Attributes in VHDL

Presented by:
Saurav Agrawal
M-Tech(2nd Sem)
Subject seminar
on
ATTRIBUTES
Priyadarshini College of Engineering , Nagpur

ATTRIBUTES
 Attributes are a feature of VHDL that allow you to extract
additional information about an object (such as a signal,
variable or type).
 Attributes also allow you to assign additional information
(such as data related to synthesis) to objects in your
design description.
 An attribute is a value, function, type, range, signal, or a
constant.
 An Attribute can be associated with certain names.

There are two classes of attributes:
 Predefined Attributes : Those that are predefined as a
part of the 1076 standard.
 User-Defined Attributes: Those that have been
introduced outside of the standard.

Predefined Attributes
There are five classes of predefined attributes.
1. Value attributes: these return a constant value
2. Function attributes: calls a function that returns a
value
3. Signal attributes: creates a new signal
4. Type attributes: returns a type name
5. Range attributes: returns a range

Value attributes
If T is any scalar type or subtype,
 T'LEFT: returns the left bound, that is, the leftmost value,
of T.
 T'RIGHT: returns the right bound, that is, the rightmost
value, of T.
 T'HIGH: returns the upper bound, that is, the value at the
highest position number, of T.
 T'LOW: returns the lower bound, that is, the value at the
lowest position number, of T.

For example, if
type ALLOWED_VALUE is range 31 downto 0;
Then the following equivalence relations are true:
 ALLOWED_VALUE'LEFT = 31
 ALLOWED_VALUE'HIGH = 31
 ALLOWED_VALUE'RIGHT = 0
 ALLOWED_VALUE'LOW = ALLOWED_VALUE'RIGHT

Function Attributes
These attributes represent functions that are called to obtain
a value.
 T'POS(V): returns the position number of the value V in
the ordered list of values of T.
 T'VAL(P): returns the value of the type that corresponds
to position P.
 T'SUCC(V): returns the value of the parameter whose
position is one larger than the position of value V in T.
 T'PRED(V): returns the value of the parameter whose
position is one less than the position of value V in type T.
 T'LEFTOF(V): returns the value of the parameter that is
to the left of value V in type T.
 T'RIGHTOF(V): returns the value of the parameter that is
to the right of value V in type T.

For example, if
type STATUS is (SILENT, SEND, RECEIVE);
subtype DELAY_TIME is TIME range 10 ns to 50 ns;
Then
 STATUS'POS(SEND) = 1
 DELAY_TIME'SUCC(21 ns) = 22 ns
 DELAY_TIME'PRED(10 ns) is an error

If A is a constrained array object, then
 A'LEFT(N): returns the left bound of the Nth dimension
of the array.
 A'R.IGHT(N): returns the right bound of the Nth
dimension.
 A'LOW(N): returns the lower bound of the Nth
dimension.
 A'HIGH(N): returns the upper bound of the Nth
dimension.
If
type COST_TYPE is array (7 downto 0,0 to 3) of
INTEGER;
variable COST_MATRIX: COST_TYPE;
 then the following equivalence relations are true:
 COST_MATRIX'LEFT(2) = 0

Signal Attributes
These attributes create new signals from the signals with
which they are associated.
 S'DELAYED (T): is a new signal that is the same type as
signal S but delayed from S by time T. If T is not
specified, a delay of 0ns is assumed.
 S'STABLE (T): is a Boolean signal that is true when
signal S has not had any event for time T. If T is not
specified, it implies current delta.
 S'QUIET (T): creates a Boolean signal that is true when
S has not been active for time T.
 S’TRANSACTION: creates a bit type signal that toggles
its value every time signal S becomes active.

Stable Attribute
Signal’stable(time) creates a signal that is True when the reference
signal has no events for time value

Type Attributes
If T is any type or subtype, T'BASE, which is the only type
attribute, returns the base type of T.
For example, if
type ALU_OPS is (ADDOP, SUBOP, MULOP, DIVOP,
ANDOP, NANDOP, OROP, NOROP);
subtype ARITH_OPS is ALU_OPS range ADDOP to
DIVOP;
then
 ARITH_OPS'BASE is ALU_OPS
 which is the base type, and it can be used in an expression
such as
 ARITH_OPS'BASE'LEFT which has the value ADDOP.

Range Attributes
If A is a constrained array object, then
 A'RANGE (N): returns the Nth index range of A (N=l, if
not specified).
 A'REVERSE.RANGE(N): returns the Nth index range
reversed.
For example, if
variable WBUS: MVL_VECTOR(7 downto 0);
then
 WBUS'RANGE returns the range 7 downto 0
while

User-Defined Attributes
User-defined attributes are constants of any type. They
are declared using attribute declarations.
attribute attribute-name: value-type;
For example,
type INCHES is range 0 to 1000
units micron;
end units;
attribute LENGTH: INCHES;

The syntax for an attribute specification is
attribute attribute-name of item-names: name-class is
expression;
Some examples of attribute specifications are
 attribute CAPACITANCE of CLK, RESET: signal is 20
pf;
 attribute LENGTH of RX_READY: signal is 3 micron;
 attribute CAPACITANCE of all: variable is 0 pf;
The value of the attribute can then be used in an
expression by referring to

architecture NAND_PLACE of NAND_GATE is
component NAND_COMP
port (IN1, IN2: in BIT; OUT1: out BIT);
end component;
type COMP_LOCATION is record
X, Y: INTEGER;
end record;
attribute PLACEMENT: COMP_LOCATION;
attribute PLACEMENT of N1: label is (50, 45);
signal PERIMETER: INTEGER;
signal A, B, Z: BIT;
begin
N1: NAND_COMP port map (A, B, Z);
PERIMETER <= 2 * (N1'PLACEMENT.X +
N1'PLACEMENT.Y);

Reference
 http://www.ics.uci.edu/~jmoorkan/vhdlref/attrib.html
 http://www.csee.umbc.edu/portal/help/VHDL/attribute.
html
 http://web.engr.oregonstate.edu/~sllu/vhdl/lec5.html
 VHDL Programming by example- Douglas L. Perry
 VHDL Primer - J. Bhaskar

Attributes in VHDL

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