28-05-2013, 04:17 PM
Using Foundation Express with VHDL
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Using Foundation Express with VHDL
Foundation Express translates a VHDL description to an internal
gate-level equivalent format. This format is then optimized for a
given FPGA technology.
This chapter discusses concepts that you need to work with VHDL.
These concepts are covered in the following sections.
• “Hardware Description Languages”
• “About VHDL”
• “Foundation Express Design Process”
• “Using Foundation Express to Compile a VHDL Design”
• “Design Methodology”
The United States Department of Defense, as part of its Very High
Speed Integrated Circuit (VHSIC) program, developed VHSIC HDL
(VHDL) in 1982. VHDL describes the behavior, function, inputs, and
outputs of a digital circuit design. VHDL is similar in style and
syntax to modern programing languages, but includes many hardware-
specific constructs.
Foundation Express reads and parses the supported VHDL syntax.
The “VHDL Constructs” chapter lists all VHDL constructs and
includes the level of support provided for each construct.
Hardware Description Languages
Hardware description languages (HDLs) are used to describe the
architecture and behavior of discrete electronic systems.
HDLs were developed to deal with increasingly complex designs. An
analogy is often made to the development of software description
languages; from machine code (transistors and solder) to assembly
language (netlists) to high-level languages (HDLs).
Top-down, HDL-based system design is most useful in large projects,
where several designers or teams of designers are working concurrently.
HDLs provide structured development. After major architectural
decisions have been made and major components and their
connections have been identified, work can proceed independently
on subprojects.
Typical Uses for HDLs
HDLs typically support a mixed-level description, where structural
or netlist constructs can be mixed with behavioral or algorithmic
descriptions. With this mixed-level capability, you can describe
system architectures at a high level of abstraction; then incrementally
refine a design into a particular component-level or gate-level implementation.
Alternatively, you can read an HDL design description
into Foundation Express, then direct the compiler to synthesize a
gate-level implementation automatically.
Advantages of HDLs
A design methodology that uses HDLs has several fundamental
advantages over a traditional gate-level design methodology. Some
of the advantages are listed below.
• You can verify design functionality early in the design process
and immediately simulate a design written as an HDL description.
Design simulation at this higher level, before implementation at
the gate level, allows you to test architectural and design decisions.
• Foundation Express synthesizes and optimizes logic so you can
automatically convert a VHDL description to a gate-level implementation
in a given technology.
This methodology eliminates the former gate-level design bottleneck
and reduces circuit design time and errors introduced when
a VHDL specification is hand-translated to gates. With Foundation
Express logic optimization, you can automatically transform
a synthesized design to a smaller and faster circuit.
About VHDL
VHDL is one of a few HDLs in widespread use today. VHDL is
recognized as a standard HDL by the Institute of Electrical and Electronics
Engineers (IEEE Standard 1076, ratified in 1987) and by the
United States Department of Defense (MIL-STD-454L).
VHDL divides entities (components, circuits, or systems) into an
external or visible part (entity name and connections) and an internal
or hidden part (entity algorithm and implementation). After you
define the external interface to an entity, other entities can use that
entity when they all are being developed. This concept of internal
and external views is central to a VHDL view of system design. An
entity is defined, relative to other entities, by its connections and
behavior. You can explore alternate implementations (architectures)
of an entity without changing the rest of the design.
Foundation Express Design Process
Foundation Express performs three functions.
• Translates VHDL to an internal format
• Optimizes the block-level representation through various optimization
methods
• Maps the design’s logical structure for a specific Xilinx technology
library
Foundation Express synthesizes VHDL descriptions according to the
VHDL synthesis policy defined in the “Design Descriptions” chapter.
The Xilinx VHDL synthesis policy has three parts; design methodology,
design style, and language constructs. You use the VHDL
synthesis policy to produce high quality VHDL-based designs.
Operands
Operands specify the data that the operator uses to compute its
value. An operand returns its value to the operator.
There are many categories of operands. The simplest operand is a
literal, such as the number 7, or an identifier, such as a variable or
signal name. An operand itself can be an expression. You create
expression operands by surrounding an expression with parentheses.