VHDL Programming

Data Types 85

PHYSICAL TYPES Physical types are used to represent physical

quantities such as distance, current, time, and so on. A physical type pro-

vides for a base unit, and successive units are then defined in terms of this

unit. The smallest unit representable is one base unit; the largest is deter-

mined by the range specified in the physical type declaration. An example

of a physical type for the physical quantity current is shown here:

TYPE current IS RANGE 0 to 1000000000

UNITS

na; --nano amps

ua = 1000 na; --micro amps

ma = 1000 ua; --milli amps

a = 1000 ma; --amps

END UNITS;

The type definition begins with a statement that declares the name of the

type (current) and the range of the type (0 to 1,000,000,000). The first unit

declared in the UNITSsection is the base unit. In the preceding example,

the base unit is na. After the base unit is defined, other units can be defined

in terms of the base unit or other units already defined. In the preceding

example, the unit uais defined in terms of the base unit as 1000 base

units. The next unit declaration is ma. This unit is declared as 1000 ua.

The units declaration section is terminated by the END UNITSclause.

More than one unit can be declared in terms of the base unit. In the pre-

ceding example, the maunit can be declared as 1000 maor 1,000,000 na. The

range constraint limits the minimum and maximum values that the phys-

ical type can represent in base units. The unit identifiers all must be unique

within a single type. It is illegal to have two identifiers with the same name.

PREDEFINED PHYSICAL TYPES

The only predefined physical type in VHDL is the physical type TIME. This

type is shown here:

TYPE TIME IS RANGE <implementation defined>

UNITS

fs; --femtosecond

ps = 1000 fs; --picosecond

ns = 1000 ps; --nanosecond

us = 1000 ns; --microsecond

ms = 1000 us; --millisecond

sec = 1000 ms; --second

min = 60 sec; --minute

hr = 60 min; --hour

END UNITS;