昨天在论坛上看到有人帖出了他写的并串转换VHDL代码，但是他自己说有问题，但是不知道怎么改。我大概看了一下，发现思路还是比较乱的。于是就写下了我自己的并串转换代码。这个并串转换代码是依靠同步状态机来实现其控制的。其实并串转换在实际的电路中使用还是比较多的，尤其在通信线路方面的复用和分解方面，原理上就是一个串并转换和并串转换的过程。举个简单的例子，计算机串口发送数据的过程，如果满足发送条件了，其实就是一个并串转换的过程了。好了，废话不说，看代码就是。

--------------------------------------------------------------------------------
-- Engineer: skycanny
-- Module Name: p2s - Behavioral
-- Tool versions: ISE7.1
-- Description: This module is designed to implement parallel to serial conversion
--------------------------------------------------------------------------------
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity p2s is
port(
reset : in std_logic;
clk : in std_logic;
start : in std_logic; --low active,data_in valid
data_in : in std_logic_vector(7 downto 0);
data_valid : out std_logic; --high active,output data valid
ready : out std_logic; --low active,ready to recieve data
q : out std_logic
);
end p2s;

architecture Behavioral of p2s is

signal reg : std_logic_vector(7 downto 0);
signal cnt : std_logic_vector(3 downto 0);
signal reg_en : std_logic;
signal shift_start : std_logic;
type state is (idle,recieve,shift,finish);
signal current_state, next_state : state;

begin
counter: process(reset,clk,shift_start)
begin
if(reset = '0') then
cnt <= (others => '0');
elsif(clk'event and clk = '1') then
if(shift_start = '0') then
cnt <= cnt 1;
else
cnt <= (others => '0');
end if;
end if;
end process counter;

fsm: block
begin
sync: process(reset,clk)
begin
if(reset= '0') then
current_state <= idle;
elsif(clk'event and clk = '1') then
current_state <= next_state;
end if;
end process sync;

comb: process(current_state,cnt,start)
begin
case current_state is
when idle =>
ready <= '0';
reg_en <= '1';
shift_start <= '1';
data_valid <= '1';
if(start = '0') then
reg_en <= '0';
next_state <= recieve;
else
next_state <= idle;
end if;
when recieve =>
reg_en <= '1';
ready <= '1';
data_valid <= '0';
shift_start <= '0';
next_state <= shift;
when shift =>
reg_en <= '1';
ready <= '1';
data_valid <= '0';
if(cnt = 8) then
shift_start <= '1';
next_state <= finish;
else
shift_start <= '0';
next_state <= shift;
end if;
when finish =>
reg_en <= '1';
ready <= '0';
data_valid <= '1';
shift_start <= '1';
next_state <= idle;
when others =>
next_state <= idle;
end case;
end process comb;

end block fsm;

data_channel: process(reset,clk)
begin
if(reset = '0') then
reg <= (others => '0');
q <= '0';
elsif(clk'event and clk = '1') then
if(reg_en = '0') then
reg <= data_in;
elsif(shift_start = '0') then
q <= reg(7);
for i in 7 downto 1 loop --shift register
reg(i) <= reg(i - 1);
end loop;
reg(0) <= '0';
else
q <= '0';
end if;
end if;
end process data_channel;

end Behavioral;

写完一看，一个并串转换居然搞了这么大，有点失败。但是整个代码已经通过了后仿真，而且思路还是比较清楚的，可靠性和稳定性方面也应该没有问题滴，呵呵。不过说老实话，里面有些信号是确实可以去掉的，不过后来就懒得改了。如果谁想要实际的工程中用的话可以改一下。好了，收工，今天还要去未央湖。。。

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