本模塊的功能是驗證實現(xiàn)和PC機進行基本的串口通信的功能。需要在PC機上安裝一個串口調(diào)試工具來驗證程序的功能。程序?qū)崿F(xiàn)了一個收發(fā)一幀10個bit（即無奇偶校驗位）的串口控制器，10個bit是1位起始位，8個數(shù)據(jù)位，1個結(jié)束位。串口的波特律由程序中定義的div_par參數(shù)決定，更改該參數(shù)可以實現(xiàn)相應(yīng)的波特率。程序當(dāng)前設(shè)定的div_par 的值是0x104，對應(yīng)的波特率是9600。用一個8倍波特率的時鐘將發(fā)送或接受每一位bit的周期時間劃分為8個時隙以使通信同步。

程序的工作過程是：串口處于全雙工工作狀態(tài)，按動key2，CPLD向PC發(fā)送皐elcome"字符串（串口調(diào)試工具設(shè)成按ASCII碼接受方式）；PC可隨時向CPLD發(fā)送0-F的十六進制數(shù)據(jù)，CPLD接受后顯示在7段數(shù)碼管上。

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

ENTITY UART IS
PORT (
clk : IN std_logic;
rst : IN std_logic;
rxd : IN std_logic; 串行數(shù)據(jù)接收端
txd : OUT std_logic; 串行數(shù)據(jù)發(fā)送端
en : OUT std_logic_vector(7 downto 0); 數(shù)碼管使能
seg_data : OUT std_logic_vector(7 DOWNTO 0); 數(shù)碼管數(shù)據(jù)
key_input : IN std_logic 按鍵輸入
);
END UART;

ARCHITECTURE arch OF UART IS
//////////////////inner reg////////////////////
SIGNAL div_reg : std_logic_vector(15 DOWNTO 0);分頻計數(shù)器，分頻值由波特率決定。分頻后得到頻率8倍波特率的時鐘
SIGNAL div8_tras_reg : std_logic_vector(2 DOWNTO 0);該寄存器的計數(shù)值對應(yīng)發(fā)送時當(dāng)前位于的時隙數(shù)
SIGNAL div8_rec_reg : std_logic_vector(2 DOWNTO 0); 寄存器的計數(shù)值對應(yīng)接收時當(dāng)前位于的時隙數(shù)
SIGNAL state_tras : std_logic_vector(3 DOWNTO 0); 發(fā)送狀態(tài)寄存器
SIGNAL state_rec : std_logic_vector(3 DOWNTO 0); 接受狀態(tài)寄存器
SIGNAL clkbaud_tras : std_logic; 以波特率為頻率的發(fā)送使能信號
SIGNAL clkbaud_rec : std_logic; 以波特率為頻率的接受使能信號
SIGNAL clkbaud8x : std_logic; 以8倍波特率為頻率的時鐘，它的作用是將發(fā)送或接受一個bit的時鐘周期分為8個時隙
SIGNAL recstart : std_logic; 開始發(fā)送標(biāo)志
SIGNAL recstart_tmp : std_logic; 開始接受標(biāo)志
SIGNAL trasstart : std_logic;
SIGNAL rxd_reg1 : std_logic; 接收寄存器1
SIGNAL rxd_reg2 : std_logic; 接收寄存器2，因為接收數(shù)據(jù)為異步信號，故用兩級緩存
SIGNAL txd_reg : std_logic; 發(fā)送寄存器
SIGNAL rxd_buf : std_logic_vector(7 DOWNTO 0);接受數(shù)據(jù)緩存
SIGNAL txd_buf : std_logic_vector(7 DOWNTO 0);發(fā)送數(shù)據(jù)緩存
SIGNAL send_state : std_logic_vector(2 DOWNTO 0);每次按鍵給PC發(fā)送"Welcome"字符串，這是發(fā)送狀態(tài)寄存器
SIGNAL cnt_delay : std_logic_vector(19 DOWNTO 0);延時去抖計數(shù)器
SIGNAL start_delaycnt : std_logic; 開始延時計數(shù)標(biāo)志
SIGNAL key_entry1 : std_logic; 確定有鍵按下曛?
SIGNAL key_entry2 : std_logic; 確定有鍵按下標(biāo)志
//////////////////////////////////////////////
C*TANT div_par : std_logic_vector(15 DOWNTO 0) := "0000000100000100";
分頻參數(shù)，其值由對應(yīng)的波特率計算而得，按此參數(shù)分頻的時鐘頻率是波倍特率的8倍，此處值對應(yīng)9600的波特率，即分頻出的時鐘頻率是9600*8
SIGNAL txd_xhdl3 : std_logic;

BEGIN
en <="01010101" ;7段數(shù)碼管使能信號賦值
txd <= txd_xhdl3;
txd_xhdl3 <= txd_reg ;

PROCESS(clk,rst)
BEGIN

IF (NOT rst = ’1’) THEN
cnt_delay <= "00000000000000000000";
start_delaycnt <= ’0’;
ELSIF(clk’EVENT AND clk=’1’)THEN
IF (start_delaycnt = ’1’) THEN
IF (cnt_delay /= "11000011010100000000") THEN
cnt_delay <= cnt_delay + "00000000000000000001";
ELSE
cnt_delay <= "00000000000000000000";
start_delaycnt <= ’0’;
END IF;
ELSE
IF ((NOT key_input=’1’) AND (cnt_delay = "00000000000000000000")) THEN
start_delaycnt <= ’1’;
END IF;
END IF;
END IF;
END PROCESS;

PROCESS(clk,rst)
BEGIN

IF (NOT rst = ’1’) THEN
key_entry1 <= ’0’;
ELSIF(clk’EVENT AND clk=’1’)THEN
IF (key_entry2 = ’1’) THEN
key_entry1 <= ’0’;
ELSE
IF (cnt_delay = "11000011010100000000") THEN
IF (NOT key_input = ’1’) THEN
key_entry1 <= ’1’;
END IF;
END IF;
END IF;
END IF;
END PROCESS;

PROCESS(clk,rst)
BEGIN

IF (NOT rst = ’1’) THEN
div_reg <= "0000000000000000";
ELSIF(clk’EVENT AND clk=’1’)THEN
IF (div_reg = div_par - "0000000000000001") THEN
div_reg <= "0000000000000000";
ELSE
div_reg <= div_reg + "0000000000000001";
END IF;
END IF;
END PROCESS;

PROCESS(clk,rst) 分頻得到8倍波特率的時鐘
BEGIN

IF (NOT rst = ’1’) THEN
clkbaud8x <= ’0’;
ELSIF(clk’EVENT AND clk=’1’)THEN
IF (div_reg = div_par - "0000000000000001") THEN
clkbaud8x <= NOT clkbaud8x;
END IF;
END IF;
END PROCESS;

PROCESS(clkbaud8x,rst)
BEGIN
IF (NOT rst = ’1’) THEN
div8_rec_reg <= "000";
ELSE IF(clkbaud8x’EVENT AND clkbaud8x = ’1’) THEN
IF (recstart = ’1’) THEN 接收開始標(biāo)志
div8_rec_reg <= div8_rec_reg + "001";接收開始后，時隙數(shù)在8倍波特率的時鐘下加1循環(huán)
END IF;
END IF;
END IF;
END PROCESS;

PROCESS(clkbaud8x,rst)
BEGIN
IF (NOT rst = ’1’) THEN
div8_tras_reg <= "000";
ELSE IF(clkbaud8x’EVENT AND clkbaud8x = ’1’) THEN
IF (trasstart = ’1’) THEN
div8_tras_reg <= div8_tras_reg + "001";發(fā)送開始后，時隙數(shù)在8倍波特率的時鐘下加1循環(huán)
END IF;
END IF;
END IF;
END PROCESS;

PROCESS(div8_rec_reg)
BEGIN
IF (div8_rec_reg = "111") THEN
clkbaud_rec <= ’1’; -在第7個時隙，接收
ELSE
clkbaud_rec <= ’0’;
END IF;
END PROCESS;

PROCESS(div8_tras_reg)
BEGIN
IF (div8_tras_reg = "111") THEN
clkbaud_tras <= ’1’; 在第7個時隙，發(fā)送使能信號有效，將數(shù)據(jù)發(fā)出
ELSE
clkbaud_tras <= ’0’;
END IF;
END PROCESS;

PROCESS(clkbaud8x,rst)
BEGIN
IF (NOT rst = ’1’) THEN
txd_reg <= ’1’;
trasstart <= ’0’;
txd_buf <= "00000000";
state_tras <= "0000";
send_state <= "000";
key_entry2 <= ’0’;
ELSE IF(clkbaud8x’EVENT AND clkbaud8x = ’1’) THEN
IF (NOT key_entry2 = ’1’) THEN
IF (key_entry1 = ’1’) THEN
key_entry2 <= ’1’;
txd_buf <= "01110111"; "w"
END IF;
ELSE
CASE state_tras IS
WHEN "0000" => 發(fā)送起始位
IF ((NOT trasstart=’1’) AND (send_state < "111") ) THEN
trasstart <= ’1’;
ELSE
IF (send_state < "111") THEN
IF (clkbaud_tras = ’1’) THEN
txd_reg <= ’0’;
state_tras <= state_tras + "0001";
END IF;
ELSE
key_entry2 <= ’0’;
state_tras <= "0000";
END IF;
END IF;
WHEN "0001" => 發(fā)送第1位
IF (clkbaud_tras = ’1’) THEN
txd_reg <= txd_buf(0);
txd_buf(6 DOWNTO 0) <= txd_buf(7 DOWNTO 1);
state_tras <= state_tras + "0001";
END IF;
WHEN "0010" => 發(fā)送第2位
IF (clkbaud_tras = ’1’) THEN
txd_reg <= txd_buf(0);
txd_buf(6 DOWNTO 0) <= txd_buf(7 DOWNTO 1);
state_tras <= state_tras + "0001";
END IF;
WHEN "0011" => 發(fā)送第3位
IF (clkbaud_tras = ’1’) THEN
txd_reg <= txd_buf(0);
txd_buf(6 DOWNTO 0) <= txd_buf(7 DOWNTO 1);
state_tras <= state_tras + "0001";
END IF;
WHEN "0100" => 發(fā)送第4位
IF (clkbaud_tras = ’1’) THEN
txd_reg <= txd_buf(0);
txd_buf(6 DOWNTO 0) <= txd_buf(7 DOWNTO 1);
state_tras <= state_tras + "0001";
END IF;
WHEN "0101" => 發(fā)送第5位
IF (clkbaud_tras = ’1’) THEN
txd_reg <= txd_buf(0);
txd_buf(6 DOWNTO 0) <= txd_buf(7 DOWNTO 1);
state_tras <= state_tras + "0001";
END IF;
WHEN "0110" => 發(fā)送第6位
IF (clkbaud_tras = ’1’) THEN
txd_reg <= txd_buf(0);
txd_buf(6 DOWNTO 0) <= txd_buf(7 DOWNTO 1);
state_tras <= state_tras + "0001";
END IF;
WHEN "0111" => 發(fā)送第7位
IF (clkbaud_tras = ’1’) THEN
txd_reg <= txd_buf(0);
txd_buf(6 DOWNTO 0) <= txd_buf(7 DOWNTO 1);
state_tras <= state_tras + "0001";
END IF;
WHEN "1000" => 發(fā)送第8位
IF (clkbaud_tras = ’1’) THEN
txd_reg <= txd_buf(0);
txd_buf(6 DOWNTO 0) <= txd_buf(7 DOWNTO 1);
state_tras <= state_tras + "0001";
END IF;
WHEN "1001" => 發(fā)送停止位
IF (clkbaud_tras = ’1’) THEN
txd_reg <= ’1’;
txd_buf <= "01010101";
state_tras <= state_tras + "0001";
END IF;
WHEN "1111" =>
IF (clkbaud_tras = ’1’) THEN
state_tras <= state_tras + "0001";
send_state <= send_state + "001";
trasstart <= ’0’;
CASE send_state IS
WHEN "000" =>
txd_buf <= "01100101"; "e"
WHEN "001" =>
txd_buf <= "01101100"; "l"
WHEN "010" =>
txd_buf <= "01100011"; "c"
WHEN "011" =>
txd_buf <= "01101111"; "o"
WHEN "100" =>
txd_buf <= "01101101"; "m"
WHEN "101" =>
txd_buf <= "01100101"; "e"
WHEN OTHERS =>
txd_buf <= "00000000";

END CASE;
END IF;
WHEN OTHERS =>
IF (clkbaud_tras = ’1’) THEN
state_tras <= state_tras + "0001";
trasstart <= ’1’;
END IF;

END CASE;
END IF;
END IF;
END IF;
END PROCESS;

PROCESS(clkbaud8x,rst) 接受PC機的數(shù)據(jù)
BEGIN
IF (NOT rst = ’1’) THEN
rxd_reg1 <= ’0’;
rxd_reg2 <= ’0’;
rxd_buf <= "00000000";
state_rec <= "0000";
recstart <= ’0’;
recstart_tmp <= ’0’;
ELSE IF(clkbaud8x’EVENT AND clkbaud8x = ’1’) THEN
rxd_reg1 <= rxd;
rxd_reg2 <= rxd_reg1;
IF (state_rec = "0000") THEN
IF (recstart_tmp = ’1’) THEN
recstart <= ’1’;
recstart_tmp <= ’0’;
state_rec <= state_rec + "0001";
ELSE
IF ((NOT rxd_reg1 AND rxd_reg2) = ’1’) THEN 檢測到起始位的下降沿，進入接受狀態(tài)
recstart_tmp <= ’1’;
END IF;
END IF;
ELSE
IF (state_rec >= "0001" AND state_rec<="1000") THEN
IF (clkbaud_rec = ’1’) THEN
rxd_buf(7) <= rxd_reg2;
rxd_buf(6 DOWNTO 0) <= rxd_buf(7 DOWNTO 1);
state_rec <= state_rec + "0001";
END IF;
ELSE
IF (state_rec = "1001") THEN
IF (clkbaud_rec = ’1’) THEN
state_rec <= "0000";
recstart <= ’0’;
END IF;
END IF;
END IF;
END IF;
END IF;
END IF;
END PROCESS;

PROCESS(rxd_buf) 將接受的數(shù)據(jù)用數(shù)碼管顯示出來
BEGIN
CASE rxd_buf IS
WHEN "00110000" =>
seg_data <= "00000011"; 0
WHEN "00110001" =>
seg_data <= "10011111"; 1
WHEN "00110010" =>
seg_data <= "00100101"; 2
WHEN "00110011" =>
seg_data <= "00001101"; 3
WHEN "00110100" =>
seg_data <= "10011001"; 4
&nbs p; WHEN "00110101" =>
seg_data <= "01001001"; 5
WHEN "00110110" =>
seg_data <= "01000001"; 6
WHEN "00110111" =>
seg_data <= "00011111"; 7
WHEN "00111000" =>
seg_data <= "00000001"; 8
WHEN "00111001" =>
seg_data <= "00001001"; 9
WHEN "01000001" =>
seg_data <= "00010001"; A
WHEN "01000010" =>
seg_data <= "11000001"; B
WHEN "01000011" =>
seg_data <= "01100011"; C
WHEN "01000100" =>
seg_data <= "10000101"; D
WHEN "01000101" =>
seg_data <= "01100001"; E
WHEN "01000110" =>
seg_data <= "01110001"; F
WHEN OTHERS =>
seg_data <= "11111111";
END CASE;
END PROCESS;
END arch;