我正在设计一种采用LUT生成输出的架构。代码如下。
with resolvedOS select outBits <=
(37 => parse(6), 36 => parse(5), 35 => parse(4), 34 => parse(3), 33 => parse(2), 32 => parse(1), 31 => parse(0), others => '0') when "00000",
(37 => parse(6), 36 => not parse(6), 35 => parse(5), 34 => parse(4), 33 => parse(3), 32 => parse(2), 31 => parse(1), 30 => parse(0), others => '0') when "00001",
(37 => parse(6), 36 downto 35 => not parse(6), 34 => parse(5), 33 => parse(4), 32 => parse(3), 31 => parse(2), 30 => parse(1), 29 => parse(0), others => '0') when "00010",
(37 => parse(6), 36 downto 34 => not parse(6), 33 => parse(5), 32 => parse(4), 31 => parse(3), 30 => parse(2), 29 => parse(1), 28 => parse(0), others => '0') when "00011",
(37 => parse(6), 36 downto 33 => not parse(6), 32 => parse(5), 31 => parse(4), 30 => parse(3), 29 => parse(2), 28 => parse(1), 27 => parse(0), others => '0') when "00100",
(37 => parse(6), 36 downto 32 => not parse(6), 31 => parse(5), 30 => parse(4), 29 => parse(3), 28 => parse(2), 27 => parse(1), 26 => parse(0), others => '0') when "00101",
(37 => parse(6), 36 downto 31 => not parse(6), 30 => parse(5), 29 => parse(4), 28 => parse(3), 27 => parse(2), 26 => parse(1), 25 => parse(0), others => '0') when "00110",
(37 => parse(6), 36 downto 30 => not parse(6), 29 => parse(5), 28 => parse(4), 27 => parse(3), 26 => parse(2), 25 => parse(1), 24 => parse(0), others => '0') when "00111",
(37 => parse(6), 36 downto 29 => not parse(6), 28 => parse(5), 27 => parse(4), 26 => parse(3), 25 => parse(2), 24 => parse(1), 23 => parse(0), others => '0') when "01000",
(37 => parse(6), 36 downto 28 => not parse(6), 27 => parse(5), 26 => parse(4), 25 => parse(3), 24 => parse(2), 23 => parse(1), 22 => parse(0), others => '0') when "01001",
(37 => parse(6), 36 downto 27 => not parse(6), 26 => parse(5), 25 => parse(4), 24 => parse(3), 23 => parse(2), 22 => parse(1), 21 => parse(0), others => '0') when "01010",
(37 => parse(6), 36 downto 26 => not parse(6), 25 => parse(5), 24 => parse(4), 23 => parse(3), 22 => parse(2), 21 => parse(1), 20 => parse(0), others => '0') when "01011",
(37 => parse(6), 36 downto 25 => not parse(6), 24 => parse(5), 23 => parse(4), 22 => parse(3), 21 => parse(2), 20 => parse(1), 19 => parse(0), others => '0') when "01100",
(37 => parse(6), 36 downto 24 => not parse(6), 23 => parse(5), 22 => parse(4), 21 => parse(3), 20 => parse(2), 19 => parse(1), 18 => parse(0), others => '0') when "01101",
(37 => parse(6), 36 downto 23 => not parse(6), 22 => parse(5), 21 => parse(4), 20 => parse(3), 19 => parse(2), 18 => parse(1), 17 => parse(0), others => '0') when "01110",
(37 => parse(6), 36 downto 22 => not parse(6), 21 => parse(5), 20 => parse(4), 19 => parse(3), 18 => parse(2), 17 => parse(1), 16 => parse(0), others => '0') when "01111",
(37 => parse(6), 36 downto 21 => not parse(6), 20 => parse(5), 19 => parse(4), 18 => parse(3), 17 => parse(2), 16 => parse(1), 15 => parse(0), others => '0') when "10000",
(37 => parse(6), 36 downto 20 => not parse(6), 19 => parse(5), 18 => parse(4), 17 => parse(3), 16 => parse(2), 15 => parse(1), 14 => parse(0), others => '0') when "10001",
(37 => parse(6), 36 downto 19 => not parse(6), 18 => parse(5), 17 => parse(4), 16 => parse(3), 15 => parse(2), 14 => parse(1), 13 => parse(0), others => '0') when "10010",
(37 => parse(6), 36 downto 18 => not parse(6), 17 => parse(5), 16 => parse(4), 15 => parse(3), 14 => parse(2), 13 => parse(1), 12 => parse(0), others => '0') when "10011",
(37 => parse(6), 36 downto 17 => not parse(6), 16 => parse(5), 15 => parse(4), 14 => parse(3), 13 => parse(2), 12 => parse(1), 11 => parse(0), others => '0') when "10100",
(37 => parse(6), 36 downto 16 => not parse(6), 15 => parse(5), 14 => parse(4), 13 => parse(3), 12 => parse(2), 11 => parse(1), 10 => parse(0), others => '0') when "10101",
(37 => parse(6), 36 downto 15 => not parse(6), 14 => parse(5), 13 => parse(4), 12 => parse(3), 11 => parse(2), 10 => parse(1), 9 => parse(0), others => '0') when "10110",
(37 => parse(6), 36 downto 14 => not parse(6), 13 => parse(5), 12 => parse(4), 11 => parse(3), 10 => parse(2), 9 => parse(1), 8 => parse(0), others => '0') when "10111",
(37 => parse(6), 36 downto 13 => not parse(6), 12 => parse(5), 11 => parse(4), 10 => parse(3), 9 => parse(2), 8 => parse(1), 7 => parse(0), others => '0') when "11000",
(37 => parse(6), 36 downto 12 => not parse(6), 11 => parse(5), 10 => parse(4), 9 => parse(3), 8 => parse(2), 7 => parse(1), 6 => parse(0), others => '0') when "11001",
(37 => parse(6), 36 downto 11 => not parse(6), 10 => parse(5), 9 => parse(4), 8 => parse(3), 7 => parse(2), 6 => parse(1), 5 => parse(0), others => '0') when "11010",
(37 => parse(6), 36 downto 10 => not parse(6), 9 => parse(5), 8 => parse(4), 7 => parse(3), 6 => parse(2), 5 => parse(1), 4 => parse(0), others => '0') when "11011",
(37 => parse(6), 36 downto 9 => not parse(6), 8 => parse(5), 7 => parse(4), 6 => parse(3), 5 => parse(2), 4 => parse(1), 3 => parse(0), others => '0') when "11100",
(37 => parse(6), 36 downto 8 => not parse(6), 7 => parse(5), 6 => parse(4), 5 => parse(3), 4 => parse(2), 3 => parse(1), 2 => parse(0), others => '0') when "11101",
(37 => parse(6), 36 downto 7 => not parse(6), 6 => parse(5), 5 => parse(4), 4 => parse(3), 3 => parse(2), 2 => parse(1), 1 => parse(0), others => '0') when "11110",
(37 => parse(6), 36 downto 6 => not parse(6), 5 => parse(5), 4 => parse(4), 3 => parse(3), 2 => parse(2), 1 => parse(1), 0 => parse(0)) when others;
通常情况下，LUT不会占用很多延迟，但对于这一点，合成结果会产生非常高的延迟。该设计在Synopsis DC中编译，使用NANGATE 45nm。
U619/ZN (NAND2_X1) 0.05 0.67 f
U618/ZN (INV_X1) 0.04 0.72 r
U613/ZN (OAI21_X1) 0.03 0.75 f
U612/ZN (OAI21_X1) 0.04 0.79 r
U610/ZN (XNOR2_X1) 0.07 0.86 r
U607/ZN (NAND3_X1) 0.09 0.95 f
U565/ZN (OAI211_X1) 0.11 1.06 r
U564/ZN (INV_X1) 0.08 1.13 f
U563/ZN (NAND2_X1) 0.09 1.22 r
U562/ZN (INV_X1) 0.06 1.28 f
U561/ZN (NAND2_X1) 0.08 1.36 r
U534/ZN (NAND2_X1) 0.04 1.41 f
U531/ZN (OR2_X1) 0.06 1.47 f
U528/ZN (NOR2_X1) 0.05 1.52 r
U521/ZN (NAND2_X1) 0.04 1.55 f
U480/ZN (NOR2_X1) 0.05 1.60 r
U474/ZN (NAND2_X1) 0.04 1.64 f
U465/ZN (NOR2_X1) 0.05 1.69 r
U458/ZN (NAND2_X1) 0.04 1.72 f
U449/ZN (OAI21_X1) 0.05 1.77 r
U448/ZN (OAI21_X1) 0.03 1.80 f
U447/ZN (AOI221_X1) 0.09 1.89 r
U446/ZN (OAI211_X1) 0.04 1.93 f
outBits[15] (out) 0.00 1.93 f
data arrival time 1.93
你们能解释一下为什么这个会有如此高的延迟吗？或者建议我使用其他一些输出延迟较小的方法？
答案 0 :(得分：2)
我不知道你的代码中有parse，但我怀疑这是问题所在。
为什么不把所有值都放到正确类型的数组中？我最近这样做了，它起作用了：
library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;
entity lut is
port (
address_i : in integer range 0 to 255;
data_o : out std_logic_vector(31 downto 0)
);
end lut;
architecture Behavioral of lut is
type rom_t is array (0 to 255) of std_logic_vector(31 downto 0);
signal rom: rom_t := (
x"00000000", x"0000005f", x"00000064", x"0000006a",
x"00000071", x"00000077", x"0000007e", x"00000086",
x"0000008e", x"00000096", x"0000009f", x"000000a9",
(...)
x"0aa4951c", x"0b46062d", x"0bf10824", x"0ca62c1d"
);
begin
data_o <= rom(address_i);
end Behavioral;
如果您不喜欢端口实体中的整数，可以使用vector，并将其更改为实体内的整数。