NAME

vasy VHDL RTL subset. See the file man1/alc_origin.1.

DESCRIPTION

This document describes the VHDL subset accepted by VASY for RTL descriptions. CONCURRENT STATEMENTS In an RTL architecture most of the concurrent statements are supported. Allowed concurrent statements are: generate statement SEQUENTIAL STATEMENTS Inside a process, all sequential statements including loops, signal assignment, variable assignment are supported. TYPE All types usefull for synthesis are accepted (IEEE-1164 and IEEE-1076.3), and all types defined in the VHDL Alliance subset (see vbe(5) for more details). OPERATORS All operators usefull for synthesis are accepted, such as arithmetic, logical and relationnal operators (IEEE-1164 and IEEE-1076.3), and those defined in the VHDL Alliance subset (see vbe(5) for more details). HARDWARE DESCRIPTION EXAMPLES A MULTIPLEXER may be described as follow:

library IEEE;
use IEEE.std_logic_1164.all;
entity mux is
port(
  sel,a,b : in std_logic;
  mux_out : out std_logic );
end mux;

architecture rtl_1 of mux is
begin
 process( sel,a,b )
 begin
   if (sel='1') then mux_out <= a;
                else mux_out <= b;
   end if;
 end process;
end rtl_1;

architecture rtl_2 of mux is
begin
  mux_out <= a when sel='1' else b;
end rtl_2;

A LATCH may be described as follow:

library IEEE;
use IEEE.std_logic_1164.all;
entity latch is
port(
  en,a : in std_logic;
  latch_out : out std_logic );
end latch;

architecture rtl_1 of latch is
begin
 process( en, a )
 begin
   if (en='1') then latch_out <= a;
   end if;
 end process;
end rtl_1;

A D-FLIP-FLOP may be described as follow:

library IEEE;
use IEEE.std_logic_1164.all;
entity d_ff is
port(
   ck,a : in std_logic;
  d_ff_out : out std_logic );
end d_ff;

architecture rtl_1 of d_ff is
begin
 process( ck )
 begin
   if (ck='1') then d_ff_out <= a;
   end if;
 end process;
end rtl_1;

architecture rtl_2 of d_ff is
begin
 process( ck )
 begin
   if (ck='1' and ck'event)
   then d_ff_out <= a;
   end if;
 end process;
end rtl_2;

architecture rtl_3 of d_ff is
begin
 process
 begin
   wait until ck='1';
   d_ff_out <= a;
 end process;
end rtl_3;

A TRISTATE BUFFER may be described as follow:

library IEEE;
use IEEE.std_logic_1164.all;
entity trs is
port(
   en,a : in std_logic;
   trs_out : out std_logic );
end trs;

architecture rtl_1 of trs is
begin
 process( en,a )
 begin
   if (en='1') then trs_out <= a;
               else trs_out <= 'Z';
   end if;
 end process;
end rtl_1;

architecture rtl_2 of d_ff is
begin
 trs_out <= a when en='1' else 'Z';
end rtl_2;

A RAM may be described as follow:

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;

entity ram is
port( clk,wr : in std_logic;
      adr : std_logic_vector(1 downto 0);
      i0  : in std_logic_vector(3 downto 0);
      o0  : out std_logic_vector(3 downto 0)
     );
end ram;

architecture rtl_1 of ram is
  type my_array is array (0 to 3) of std_logic_vector(3 downto 0);
  signal s : my_array;
begin
 process
 begin
   wait until (clk='0' and clk'event);
   if (wr='1')
   then s(to_integer(unsigned(adr))) <= I0;
   end if;
 end process;
 o0 <= s(to_integer(unsigned(adr)));
end rtl_1;

A ROM may be described as follow:

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;

entity rom is
port( adr : in  std_logic_vector(1 downto 0);
      o0  : out std_logic_vector(3 downto 0)
    );
end rom;

architecture rtl_1 of rom is
  subtype my_word is std_logic_vector(3 downto 0);
  type my_array is array (0 to 3) of my_word;
  constant s : my_array := ( "0000", "0001", "0010", "0011" );
begin
  o0 <= s(to_integer(unsigned(adr)));
end rtl_1;

A PRIORITY DECODER may be described as follow:

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;

entity decod is
port( A : in std_logic_vector(3 downto 0);
      B : out std_logic_vector(2 downto 0));
end decod;
architecture rtl_1 of decod is
begin
 process( a )
 begin
   b <= "111";
   for i in a'range   -- Static For Loop are unrolled !
   loop
     exit when a(i)='1';
     b <= std_logic_vector(to_unsigned(i,3));
   end loop;
 end process;
end rtl_1;

SEE ALSO

vasy(1), vbe(5), vhdl(5), vst(5), boom(1), loon(1), boog(1), asimut(1), proof(1) See the file man1/alc_bug_report.1.