src/uart_rx.vhd

   1 library ieee;
   2 use ieee.std_logic_1164.all;
   3 use ieee.numeric_std.all;
   4 use work.gen_pkg.all;
   5
   6 entity uart_rx is
   7         generic (
   8                 CLK_FREQ : integer := 33000000;
   9                 BAUDRATE : integer := 115200
  10         );
  11         port(
  12                 sys_clk : in std_logic;
  13                 sys_res_n : in std_logic;
  14                 rxd : in std_logic;
  15                 rx_data : out std_logic_vector(7 downto 0);
  16                 rx_new : out std_logic
  17         );
  18 end entity uart_rx;
  19
  20 architecture beh of uart_rx is
  21         constant BAUD : integer := CLK_FREQ/BAUDRATE;
  22
  23         type STATE_UART_RX is (IDLE, STARTBIT, DBITS, STOPBIT);
  24         signal state : STATE_UART_RX;
  25
  26         signal startbitdetection : std_logic_vector(1 downto 0);
  27         signal bitcnt : integer range 0 to 7;
  28         signal baudcnt : integer range 0 to BAUD + 2;
  29         signal rx_data_int : std_logic_vector(7 downto 0);
  30 begin
  31         rx_data <= rx_data_int;
  32         process(sys_clk, sys_res_n)
  33         begin
  34                 if (sys_res_n = '0') then
  35                         state <= IDLE;
  36                         rx_data_int <= (others => '0');
  37                         rx_new <= '0';
  38                         bitcnt <= 0;
  39                         baudcnt <= 0;
  40                         startbitdetection <= b"11";
  41                 elsif rising_edge(sys_clk) then
  42                         startbitdetection <= startbitdetection(0) & rxd;
  43                         rx_new <= '0';
  44                         baudcnt <= baudcnt + 1;
  45                         case state is
  46                                 when IDLE =>
  47                                         baudcnt <= 0;
  48                                         -- bei fallender flanke koennte starbit folgen
  49                                         if startbitdetection = b"10" then
  50                                                 state <= STARTBIT;
  51                                         end if;
  52                                 when STARTBIT =>
  53                                         rx_data_int <= (others => '0');
  54
  55                                         -- halbe BAUDTIME warten, um immer in der mitte abzutasten
  56                                         -- vgl. http://upload.wikimedia.org/wikipedia/de/d/de/RS-232_timing.png
  57                                         if baudcnt = BAUD/2 then
  58                                                 baudcnt <= 0;
  59                                                 if rxd = '0' then
  60                                                         -- starbit (= '0')? dann kommen daten
  61                                                         state <= DBITS;
  62                                                         bitcnt <= 0;
  63                                                         baudcnt <= 0;
  64                                                 else
  65                                                         -- sonst war das nix...
  66                                                         state <= IDLE;
  67                                                 end if;
  68                                         end if;
  69                                 when DBITS =>
  70                                         if baudcnt = BAUD then
  71                                                 baudcnt <= 0;
  72                                                 -- bitorder beachten
  73                                                 rx_data_int <= rxd & rx_data_int(7 downto 1);
  74
  75                                                 if bitcnt = 7 then
  76                                                         state <= STOPBIT;
  77                                                 else
  78                                                         bitcnt <= bitcnt + 1;
  79                                                 end if;
  80                                         end if;
  81                                 when STOPBIT =>
  82                                         if baudcnt = BAUD then
  83                                                 state <= IDLE;
  84                                                 if rxd = '1' then
  85                                                         rx_new <= '1';
  86                                                 end if;
  87                                         end if;
  88                         end case;
  89                 end if;
  90         end process;
  91 end architecture beh;