一个老外写的DDS（VHDL)的例子,很经典
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-- DDFS.vhd ------------------------------------- -- Direct Digital Freq. Synthesis -- ------------------------------------- -- (c) Bert Cuzeau, ALSE - info@alse-fr.com -- May be reproduced provided that copyright above remains. -- We use one of the symetries in the sine function, -- so the lookup table is re-used twice (128 entries table) -- The Sine Table is built by a C program... ------------------------------------- -- Design IOs : -- Clk : Global Clock input -- Rst : Global Reset input -- Freq_data : 8-bit frequency control vector -- from DIP switches on the board. -- Dout : is signed 8-bit output to the DAC. -- ----------------------------------------------- library IEEE; use IEEE.std_logic_1164.all; use IEEE.numeric_std.all; -- ----------------------------------------------- Entity DDFS is -- ----------------------------------------------- Port ( CLK : in std_logic; RST : in std_logic; Freq_Data : in std_logic_vector (7 downto 0); Dout : out std_logic_vector (7 downto 0) ); end DDFS; -- ----------------------------------------------- Architecture RTL of DDFS is -- ----------------------------------------------- signal Address : unsigned (6 downto 0); signal Result : std_logic_vector (7 downto 0); signal Accum : unsigned (28 downto 0); -- we want very low Frequencies ! signal Sign : std_logic; begin -- Signed Accumulator -- ------------------ Acc: process (CLK,RST) begin if RST='1' then Accum '0'); elsif rising_edge(CLK) then Accum <= Accum + unsigned(Freq_Data); end if; END process acc; Sign <= Accum(Accum'high); -- MSB -- Lookup Table Index calculation -- ------------------------------ Address <= unsigned(Accum(Accum'high-1 downto Accum'high-Address'length)); -- SINE Look-Up TABLE -- ------------------- -- Inference of an Asynchronous Rom. -- A synchronous one would be better, but we register the output. -- This table has been built by GENVEC.exe (C program) -- We use only positive values ! (sign comes from quadrant info) -- This could be further optimized by coding only one quadrant... lookup: process (Address) subtype SLV8 is std_logic_vector (7 downto 0); type Rom128x8 is array (0 to 127) of SLV8; -- 0 to 2**Address'length - 1 constant Sinus_Rom : Rom128x8 := ( x"00", x"03", x"06", x"09", x"0c", x"0f", x"12", x"15", x"18", x"1b", x"1e", x"21", x"24", x"27", x"2a", x"2d", x"30", x"33", x"36", x"39", x"3b", x"3e", x"41", x"43", x"46", x"49", x"4b", x"4e", x"50", x"52", x"55", x"57", x"59", x"5b", x"5e", x"60", x"62", x"64", x"66", x"67", x"69", x"6b", x"6c", x"6e", x"70", x"71", x"72", x"74", x"75", x"76", x"77", x"78", x"79", x"7a", x"7b", x"7b", x"7c", x"7d", x"7d", x"7e", x"7e", x"7e", x"7e", x"7e", x"7f", x"7e", x"7e", x"7e", x"7e", x"7e", x"7d", x"7d", x"7c", x"7b", x"7b", x"7a", x"79", x"78", x"77", x"76", x"75", x"74", x"72", x"71", x"70", x"6e", x"6c", x"6b", x"69", x"67", x"66", x"64", x"62", x"60", x"5e", x"5b", x"59", x"57", x"55", x"52", x"50", x"4e", x"4b", x"49", x"46", x"43", x"41", x"3e", x"3b", x"39", x"36", x"33", x"30", x"2d", x"2a", x"27", x"24", x"21", x"1e", x"1b", x"18", x"15", x"12", x"0f", x"0c", x"09", x"06", x"03" ); begin Result '0'); elsif rising_edge(CLK) then if Sign='1' then Dout <= Result; else Dout <= std_logic_vector (- signed(Result)); end if; end if; end process outreg; end RTL; |
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