For a more detailed treatment, please consult any of the many good books on this topic. Several of these books are listed in the reference list. Department of Defense and the IEEE sponsored the development of this hardware description language with the goal to develop very high-speed integrated circuit. The other widely used hardware description language is Verilog. Both are powerful languages that allow you to describe and simulate complex digital systems.
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For a more detailed treatment, please consult any of the many good books on this topic. Several of these books are listed in the reference list. Department of Defense and the IEEE sponsored the development of this hardware description language with the goal to develop very high-speed integrated circuit.
The other widely used hardware description language is Verilog. Both are powerful languages that allow you to describe and simulate complex digital systems. ABEL is less powerful than the other two languages and is less popular in industry. Although these languages look similar as conventional programming languages, there are some important differences.
A hardware description language is inherently parallel, i. A HDL program mimics the behavior of a physical, usually digital, system.
It also allows incorporation of timing specifications gate delays as well as to describe a system as an interconnection of different components. A digital system can be represented at different levels of abstraction . This keeps the description and design of complex systems manageable. Figure 1 shows different levels of abstraction.
Figure 1: Levels of abstraction: Behavioral, Structural and Physical The highest level of abstraction is the behavioral level that describes a system in terms of what it does or how it behaves rather than in terms of its components and interconnection between them. A behavioral description specifies the relationship between the input and output signals.
This could be a Boolean expression or a more abstract description such as the Register Transfer or Algorithmic level. A structural description could be compared to a schematic of interconnected logic gates.
It is a representation that is usually closer to the physical realization of a system. For the example above, the structural representation is shown in Figure 2 below.
VHDL allows one to describe a digital system at the structural or the behavioral level. The behavioral level can be further divided into two kinds of styles: Data flow and Algorithmic. The dataflow representation describes how data moves through the system. This is typically done in terms of data flow between registers Register Transfer level.
The data flow model makes use of concurrent statements that are executed in parallel as soon as data arrives at the input. On the other hand, sequential statements are executed in the sequence that they are specified.
VHDL allows both concurrent and sequential signal assignments that will determine the manner in which they are executed. Examples of both representations will be given later. Each entity is modeled by an entity declaration and an architecture body. One can consider the entity declaration as the interface to the outside world that defines the input and output signals, while the architecture body contains the description of the entity and is composed of interconnected entities, processes and components, all operating concurrently, as schematically shown in Figure 3 below.
In a typical design there will be many such entities connected together to perform the desired function. Figure 3: A VHDL entity consisting of an interface entity declaration and a body architectural description. VHDL uses reserved keywords that cannot be used as signal names or identifiers. Keywords and user-defined identifiers are case insensitive. Lines with comments start with two adjacent hyphens -- and will be ignored by the compiler.
VHDL also ignores line breaks and extra spaces. VHDL is a strongly typed language which implies that one has always to declare the type of every object that can have a value, such as signals, constants and variables. The entity declaration defines the NAME of the entity and lists the input and output ports.
VHDL Primer, A, 3rd Edition
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A VHDL Synthesis Primer / Edition 2
A Simplified Blackjack Program. Pearson offers special pricing when you package your text with other student resources. The book presents a subset of VHDL consisting of commonly used features that make it both simple and easy to use. More on Signal Assignment Statement. VHDL Primer, A, 3rd Edition Instructor resource file download The work is protected by local and international copyright laws and is provided solely for the use of z in teaching their courses and assessing student learning. Converting Real and Integer to Time. A Generic Binary Multiplier.