Provided by: alliance_5.1.1-3_amd64 bug

NAME

       vbe
       VHDL behavioural subset.

DESCRIPTION

       This document describes the ALLIANCE VHDL subset for behavioural data flow descriptions.

       CONCURRENT STATEMENTS
       In a data flow architecture only concurrent statements (except process) are supported. All
       sequential statements including loops, signal assignment, etc .. are to be banished.

       Allowed concurrent statements are:
              simple signal assignment
              conditional signal assignment
              selected signal assignment
              concurrent assert statement
              block statement

       BUSES
       When using concurrent statements, an ordinary signal can be assigned only once.  The value
       of  the  signal  must  be  explicitly  defined by the signal assignment (for example, in a
       selected signal assignment the value of the target signal is to be defined for every value
       that the select expression can take).

       The  above  constraint  may  be  felt  as  a  hard  restriction when designing distributed
       controled hardware (precharged line, distributed multiplexer, etc ...).  To  hurdle  this,
       VHDL uses a special feature: guarded-resolved signals.

       A  resolved signal is a signal declared with a resolved subtype (see vhdl(5)).  A resolved
       subtype is a type combined with a resolution function. A resolved signal can  be  assigned
       by  multiple  signal  assignments.  Depending  on the value of each driver, the resolution
       function determines the effective value of the signal.

       A guarded signal is a resolved signal with drivers that can  be  disconected.   A  guarded
       signal must be assigned inside a block statement through a guarded signal assignment.

       A distributed multiplexer may be described as :

       signal Distributed_Mux : mux_bit bus;

       begin

       first_driver_of_mux : block (Sel1 = '1')
       begin
         Distributed_Mux <= guarded Data1;
       end block;

       second_driver_of_mux : block (Sel2 = '1')
       begin
         Distributed_Mux <= guarded Data2;
       end block;

       LATCHES and REGISTERS
       Sequential  elements must be explicitly declared using the type reg_bit or reg_vector (and
       must be of kind register). A sequential element must be assigned inside a block  statement
       by a guarded signal assignment.

       Rising edge triggered D flip flop :

       signal Reg : reg_bit register;

       begin

       flip_flop : block (ck = '1' and not ck'STABLE)
       begin
         Reg <= guarded Din;
       end block;

       Level sensitive latch:

       signal Reg : reg_bit register;

       begin

       latch : block (ck = '1')
       begin
         Lat <= guarded Din;
       end block;

       In  both  cases, the guard expression must depend only on one signal if the description is
       to be processed by the logic synthetizer (boom + boog).

       The following operators are only supported: not, and, or, xor, nor, nand, &, =, /=

       They can be applied on all types supported by the subset. Other  standard  VHDL  operators
       (+, -, >, <, ...) have not been implemented in the present release.

       TIMING
       Timing  information  can  be  specified  in  behavioural descriptions using after clauses.
       However, those delays are currently only used for simulation. After clauses are  supported
       but not used for synthesis and formal proof.

       After  clauses  in block statements (for guarded signal assignments) are not supported for
       sequential elements (signals of kind register), but supported for bus elements (signals of
       kind  bus).  This  is  because  the  VHDL  default disconnection time is null and this can
       generate unexpected behavior for sequential elements.

       In selected signal assignment, only uniform delays are supported (the same After clause in
       all assignments).

       Transport option is not supported. All delays are inertial delays.

       ASSERT STATEMENT
       Only two severity levels are supported in concurrent assert statements:

       warning        print a warning message if the assert condition is not satisfied.

       error          print an error message if the assert condition is not satisfied. Then, stop
                      the simulation.

       Assert statements are ignored by the logic synthesis tool.

       DON'T CARE
       A special feature has been introduced in order to allow "don't  care"  specification  when
       the  logic  synthtizer  is  targeted ( Beware : this feature is incompatible with the IEEE
       VHDL standard !!).

       An output can be assigned to the value 'D' (don't care). This is taken into account by the
       logic  synthesis  tool in the optimization process. When the value of an output is 'D' the
       logic synthesis tool may turn it into a '1' or a '0'.

       A 'D' value is understood as a '0' by the logic simulator (asimut).

       ARRAIES
       Arraies other than bit_vector, reg_vector, mux_vector and wor_vector are not supported.

EXAMPLES

       Here is the description of an adder with an accumulator register.

       entity add_accu is
       port (
         clk      : in  bit;
         command  : in  bit;
         data_in  : in  bit_vector (31 downto 0);
         data_out : out bit_vector (31 downto 0);
         cry_out  : out bit;
         vdd      : in  bit;
         vss      : in  bit
         );
       end add_accu;

       architecture data_flow of add_accu is

       signal eff_data  : bit_vector (31 downto 0);      -- effective operande
       signal adder_out : bit_vector (31 downto 0);      -- adder's result
       signal adder_cry : bit_vector (32 downto 0);      -- adder's carry
       signal accum_reg : reg_vector (31 downto 0) register;  -- accumulator

       constant initialize : bit := '0';
       constant accumulate : bit := '1';

       begin

         -- select the effective operand

         with command select
         eff_data <= X"0000_0000" when initialize,
                     accum_reg    when accumulate;

         -- compute the result out of the adder

         adder_out               <= eff_data xor data_in xor adder_cry;
         adder_cry (0)           <= '0';
         adder_cry (32 downto 1) <= (eff_data and adder_cry (31 downto 0)) or
                                    (data_in  and adder_cry (31 downto 0)) or
                                    (aff_data and data_in                ) ;

         -- write the result into the register on the rising edge of clk

         write : block (clk = '1' and not clk'STABLE)
         begin
           accum_reg <= guarded adder_out;
         end block;

         -- assign outputs

         cry_out  <= adder_cry (32);
         data_out <= accum_reg     ;

         -- check power supply

         assert (vdd = '1' and vss = '0')
         report "power sypply is missing"
         severity ERROR;

       end;

SEE ALSO

       vhdl(5), vst(5), boom(1), loon(1), boog(1), asimut(1), proof(1)