ssdt.vhd

同步串行数据发送电路SSDT的基本功能是将并行数据转换成串行数据并进行同步发送。系统写入和读出时序完全兼容Intel8086时序。 系统以同步信号开始连续发送四个字节

Library IEEE;
USE IEEE.std_logic_1164.all; 
USE IEEE.STD_LOGIC_UNSIGNED.all;

Entity SSDT IS
Port(  
      CLK2M:in std_logic; 
      FS:in std_logic;     
      RST:in std_logic;             
      CS:in std_logic; 
      WR:in std_logic;
      RD:in std_logic;	 
      A0:in std_logic;
      D_IN: in std_logic_vector(7 downto 0);
      D_OUT: out std_logic_vector(7 downto 0);
      TXD: out std_logic );   
End SSDT;

Architecture rtl of SSDT  is 
  Type state is(IDLE,TRAN,FREE);  
  signal C_STATE, N_STATE: STATE;
  signal INSERT_ZERO: std_logic;
  signal TXD_SHIFT: std_logic_vector(4 downto 0);
  signal TSR:std_logic_vector(7 downto 0);
  signal TSR_NUM:std_logic_vector(7 downto 0);	 
  signal BYTE_NUM:std_logic_vector(1 downto 0);
  signal TSR_EMPTY:std_logic;
  signal THR:std_logic_vector(7 downto 0);
  signal THR_EMPTY:std_logic;
  signal IDLER:std_logic_vector(7 downto 0);
  
  signal D_OUT_reg:std_logic_vector(7 downto 0);
  signal TXD_reg:std_logic;
  signal TRAN_IDLE_reg:std_logic;
 
Begin  	  
Process(RST,CLK2M)	   	                    --produce the C_STATE;
Begin	  
    if RST='0' then
        C_STATE<=FREE;
    elsif( CLK2M'event and CLK2M='1' )then 
        C_STATE<=N_state;

    end if;	 
End process; 	
  
Process(FS,C_STATE,TSR_NUM,BYTE_NUM)			       --produce the N_STATE:OK
Begin	 	  
case C_STATE is
    when FREE=>
        if FS='0' then	 
	    N_STATE<=TRAN; 
	else 
	    N_STATE<=FREE;
	end if;
    when TRAN=>
        if (TSR_NUM="11111111" and BYTE_NUM="11" ) then
            if(not(TSR(7)='1' and TXD_SHIFT="11110")) then 
	        N_STATE<=IDLE;
	    else 
	        N_STATE<=TRAN;
	    end if;
	elsif(TRAN_IDLE_reg='1') then
	    N_STATE<=IDLE;
	else
            N_STATE<=TRAN;
        end if;
    when others=>                                  -- idle
        if FS='0' then	 
	    N_STATE<=TRAN; 
	else 
	    N_STATE<=IDLE;
	end if;
end case; 
end process;


Process(CLK2M)			     -- TRAN_IDLE_reg OK  
Begin 
    if(ClK2M'event and CLK2M='1') then
        if (TSR_NUM="11111111" and BYTE_NUM="11" and (TSR(7)='1' and TXD_SHIFT="11110") ) then 
	    TRAN_IDLE_reg<='1';
        else
            TRAN_IDLE_reg<='0';
        end if;
    end if;
End Process;    

Process(TXD_SHIFT,C_STATE)			     -- INSERT_ZERO：表示插入0的标志 OK  
Begin 
    if (TXD_SHIFT="11111")  then
        INSERT_ZERO<='1';			   
    else
        INSERT_ZERO<='0';
    end if;
End Process;  
                                      

Process(CLK2M,RST)			     -- TXD and TXD_SHIFT:发送寄存器  OK
Begin 	 
if(RST='0') then  
     TXD_reg<='0';
     TXD_SHIFT<="00000";
elsif(ClK2M'event and CLK2M='1') then
    if (FS='0')  then 
       TXD_reg<=TSR(7);
       TXD_SHIFT<=TSR(7)&TXD_SHIFT(4 downto 1);
    else 
        if(C_STATE=TRAN and INSERT_ZERO='0') then
            TXD_reg<=TSR(7);
            TXD_SHIFT<=TSR(7)&TXD_SHIFT(4 downto 1);
        elsif(C_STATE=TRAN and INSERT_ZERO='1') then
            TXD_reg<='0';
            TXD_SHIFT<='0'&TXD_SHIFT(4 downto 1);
        else
            TXD_reg<=IDLER(7);
            TXD_SHIFT<='0'&TXD_SHIFT(4 downto 1);
        end if;
    end if;
end if;
End Process;
TXD<=TXD_reg;


Process(CLK2M)			     -- TSR:移位寄存器  OK
Begin 	   
if(ClK2M'event and CLK2M='1') then
    if (FS='0')  then 
       TSR<=TSR(6 downto 0)&'0';
    else 
        if(C_STATE=TRAN) then
            if(TSR_EMPTY='1' and THR_EMPTY='0' ) then
                TSR<=THR;
            elsif(INSERT_ZERO='0') then
                TSR<=TSR(6 downto 0)&'0';
            end if;
        elsif(C_STATE=FREE or C_STATE=IDLE) then
            if(TSR_EMPTY='1' and THR_EMPTY='0' ) then
                TSR<=THR;
            end if;
        end if;
    end if;
end if;
End Process;

Process(CLK2M,RST)			     -- TSR_NUM:实际是个移位寄存器 OK
Begin   
if(RST='0') then
    TSR_NUM<="00000000";  		   
elsif (ClK2M'event and CLK2M='1') then
    if (FS='0')  then 
         TSR_NUM<="11000000";
    elsif(C_STATE=TRAN and INSERT_ZERO='0') then
       case TSR_NUM is 
           when "10000000"=>  TSR_NUM<="11000000";
           when "11000000"=>  TSR_NUM<="11100000";
           when "11100000"=>  TSR_NUM<="11110000";
           when "11110000"=>  TSR_NUM<="11111000";
           when "11111000"=>  TSR_NUM<="11111100";
           when "11111100"=>  TSR_NUM<="11111110";
           when "11111110"=>  TSR_NUM<="11111111";
           when others=>  TSR_NUM<="10000000";        --11111111
       end case;           
    end if;
end if;
End Process;

Process(CLK2M)			     -- BYTE_NUM:寄存器 OK
Begin     		   
if (ClK2M'event and CLK2M='1') then
    if (FS='0')  then 
          BYTE_NUM<="00";
    elsif(TSR_NUM= "11111111" ) then
        case BYTE_NUM is 
            when "00"=>  BYTE_NUM<="01";
            when "01"=>  BYTE_NUM<="10";
            when "10"=>  BYTE_NUM<="11";
            when others=>  BYTE_NUM<="11";       --"11"
        end case;           
    end if;
end if;
End Process;

Process(TSR_NUM)			     -- TSR_EMPTY：表示一个字节8位传送完成的标志 OK
Begin 
    if(RST='0') then 
        TSR_EMPTY<='1';
    elsif (TSR_NUM="11111111")  then
        TSR_EMPTY<='1';			   
    else        
        TSR_EMPTY<='0';
    end if;
End Process;
  
Process(CLK2M)			     -- THR:移位寄存器  OK
Begin 	   
if(RST='0') then 
    THR<="00000000";
elsif (ClK2M'event and CLK2M='1') then
    if(A0='0' and CS='0' and THR_EMPTY='1' and WR='0') then 
        THR<=D_IN;
    end if;
end if;
End Process;

Process(CLK2M,FS)			     -- THR_EMPTY:寄存器  OK
Begin 	   
if(RST='0') then 
    THR_EMPTY<='1';
elsif (ClK2M'event and CLK2M='1') then
    if(A0='0' and CS='0' and WR='0' and THR_EMPTY='1') then 
        THR_EMPTY<='0';
    elsif(TSR_EMPTY='1' and THR_EMPTY='0') then
        THR_EMPTY<='1';
    end if;
end if;
End Process;

Process(CLK2M)			     -- THR_EMPTY Read Out:  OK
Begin 	   
if (ClK2M'event and CLK2M='1') then
    if(A0='0' and CS='0' and RD='0') then 
        if(THR_EMPTY='1') then  
            D_OUT_reg<="00000000";                      -- Empty
        elsif(THR_EMPTY='0') then
            D_OUT_reg<="11111111";                      -- Full
        end if;
    end if;
end if;
End Process;
D_OUT<=D_OUT_reg;


Process(CLK2M,FS)			     -- IDLER:移位寄存器  OK
Begin 	   
if(RST='0') then 
    IDLER<="01111111";
elsif (ClK2M'event and CLK2M='1') then
    if (FS='0')  then 
       IDLER<="01111111";
    elsif(C_STATE=IDLE) then
       IDLER<=IDLER(6 downto 0)&IDLER(7);
    end if;
end if;
End Process;
	  
end rtl;