Signal declarations

signal identifier_list : subtype_indication [ signal_kind ] [ := expression ] ;

• entity_declarative_part
• architecture_declarative_part
• package
• block_declarative_part
• procedure_declarative_part

identifier {, identifier }

[ resolution_function_ name ] type_mark [ constraint ]

• register
• bus

• relation { and relation }
• relation { or relation }
• relation { xor relation }
• relation [ nand relation ]
• relation [ nor relation ]
• relation { xnor relation }

The signals declared with REGISTER or BUS are 'guarded signals'.

'Guarded signals' have to be 'resolved signals'.

Guarded signal assignments have the following meaning for 'guarded signals':

IF guard_expression THEN
     sig_name <= sig_waveform ;
ELSE
     sig_name <= NULL ;
END IF ;

The assignment of NULL to the signal sig_name means that the driver of this signal assignment is switched off. This has the following consequences for the 'resolved signal':

• If not all drivers were switched off the resulting signal is only determined by the drivers which were not switched off.
• If all drivers were switched off the last available signal value is kept in case of the signal declaration REGISTER .
• If all drivers were switched off the default value stated in the `resolution function` is used in case of the signal declaration BUS .

The signal driver is switched off immediately, i.e. without delay, after the guard_expression has taken on the value false except if an explicit delay time was planned to be given to the signal by the disconnection-statement after it was declared a controlled signal. An extensive example on this can be found in the chapter Disconnection specification.

The signals clk_1 and clk_2 of the physical type time are declared.

SIGNAL clk_1, clk_2 : time ;

• The signal address of the type bit8 is declared and initialized with the value “00110110”.
• The signal bittab is declared by the type bit_vector and initialized with the value “000111Z1Z”.
• The signal tab is declared by the type table_type and initialized with the values 2, 3, 4, -2 and 0.

SIGNAL address : bit8 := "00110110" ;
 
SIGNAL bittab : bit_vector(1 TO 9)
      := ( 1 TO 3 => '0', 7 | 9 => 'Z',
          OTHERS => '1' ) ;
 
SIGNAL tab : table_type( 0 TO 4 )
      := ( 2, 3, 4, -2, 0 ) ;

The signal address_bus is declared as a guarded signal (BUS) of the type resolved_bit_vector and initialized with the value “10011001”.

SIGNAL address_bus : resolved_bit_vector
      BUS := "10011001"; 

The signal resolved_s is declared as a guarded signal (REGISTER) of the type zbit . In addition to that the signal is also assigned a resolution function resolution_function .

SIGNAL resolved_s : resolution_function
      zbit REGISTER ;