UART串口收发协议以及时序分析

发表于 2013/6/25 20:48:13 阅读（12941）

UART串口收发协议以及时序分析

/**************************************************************************

.......IDLE.Start................................UART DATA..................................End...IDLE...

________ ______________

|____< D0 >< D1 >< D2 >< D3 >< D4 >< D5 >< D6 >< D7 >

Bit0 Bit1 Bit2 Bit3 Bit4 Bit5 Bit6 Bit7 Bit08 Bit9

**************************************************************************/

1. 任意分频实现精确波特率

详见：http://blog.chinaaet.com/detail/21535.html

\发送&接收数据时序图

2. 串口数据接收时序分析：

（1）状态机：R_IDLE; R_START; R_SAMPLE; R_STOP

//---------------------------------------

//parameter of uart transfer

localparam R_IDLE = 2'd0; //detect if the uart data is begin

localparam R_START = 2'd1; //uart transfert start mark bit

localparam R_SAMPLE = 2'd2; //uart 8 bit data receive

localparam R_STOP = 2'd3; //uart transfer stop mark bit

reg [1:0] rxd_state;

（2）数据采样在smp_cnt==7即数据中点！

//----------------------------------
//uart data receive in center point
reg	[7:0]	rxd_data_r;
always@(posedge clk or negedge rst_n)
begin
	if(!rst_n)
		rxd_data_r <= 0;
	else if(rxd_state == R_SAMPLE)
		begin
		if(clken_16bps == 1 && smp_cnt == 4'd7)	//sample center point
			case(rxd_cnt)
			4'd1:	rxd_data_r[0] <= rxd_sync;
			4'd2:	rxd_data_r[1] <= rxd_sync;
			4'd3:	rxd_data_r[2] <= rxd_sync;
			4'd4:	rxd_data_r[3] <= rxd_sync;
			4'd5:	rxd_data_r[4] <= rxd_sync;
			4'd6:	rxd_data_r[5] <= rxd_sync;
			4'd7:	rxd_data_r[6] <= rxd_sync;
			4'd8:	rxd_data_r[7] <= rxd_sync;
			default:;
			endcase
		else
			rxd_data_r <= rxd_data_r;
		end
	else if(rxd_state == R_STOP)
		rxd_data_r <= rxd_data_r;
	else
		rxd_data_r <= 0;
end

（3）接收完停止标志位后：rxd_flag有效，以及rxd_data更新！

//----------------------------------
//update uart receive data and receive flag signal
always@(posedge clk or negedge rst_n)
begin
	if(!rst_n)
		rxd_data <= 0;
	else if(clken_16bps == 1 &&  rxd_cnt == 4'd9 && smp_cnt == 4'd15)	//Start + 8 Bit + Stop Bit
		rxd_data <= rxd_data_r;
	else
		rxd_data <= rxd_data;
end

always@(posedge clk or negedge rst_n)
begin
	if(!rst_n)
		rxd_flag <= 0;
	else if(clken_16bps == 1 &&  rxd_cnt == 4'd9 && smp_cnt == 4'd15)	//Start + 8 Bit + Stop Bit
		rxd_flag <= 1;
	else
		rxd_flag <= 0;
end

3. 串口数据发送时序分析：

（1）状态机：R_IDLE; R_SEND（R_SEND发送起始位，数据位，结束位）

//---------------------------------------

//parameter of uart transfer

localparam T_IDLE = 2'b0; //test the flag to transfer data

localparam T_SEND = 2'b1; //uart transfer data

reg [1:0] txd_state;

（2）数据与rxd_cnt：0-9完全对齐，因此采用了组合逻辑输出，如下：

//--------------------------------------
//uart 8 bit data transfer
always@(*)
begin
	if(txd_state == T_SEND)
		case(txd_cnt)
		4'd0:	txd = 0;
		4'd1:	txd = txd_data[0];
		4'd2:	txd = txd_data[1];
		4'd3:	txd = txd_data[2];
		4'd4:	txd = txd_data[3];
		4'd5:	txd = txd_data[4];
		4'd6:	txd = txd_data[5];
		4'd7:	txd = txd_data[6];
		4'd8:	txd = txd_data[7];
		4'd9:	txd = 1;
		default:txd = 1;
		endcase
	else
		txd = 1'b1;	//default state
end

（3）在发送完停止位后：txd_flag有效！

//-------------------------------------
//Capture the falling of data transfer over
always@(posedge clk or negedge rst_n)
begin
	if(!rst_n)
		txd_flag <= 0;
	else if(clken_16bps == 1 &&  txd_cnt == 4'd9 && smp_cnt == 4'd15)	//Totally 8 data is done
		txd_flag <= 1;
	else
		txd_flag <= 0;
end