串口的波特律由程序中定义的p_par参数决定，更改该参数可以实现相应的波特率。程序当前设定的p_par 的值是0x104，对应的波特率是9600。用一个8倍波特率的时钟将发送或接受每一位bit的周期时间，划分为8个时隙以使通信同步.
程序的工作过程是：串口处于全双工工作状态，按动key2，CPLD向PC发送皐elcome"字符串（串口调试工具设成按ASCII码接受方式）；PC可随时向CPLD发送0-F的十六进制数据，CPLD接受后显示在7段数码管上。
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; --串行数据接收端
txd : OUT std_logic; --串行数据发送端
en : OUT std_logic_vector(7 downto 0); -- 数码管使能
seg_data : OUT std_logic_vector(7 DOWNTO 0); --数码管数据
key_input : IN std_logic --按键输入
);
END UART;
ARCHITECTURE arch OF UART IS
--//////////////////inner reg////////////////////
SIGNAL p_reg : std_logic_vector(15 DOWNTO 0);--分频计数器，分频值由波特率决定。分频后得到频率8倍波特率的时钟
SIGNAL p8_tras_reg : std_logic_vector(2 DOWNTO 0);--该寄存器的计数值对应发送时当前位于的时隙数
SIGNAL p8_rec_reg : std_logic_vector(2 DOWNTO 0); --寄存器的计数值对应接收时当前位于的时隙数
SIGNAL state_tras : std_logic_vector(3 DOWNTO 0); -- 发送状态寄存器
SIGNAL state_rec : std_logic_vector(3 DOWNTO 0); -- 接受状态寄存器
SIGNAL clkbaud_tras : std_logic; --以波特率为频率的发送使能信号
SIGNAL clkbaud_rec : std_logic; --以波特率为频率的接受使能信号
SIGNAL clkbaud8x : std_logic; --以8倍波特率为频率的时钟，它的作用是将发送或接受一个bit的时钟周期分为8个时隙
SIGNAL recstart : std_logic; -- 开始发送标志
SIGNAL recstart_tmp : std_logic; --开始接受标志
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，因为接收数据为异步信号，故用两级缓存
SIGNAL txd_reg : std_logic; --发送寄存器
SIGNAL rxd_buf : std_logic_vector(7 DOWNTO 0);--接受数据缓存
SIGNAL txd_buf : std_logic_vector(7 DOWNTO 0);--发送数据缓存
SIGNAL send_state : std_logic_vector(2 DOWNTO 0);--每次按键给PC发送"Welcome"字符串，这是发送状态寄存器
SIGNAL cnt_delay : std_logic_vector(19 DOWNTO 0);--延时去抖计数器
SIGNAL start_delaycnt : std_logic; --开始延时计数标志
SIGNAL key_entry1 : std_logic; --确定有键按下曛?
SIGNAL key_entry2 : std_logic; --确定有键按下标志
--//////////////////////////////////////////////
CONSTANT p_par : std_logic_vector(15 DOWNTO 0) := "0000000100000100";
--分频参数，其值由对应的波特率计算而得，按此参数分频的时钟频率是波倍特率的8倍，此处值对应9600的波特率，即分频出的时钟频率是9600*8
SIGNAL txd_xhdl3 : std_logic;
BEGIN
en <="01010101" ;--7段数码管使能信号赋值
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
p_reg <= "0000000000000000";
ELSIF(clk'EVENT AND clk='1')THEN
IF (p_reg = p_par - "0000000000000001") THEN
p_reg <= "0000000000000000";
ELSE
p_reg <= p_reg + "0000000000000001";
END IF;
END IF;
END PROCESS;
PROCESS(clk,rst) --分频得到8倍波特率的时钟
BEGIN
IF (NOT rst = '1') THEN
clkbaud8x <= '0';
ELSIF(clk'EVENT AND clk='1')THEN
IF (p_reg = p_par - "0000000000000001") THEN
clkbaud8x <= NOT clkbaud8x;
END IF;
END IF;
END PROCESS;
PROCESS(clkbaud8x,rst)
BEGIN
IF (NOT rst = '1') THEN
p8_rec_reg <= "000";
ELSE IF(clkbaud8x'EVENT AND clkbaud8x = '1') THEN
IF (recstart = '1') THEN --接收开始标志
p8_rec_reg <= p8_rec_reg + "001";--接收开始后，时隙数在8倍波特率的时钟下加1循环
END IF;
END IF;
END IF;
END PROCESS;
PROCESS(clkbaud8x,rst)
BEGIN
IF (NOT rst = '1') THEN
p8_tras_reg <= "000";
ELSE IF(clkbaud8x'EVENT AND clkbaud8x = '1') THEN
IF (trasstart = '1') THEN
p8_tras_reg <= p8_tras_reg + "001";--发送开始后，时隙数在8倍波特率的时钟下加1循环
END IF;
END IF;
END IF;
END PROCESS;
PROCESS(p8_rec_reg)
BEGIN
IF (p8_rec_reg = "111") THEN
clkbaud_rec <= '1'; ---在第7个时隙，接收
ELSE
clkbaud_rec <= '0';
END IF;
END PROCESS;
PROCESS(p8_tras_reg)
BEGIN
IF (p8_tras_reg = "111") THEN
clkbaud_tras <= '1'; --在第7个时隙，发送使能信号有效，将数据发出
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" => --发送起始位
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" => --发送第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" => --发送第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" => --发送第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" => --发送第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" => --发送第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" => --发送第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" => --发送第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" => --发送第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" => --发送停止位
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机的数据
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 --检测到起始位的下降沿，进入接受状态
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) --将接受的数据用数码管显示出来
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;