====== Scalar types ======
===== Definitions =====
==== scalar_type_definition ====
  * [[.:bnf#enumeration_type_definition]]
  * [[.:bnf#integer_type_definition]]
  * [[.:bnf#floating_type_definition]]
  * [[.:bnf#physical_type_definition]]

===== Examples =====
Different types are declared.
  * Types //Bit// and //switch_level// are declared as enumeration types through characters and can only have the values ' //0//' or ' //1//' and '//X// '. One has to distinguish between these values '//0// ' and '//1//' and the integer values //0// and //1// . Both characters are [[.:use-statements#overloading_resolution|overloaded]].
  * Types //states// and //colours// are declared as enumeration types by self-explanatory element names.

<code vhdl>
TYPE Bit IS ( '0', '1' ) ;

TYPE switch_level IS ( '0', '1', 'X' ) ;

TYPE states IS (out, sleeping, working);

TYPE colors IS ( red, green, blue );
</code>

----

  * Integer types are generated by stating integral numbers in the range declaration (RANGE).
  * The range can be declared rising (TO) or falling (DOWNTO).
  * Real types are created through realistic range declarations.
  * Subtypes can be declared by stating the base type and the selected range.

<code vhdl>
TYPE byte_length_integer IS
             RANGE 0 TO 255 ;

TYPE word_index IS RANGE 31 DOWNTO 0 ;

TYPE my_real IS RANGE 0.0 TO 9.99 ;

SUBTYPE high_bit_low IS byte_length_integer
      RANGE 0 TO 127 ;
</code>

----

With physical types a real-type is defined first. Afterwards the basic unit and its derivative units are declared.

As examples the definitions of the units of length (//distance//) and the units of time (//time//) are shown here.

The physical unit //time// is a predefined type.

<code vhdl>
TYPE distance IS RANGE 0 TO 1E16
   UNITS
      -- Basiseinheit :
         A -- Angström
      -- Metrische Einheiten :
         nm  =  10 A ;    -- Nanometer
         um  =  1000 nm ;  -- Mikrometer
         mm  =  1000 um ;  -- Millimeter
         cm  =  10 mm ;  -- Centimeter
         m  =  1000 mm ;  -- Meter
         km  =  1000 m ;   -- Kilometer
   END UNITS ;

TYPE time IS RANGE -1E18 TO 1E18
   UNITS
      fs ;  --Femtosekunde
      ps   = 1000 fs ;  -- Picosecond
      ns   = 1000 ps ;  -- Nanosecond
      us   = 1000 ns ;  -- Microsecond
      ms   = 1000 us ;  -- Millisecond
      sec  = 1000 ms ;  -- Second
      min  = 60 sec ;   -- Minute
   END UNITS
</code>