Diary/2013-6-29

続・VHDLでの除算について

パス遅延65.6ns，つまり最大動作周波数が15MHzくらいになる，というのは
受け入れ難いな，とは思うけど，
ちゃんと合成可能なライブラリが用意されているのは素晴しい．

VHDLでの除算

VHDLでは，除算で商と剰余を求めるには，それぞれ/とmodという演算子が
あることは知っていたけど，シミュレーション用であって，
合成できないと信じていた．
@kibayosさんに尋ねられたので，エラーメッセージを見てもらおうと合成してみると...
合成できるのね．使ったツールはISE 14.4．

HDLのソースコードは，素直に↓の通り．
singedじゃなくてunsingedでも合成できる．

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity arith is
  port (
    a : in signed(31 downto 0);
    b : in signed(31 downto 0);
    q : out signed(31 downto 0);
    r : out signed(31 downto 0)
    );
end arith;

architecture RTL of arith is

begin  -- RTL
  process (a, b)
  begin  -- process
    q <= a / b;
    r <= a mod b;
  end process;
end RTL;

対象をXC7K325T-2とした時のログからの抜粋ですが，
パス遅延は，

  Maximum combinational path delay: 65.661ns

で，合成レポートは↓のような感じ．
P&Rまで行ったときの占有スライス数は958でした．
合成したモジュールのSchematicを見ようとしたのですが，
VMWare上のWindowsで動かしているISEには荷が重いようで，断念．

=========================================================================
Advanced HDL Synthesis Report

Macro Statistics
# Adders/Subtractors                                   : 72
 32-bit adder                                          : 1
 32-bit adder carry in                                 : 66
 32-bit subtractor                                     : 4
 33-bit adder                                          : 1
# Comparators                                          : 66
 32-bit comparator greater                             : 1
 32-bit comparator lessequal                           : 1
 33-bit comparator greater                             : 1
 33-bit comparator lessequal                           : 1
 34-bit comparator greater                             : 1
 34-bit comparator lessequal                           : 1
 35-bit comparator greater                             : 1
 35-bit comparator lessequal                           : 1
 36-bit comparator greater                             : 1
 36-bit comparator lessequal                           : 1
 37-bit comparator greater                             : 1
 37-bit comparator lessequal                           : 1
 38-bit comparator greater                             : 1
 38-bit comparator lessequal                           : 1
 39-bit comparator greater                             : 1
 39-bit comparator lessequal                           : 1
 40-bit comparator greater                             : 1
 40-bit comparator lessequal                           : 1
 41-bit comparator greater                             : 1
 41-bit comparator lessequal                           : 1
 42-bit comparator greater                             : 1
 42-bit comparator lessequal                           : 1
 43-bit comparator greater                             : 1
 43-bit comparator lessequal                           : 1
 44-bit comparator greater                             : 1
 44-bit comparator lessequal                           : 1
 45-bit comparator greater                             : 1
 45-bit comparator lessequal                           : 1
 46-bit comparator greater                             : 1
 46-bit comparator lessequal                           : 1
 47-bit comparator greater                             : 1
 47-bit comparator lessequal                           : 1
 48-bit comparator greater                             : 1
 48-bit comparator lessequal                           : 1
 49-bit comparator greater                             : 1
 49-bit comparator lessequal                           : 1
 50-bit comparator greater                             : 1
 50-bit comparator lessequal                           : 1
 51-bit comparator greater                             : 1
 51-bit comparator lessequal                           : 1
 52-bit comparator greater                             : 1
 52-bit comparator lessequal                           : 1
 53-bit comparator greater                             : 1
 53-bit comparator lessequal                           : 1
 54-bit comparator greater                             : 1
 54-bit comparator lessequal                           : 1
 55-bit comparator greater                             : 1
 55-bit comparator lessequal                           : 1
 56-bit comparator greater                             : 1
 56-bit comparator lessequal                           : 1
 57-bit comparator greater                             : 1
 57-bit comparator lessequal                           : 1
 58-bit comparator greater                             : 1
 58-bit comparator lessequal                           : 1
 59-bit comparator greater                             : 1
 59-bit comparator lessequal                           : 1
 60-bit comparator greater                             : 1
 60-bit comparator lessequal                           : 1
 61-bit comparator greater                             : 1
 61-bit comparator lessequal                           : 1
 62-bit comparator greater                             : 1
 62-bit comparator lessequal                           : 1
 63-bit comparator greater                             : 1
 63-bit comparator lessequal                           : 1
 64-bit comparator greater                             : 1
 64-bit comparator lessequal                           : 1
# Multiplexers                                         : 2087
 1-bit 2-to-1 multiplexer                              : 2080
 32-bit 2-to-1 multiplexer                             : 6
 33-bit 2-to-1 multiplexer                             : 1
# Xors                                                 : 2
 1-bit xor2                                            : 2

=========================================================================