rcvr.v

利用VHDL实现CPLD（EPM240T100C5）的串口接收程序

/******************************************************************************
*
*    File Name:  rcvr.v
*      Version:  1.1
*         Date:  January 22, 2000
*        Model:  Receiver Chip
*
*      Company:  think studio
*
******************************************************************************/

`timescale 1 ns / 1 ns

module rcvr (dout,data_ready,framing_error,parity_error,rxd,clk50,rst) ;

input rxd ;
input clk50 ;
input rst ;
output [7:0] dout ;
output data_ready ;
output framing_error ;
output parity_error ;

reg rxd1 ;
reg rxd2 ;
reg clk1x_enable ;
reg [3:0] clkdiv ;
reg [7:0] rsr ;
reg [7:0] rbr ;
reg [3:0] no_bits_rcvd ;

reg data_ready ;

reg parity ;
reg parity_error ;
reg framing_error ;

reg clk16x;
reg[7:0] clkdiv2;
reg clk;

wire clk1x ;

//assign dout = !rdn ? rbr : 8'bz ;
assign dout = rbr;

always @(posedge clk50)
begin
	clk <= !clk;
end

always @(posedge clk or posedge rst)
begin
	if (rst)
	begin
	clk16x <= 1'b0 ;
	clkdiv2 <= 8'h00 ;
	end
	else
	begin
		if(clkdiv2 == 8'h4b)
		begin
			clk16x <= !clk16x ;
			clkdiv2 <= 8'h00 ;
		end
		else
			clkdiv2 <= clkdiv2 + 1;
	end
end

always @(posedge clk16x or posedge rst)
begin
if (rst)
begin
rxd1 <= 1'b1 ;
rxd2 <= 1'b1 ;
end
else 
begin
rxd1 <= rxd ;
rxd2 <= rxd1 ;
end
end

always @(posedge clk16x or posedge rst)
begin
if (rst)
clk1x_enable <= 1'b0;
else if (!rxd1 && rxd2)
clk1x_enable <= 1'b1 ;
else if (no_bits_rcvd == 4'b1100)
clk1x_enable <= 1'b0 ;
end

always @(posedge clk16x or posedge rst)
begin
if (rst)
data_ready = 1'b0 ;
else
if (no_bits_rcvd == 4'b1011)
data_ready = 1'b1 ;
end

always @(posedge clk16x or posedge rst)
begin
if (rst)
clkdiv = 4'b0000 ;
else if (clk1x_enable)
clkdiv = clkdiv +1 ;
end

assign clk1x = clkdiv[3] ;

always @(posedge clk1x or posedge rst)
if (rst)
begin
rsr <= 8'b0 ;
rbr <= 8'b0 ;
parity <= 1'b1 ;
framing_error = 1'b0 ;
parity_error = 1'b0 ;
end
else 
begin
if (no_bits_rcvd >= 4'b0001 && no_bits_rcvd <= 4'b1000) 
begin
rsr[0] <= rxd2 ;
rsr[7:1] <= rsr[6:0] ;
parity <= parity ^ rsr[7] ;
end
else if (no_bits_rcvd == 4'b1001)
begin
rbr <= rsr ;
end
else if (!parity) 
parity_error = 1'b1 ;
else if ((no_bits_rcvd == 4'b1010) && (rxd2 != 1'b1))
framing_error = 1'b1 ;
else
framing_error = 1'b0 ;
end


always @(posedge clk1x or posedge rst or negedge clk1x_enable)
if (rst)
no_bits_rcvd = 4'b0000;
else
if (!clk1x_enable)
no_bits_rcvd = 4'b0000 ;
else
no_bits_rcvd = no_bits_rcvd + 1 ;

endmodule