串行通訊是指外部設備與計算機間只使用一根數據線（另外須要地線，還可能須要控制線）進行數據傳送的方式。數據在一根數據線上按位依次傳送。因爲CPU與接口之間按並行方式傳輸，接口與外設之間按串行方式傳輸，所以串行通訊的基本功能是：在發送時，把CPU送來的並行數據轉換爲串行數據，逐位依次發送出去；在接受時，把外部設備發送過來的的串行數據逐位地接受，組裝成並行數據，並行地送給CPU去處理。異步
異步通訊是以一個字符做爲傳輸方式，通訊中兩個字符間的時間間隔多少不固定，但同一個字符的兩個相鄰位的時間間隔固定。通訊過程當中沒有時鐘線。code
UART使用的是串行異步通訊。blog
基本的UART通訊協議：接口
波特率：表示每秒鐘傳輸字符數的多少input
起始位：通常爲邏輯「0」it
數據位：通常位6~8位之間可變，數據低位在前，高位在後class
校驗位：能夠爲無校驗位，奇校驗，偶校驗，常「0」或常「1」之間可選module
中止位：必須爲邏輯「1」sed
空閒位：處於邏輯「1」狀態並行
UART發送功能模塊：
`define NONE 0 //無校驗位
`define ODD 1 //奇校驗
`define EVEN 2 //偶校驗
`define MARK 3 //常「1」
`define SPACE 4 //常「0」
`define STOPBITS_ONE 0 //1位中止位
`define STOPBITS_ONEHALF 1 //1.5位中止位
`define STOPBITS_TWO 2 //2位中止位
module uart_tx #(
parameter CLK_FREQ = 50_000_000, //時鐘頻率
parameter BPS = 9600, //波特率
parameter PARITY = `NONE, //數據位寬
parameter DATA_BITS = 8, //校驗位
parameter STOP_BITS = `STOPBITS_ONE //中止位位寬
)
(
input clk,
input rst_n,
input [DATA_BITS-1:0] data,
input enable, //定義一個發送使能信號
output reg tx,
output wire free //定義一個發送端空閒狀態
);
reg [DATA_BITS-1:0] data_2 = 0;
reg flag = 0; //定義一個flag信號來鎖住enable
wire stop_flag; //定義一個傳輸中止脈衝
reg [31:0] cnt = 0;
localparam [31:0] CNT_MAX = CLK_FREQ/BPS;
reg [2:0] cstate = 0;
localparam [2:0]
FSM_IDEL = 0, //空閒位
FSM_START = 1, //開始位
FSM_DATA = 2, //數據位
FSM_PARITY = 3, //校驗位
FSM_STOP = 4; //中止位
reg ifparity = 0; //有無校驗位
reg parity_value = 0; //校驗位的值
reg [3:0] num = 0; //定義一個計數num
assign free = ~flag;
assign stop_flag = (cstate == FSM_STOP && ((STOP_BITS == `STOPBITS_ONE && cnt == CNT_MAX-1) || (STOP_BITS == `STOPBITS_ONEHALF && num == 1 && cnt == CNT_MAX>>1) || (STOP_BITS == `STOPBITS_TWO && num == 1 && cnt == CNT_MAX-1))) ? 1'b1 : 1'b0;
always @(posedge clk or negedge rst_n)
if (!rst_n)
data_2 <= 0;
else if (enable && flag == 0)
data_2 <= data;
else data_2 <= data_2;
always @(posedge clk or negedge rst_n)
if (!rst_n)
flag <= 0;
else if (enable)
flag <= 1;
else if (stop_flag)
flag <= 0;
else flag <= flag;
always @(posedge clk or negedge rst_n)
if (!rst_n)
cnt <= 0;
else if (cnt == CNT_MAX-1 || flag == 0 )
cnt <= 0;
else if (flag)
cnt <= cnt + 1;
else cnt <= cnt;
always @(posedge clk or negedge rst_n)
if (!rst_n)
cstate <= FSM_IDEL;
else case (cstate)
FSM_IDEL : begin
tx <= 1;
if (flag)
cstate <= FSM_START;
else cstate <= FSM_IDEL;
end
FSM_START : begin
tx <= 0;
if (cnt == CNT_MAX-1)
cstate <= FSM_DATA;
else cstate <= FSM_START;
end
FSM_DATA : begin
tx <= data_2 [num];
if (cnt == CNT_MAX-1 && num == DATA_BITS-1)
begin
num <= 0;
cstate <= ifparity ? FSM_PARITY : FSM_STOP;
end
else if (cnt == CNT_MAX-1)
num <= num + 1;
else num <= num;
end
FSM_PARITY : begin
tx <= parity_value;
if (cnt == CNT_MAX-1)
cstate <= FSM_STOP;
else cstate <= FSM_PARITY;
end
FSM_STOP : begin
tx <= 1;
if (STOP_BITS == `STOPBITS_ONE && cnt == CNT_MAX-1)
cstate <= FSM_IDEL;
else if (STOP_BITS == `STOPBITS_ONEHALF) begin
if (num == 1 && cnt == CNT_MAX>>1)
begin cstate <= FSM_IDEL; num <= 0; end
else if (cnt == CNT_MAX-1)
num <=1;
else cstate <= FSM_STOP;
end
else if (STOP_BITS == `STOPBITS_TWO) begin
if (num == 1 && cnt == CNT_MAX-1)
begin cstate <= FSM_IDEL; num <= 0; end
else if (cnt == CNT_MAX-1)
num <=1;
else cstate <= FSM_STOP;
end
end
default : cstate <= FSM_IDEL;
endcase
always @(*)
if (!rst_n)
begin
ifparity = 0;
parity_value = 0;
end
else case (PARITY)
`NONE : begin
ifparity = 0;
parity_value = 0;
end
`ODD : begin
ifparity = 1;
parity_value = ~(^data_2);
end
`EVEN : begin
ifparity = 1;
parity_value = ^data_2;
end
`MARK : begin
ifparity = 1;
parity_value = 1;
end
`SPACE : begin
ifparity = 1;
parity_value = 0;
end
default : begin
ifparity = 0;
parity_value = 0;
end
endcase
UART接收功能模塊：
`define NONE 0 //無校驗位
`define ODD 1 //奇校驗
`define EVEN 2 //偶校驗
`define MARK 3 //常"1"
`define SPACE 4 //常"0"
`define STOPBITS_ONE 0 //1位中止位
`define STOPBITS_ONEHALF 1 //1.5位中止位
`define STOPBITS_TWO 2 //2位中止位
module uart_rx #(
parameter BPS = 9600, //波特率
parameter CLK_FREQ = 50_000_000, //時鐘頻率
parameter DATA_BITS = 8, //數據位寬
parameter PARITY = `NONE, //校驗位
parameter STOP_BITS = `STOPBITS_ONE //中止位位寬
)
(
input clk,
input rst_n,
input rx,
output reg [DATA_BITS-1:0] data,
output reg done,
output reg err
);
reg [1:0] in;
always @(posedge clk or negedge rst_n)
if (!rst_n)
in <= 0;
else in <= {in[0],rx};
wire en;
assign en = (in[1] == 1 && in[0] == 0) ? 1 : 0;
reg flag = 0; //定義一個接收信號標誌位
always @(posedge clk or negedge rst_n)
if (!rst_n)
flag <= 0;
else if (en)
flag <= 1;
else if (cstate == FSM_STOP && sampling)
flag <= 0;
else flag <= flag;
reg [31:0] cnt = 0;
localparam [31:0] CNT_MAX = CLK_FREQ/BPS;
always @(posedge clk or negedge rst_n)
if (!rst_n)
cnt <= 0;
else if (cnt == CNT_MAX-1 || flag == 0)
cnt <= 0;
else if (flag)
cnt <= cnt + 1;
else cnt <= cnt;
reg sampling = 0; //定義一個採樣脈衝信號
always @(posedge clk or negedge rst_n)
if (!rst_n)
sampling = 0;
else if (cnt == CNT_MAX>>1)
sampling = 1;
else sampling = 0;
reg [3:0] num = 0; //定位計數num
reg ifparity = 0; //有無校驗位
reg parity_value = 0; //定義校驗位的值
reg parity_rx = 0; //定義接受信號中校驗位的值
reg [2:0] cstate = 0;
localparam [2:0]
FSM_IDEL = 0, //空閒位
FSM_START = 1, //開始位
FSM_DATA = 2, //數據位
FSM_PARITY = 3, //校驗位
FSM_STOP = 4; //中止位
always @(posedge clk or negedge rst_n)
if (!rst_n) begin
cstate <= FSM_IDEL;
parity_rx <= 0;
done <= 0;
err <= 0;
data <= 0;
num <= 0;
end
else case (cstate)
FSM_IDEL : if (flag)
cstate <= FSM_START;
else cstate <= FSM_IDEL;
FSM_START : begin
done <= 0;
if (sampling)
cstate <= FSM_DATA;
else cstate <= FSM_START;
end
FSM_DATA : if (sampling && num <= DATA_BITS-1)
begin
data[num] <= rx;
num <= num + 1;
end
else if (num == DATA_BITS) begin
num <= 0;
cstate <= ifparity ? FSM_PARITY : FSM_STOP ;
end
FSM_PARITY : if (sampling)
begin
parity_rx <= rx;
cstate <= FSM_STOP;
end
else cstate <= FSM_PARITY;
FSM_STOP : if (sampling)
begin
done <= 1;
cstate <= FSM_IDEL;
err <= ifparity ? (parity_rx == parity_value ? 0 : 1) : 0;
end
else cstate <= FSM_STOP;
default : begin
cstate <= FSM_IDEL;
parity_rx <= 0;
done <= 0;
err <= 0;
end
endcase
always @(*)
if (!rst_n) begin
ifparity = 0;
parity_value = 0;
end
else case (PARITY)
`NONE : begin ifparity = 0; parity_value = 0; end
`ODD : begin ifparity = 1; parity_value = ~(^data); end
`EVEN : begin ifparity = 1; parity_value = ^data; end
`MARK : begin ifparity = 1; parity_value = 1; end
`SPACE : begin ifparity = 1; parity_value = 0; end
default : begin ifparity = 0; parity_value = 0; end
endcase
endmodule