rxunit.vhd

依元素开发板的示例程序

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

entity RxUnit is
  port (
     Clk    : in  Std_Logic;  -- system clock signal
     Reset  : in  Std_Logic;  -- Reset input
     Enable : in  Std_Logic;  -- Enable input
     RxD    : in  Std_Logic;  -- RS-232 data input
     RxAv   : out Std_Logic;  -- Byte available
     In_data  : out Std_Logic_Vector(7 downto 0)); -- Byte received
end entity;

architecture Behaviour of RxUnit is
  signal RReg    : Std_Logic_Vector(7 downto 0); -- receive register  
  signal RRegL   : Std_Logic;                    -- Byte received
begin
  -- RxAv process
  RxAvProc : process(RRegL,Reset)
  begin
     if Reset = '0' then
        RxAv <= '0';  -- Negate RxAv when RReg read     
     elsif Rising_Edge(RRegL) then
        RxAv <= '1';  -- Assert RxAv when RReg written
     end if;
  end process;
  
  -- Rx Process
  RxProc : process(Clk,Reset,Enable,RxD,RReg)
  variable BitPos : INTEGER range 0 to 10;   -- Position of the bit in the frame
  variable SampleCnt : INTEGER range 0 to 3; -- Count from 0 to 3 in each bit 
  begin
     if Reset = '0' then -- Reset
        RRegL <= '0';
        BitPos := 0;           
     elsif Rising_Edge(Clk) then
        if Enable = '1' then
           case BitPos is
              when 0 => -- idle
                 RRegL <= '0';
                 if RxD = '0' then -- Start Bit
                    SampleCnt := 0;
                    BitPos := 1;
                 end if;
              when 10 => -- Stop Bit
                 BitPos := 0;    -- next is idle
                 RRegL <= '1';   -- Indicate byte received
                 In_data <= RReg;  -- Store received byte
              when others =>
                 if SampleCnt = 1 then -- Sample RxD on 1
                    RReg(BitPos-2) <= RxD; -- Deserialisation
                 end if;
                 if SampleCnt = 3 then -- Increment BitPos on 3
                    BitPos := BitPos + 1;
                 end if;
           end case;
           if SampleCnt = 3 then -- Optionaly (SampleCnt can be free running)
              SampleCnt := 0;
           else   
              sampleCnt := SampleCnt + 1;
           end if;
           
        end if;
     end if;
  end process;
end Behaviour;