`include "param.v"
module uart_rx(
    input   wire        sys_clk     ,
    input   wire        sys_rst_n   ,
    input   wire        rx_din      ,//数据串行输入

    output  wire [7: 0] rx_dout     ,//数据并行输出
    output  wire        rx_vld      //输出信号有效
);
parameter BAUD = `SYS_FRQ / `MAX;//获取1bit时钟振动的次数
reg din_reg0;//同步外来信号，和本时钟域同步
reg din_reg1;//打拍，获取下降沿
wire nedge;//下降沿标志

reg rx_flag;//开始接受标志

//对bit进行计数寄存器
reg [3: 0] cnt_bit    ;
wire       add_cnt_bit;
wire       end_cnt_bit;
//对波特率进行计数寄存器
reg [12: 0] cnt_bps     ;
wire        add_cnt_bps ;
wire        end_cnt_bps ; 

//数据寄存器
reg [7: 0]  data_reg;
always @(posedge sys_clk or negedge sys_rst_n) begin
    if(!sys_rst_n)begin
        din_reg0 <= 1'b1;
        din_reg1 <= 1'b1;
    end
    else begin
        din_reg0 <= rx_din;
        din_reg1 <= din_reg0;
    end 
end
assign nedge = ~din_reg0 && din_reg1;//下降沿

//rx_flag信号约束
always @(posedge sys_clk or negedge sys_rst_n)begin
    if(!sys_rst_n)begin
        rx_flag <= 1'b0;
    end 
    else if(nedge)begin//检测到下降沿
        rx_flag <= 1'b1;
    end 
    else if(end_cnt_bit)begin//传输完成标志
        rx_flag <= 1'b0;
    end
    else begin
        rx_flag <= rx_flag;//其他时刻保持
    end  
end 
//cnt_bps计数器设计
always @(posedge sys_clk or negedge sys_rst_n)begin
    if(!sys_rst_n)begin
        cnt_bps <= 13'd0;
    end 
    else if(add_cnt_bps)begin
        if(end_cnt_bps)begin
            cnt_bps <= 13'd0;
        end 
        else begin
            cnt_bps <= cnt_bps + 1'd1;
        end 
    end 
    else begin
        cnt_bps <= cnt_bps;
    end 
end 
assign add_cnt_bps = rx_flag;
assign end_cnt_bps = add_cnt_bps && cnt_bps == BAUD - 1'd1;
//cnt_bit计数器设计
always @(posedge sys_clk or negedge sys_rst_n)begin
    if(!sys_rst_n)begin
        cnt_bit <= 4'd0;
    end 
    else if(add_cnt_bit)begin
        if(end_cnt_bit)begin
            cnt_bit <= 4'd0;
        end 
        else begin
            cnt_bit <= cnt_bit + 1'd1;
        end 
    end 
    else begin
        cnt_bit <= cnt_bit;
    end 
end 
assign add_cnt_bit = end_cnt_bps;//比特计数开始条件
assign end_cnt_bit = add_cnt_bit && cnt_bit == 4'd8;//0-8
//对data_reg进行约束
always @(posedge sys_clk or negedge sys_rst_n)begin
    if(!sys_rst_n)begin
        data_reg <= 8'hx;
    end 
    else if((cnt_bit >= 4'd1 && cnt_bit <= 4'd8) && (cnt_bps == BAUD >> 1))begin
        data_reg[cnt_bit - 1'd1] = din_reg0;//对输入数据进行采样
    end 
    else begin
        data_reg <= data_reg;
    end
end 
assign rx_dout = data_reg;//数据输出
assign rx_vld = end_cnt_bit;//串转并结束标志
endmodule