// Copyright 2021 cpu-dev
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

module rx(clk, rx_en, rx, data, end_flag, clk_count_bit);
    input wire clk;
    input wire rx_en;
    input wire rx;
    output reg[7:0] data = 8'b00000000;
    output reg end_flag = 1'b0;
    input wire [31:0] clk_count_bit;

    wire [31:0] clk_begin_to_receive;

    reg[1:0] state = 2'b00;
    reg[31:0] clk_count = 32'd0;
    reg[2:0] bit_count = 3'd0;
    reg[3:0] recent = 4'b1111;
    wire update_flag;

    assign clk_begin_to_receive = clk_count_bit + clk_count_bit / 2 - 4;

    assign update_flag = (state == 2'b01) 
        ? clk_count == clk_begin_to_receive
        : clk_count == clk_count_bit - 32'd1;
    
    always @(posedge clk) begin
        case(state)
            2'b00: begin
                clk_count <= 32'd0;
                bit_count <= 3'd0;
                end_flag <= 1'b0;
                recent = {recent[2:0], rx};
                state = (recent == 4'b0000) & rx_en ? 2'b01 : state;
            end
            2'b01: begin
                clk_count <= clk_count + 32'd1;
                if(update_flag) begin
                    state = 2'b11;
                    clk_count <= 32'd0;
                    data[2'd0] <= rx;
                    bit_count <= 3'd1;
                end
            end
            2'b11: begin
                clk_count <= clk_count + 32'd1;
                if(update_flag) begin
                    state <= (bit_count == 3'd7) ? 2'b10 : state;
                    data[bit_count] <= rx;
                    bit_count <= bit_count + 3'd1;
               clk_count <= 32'd0;
                end
            end
            2'b10: begin
                clk_count <= clk_count + 32'd1;
                if(update_flag) begin
                    state <= 2'b00;
                    end_flag <= 1'b1;
                end
            end
        endcase
    end
endmodule