VASY(5)

NAME

vasy VHDL RTL subset.

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:
    block
    concurrent assertion
    process
    concurrent signal assignment
    component instantiation statement
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)