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module uart_rx
#(
parameter N_DATA_BITS = 8, // 串口數據位個數, 取值范圍: 5, 6, 7, 8, 默認值: 8位
HOW_MANY_TICKS_FOR_STOP_BIT = 16
)
(
input wire clk,
input wire reset,
input wire rx,
input wire s_tick,
output reg rx_done_tick,
output wire [7:0] dout
);
// symbolic state declaration for UART receiver
localparam [1:0]
idle = 2'b00,
start = 2'b01,
data = 2'b10,
stop = 2'b11;
// state register
reg [1:0] state_reg, state_next;
// signal declaration
reg [7:0] b_reg, b_next; // 數據位在寄存器內每個周期向右移1bit
reg [2:0] n_reg, n_next;
reg [3:0] s_reg, s_next; // 作為s_tick的計數器, 遞增計數
// body
// FSMD state and data registers
always @(posedge clk, posedge reset)
begin
if (reset)
begin
state_reg <= idle;
b_reg <= 8'b0;
n_reg <= 3'd0;
s_reg <= 4'd0;
end
else
begin
state_reg <= state_next;
b_reg <= b_next;
n_reg <= n_next;
s_reg <= s_next;
end
end
// FSMD next-state logic
always @*
begin
rx_done_tick = 1'b0;
state_next = state_reg;
b_next = b_reg;
n_next = n_reg;
s_next = s_reg;
case (state_reg)
/* 空閑位階段 */
idle:
if (rx == 1'b0)
begin
state_next = start; // 當rx首次出現低電平時空閑位結束, 準備進入下一狀態讀起始位
s_next = 0;
end
/* 起始位階段 */
start:
if (s_tick)
begin
s_next = s_reg + 1;
if (s_reg == 7)
begin
s_next = 0;
state_next = data; // 準備開始讀數據位
n_next = 0;
end
end
/* 數據位階段 */
data:
if (s_tick)
begin
s_next = s_reg + 1;
if (s_reg == (16-1))
begin
s_next = 0;
b_next = {rx, b_reg[7:1]};
n_next = n_reg + 1;
if (n_reg == (N_DATA_BITS-1))
begin
state_next = stop;
//n_next = n_reg;
end
end
end
/* 停止位階段 */
stop:
if (s_tick)
begin
s_next = s_reg + 1;
if (s_reg == (HOW_MANY_TICKS_FOR_STOP_BIT-1))
begin
rx_done_tick = 1'b1;
state_next = idle;
//s_next = s_reg;
end
end
endcase
end
// output
assign dout = b_reg;
endmodule

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