,tx_clk_en,rx_clk_en,baud_sel,tx_clk,rx_clk);
tx tx1(clk,rst_n,tx_en,tx_clk,tx_data,tx_clk_en,txd,tbusy,tdone);
rx rx1(clk,rst_n,rx_en,rx_clk,rxd,rx_clk_en,rx_data,rbusy,rdone);

endmodule

测试
uart_test.v

module uart_test (
    input wire clk,
    input wire rst_n,
    input wire rxd,
    output wire txd
);
reg rx_en = 1;
reg tx_en;
wire rbusy,tbusy;
wire rdone,tdone;
reg[7:0] tx_data = 0;
wire[7:0] rx_data ;
// reg received = 1'b0;
reg sent = 1'b0;
reg[1:0] baud_sel = 0;


always @(posedge clk or negedge rst_n) begin
    if (!rst_n) begin
        rx_en = 0;
        tx_en = 0;
    end else  if(!tbusy && !rbusy && !sent) begin
        tx_data <= tx_data+1 ;
        tx_en <= 1'b1;
        sent <= 1;
        $display("sent %d",tx_data);
    end else if (rdone) begin
        sent <= 0;
        $display("received %d",rx_data);
    end else begin
        tx_en <= 1'b0;
        rx_en <= 1'b1;
    end
end

uart uart1(clk, rst_n, rx_en,rxd,tx_en,rx_data, tx_data,baud_sel,txd,rbusy,rdone,tbusy,tdone);

endmodule

testbench

module test;

reg rst_n=1 ;

initial begin
    $dumpfile("test.vcd");
    $dumpvars(0, test);

    #10 rst_n = !rst_n;
    #30 rst_n = !rst_n;
    #500000 $finish;
end

reg clk = 0;
always #1 clk = !clk;

wire txd;
wire rxd;
uart_test uart1(clk,rst_n,rxd,txd);
// always @(posedge clk)
//    rxd <= txd;
assign rxd = txd;

endmodule

gtkwave 波形

方便模拟测试，发送转接收，对比结果。