VHDL Coding Basics VHDL – Library. ▫ Include library library IEEE;. ▫ Define the library .. VHDL Tutorial. ▫ Jan Van der Spiegel, University of Pennsylvania. Jan Van der Spiegel, VHDL Tutorial, University of Pennsylvania, Philadelphia, USA, ∼ese/vhdl/ [RAB] J. Rabaey, A. Chandrakasan, and B. Nikolic, Digital Integrated Circuits, 2nd ed. Prentice [SPI] J. Van der Spiegel, VHDL Tutorial. University of.
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Department of Electrical and Systems Engineering. Levels of representation and abstraction. Signals, Variables and Constants. Basic Loop statement Dataflow Modeling — Concurrent Statements. For a more detailed treatment, please consult any of the many tutorisl 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.
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. As an example, let us consider a simple circuit that warns car passengers when the door is open or the seatbelt is not used whenever the car key is inserted in the ignition lock At the behavioral level this could be expressed as.
The structural level, on the other hand, describes a system as a collection of gates and components that are interconnected to perform a desired function. 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 dr 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.
A digital system in VHDL consists of a design entity that can contain other entities that are then considered components of the top-level entity.
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.
A VHDL entity consisting of an interface entity declaration and a body architectural description. VHDL uses reserved keyword s 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.
The general tugorial is as follows. An entity always starts with the keyword entityfollowed by its name and the keyword is.
Next are the port declarations using the keyword port. An entity declaration always ends with the keyword endoptionally  followed by the name of the entity. A generic can have a default value. The syntax for a generic follows.
For the example of Figure 2 above, the entity declaration looks as follows. Notice the use and placement of semicolons! Inputs are denoted by the keyword inand outputs by the keyword out. Since VHDL is a strongly typed language, each port has a defined type.
This is the preferred type of digital fan. L for weak 0, H for weak 1, W for weak unknown – see section on Enumerated Types. Drr type defines the set of values an object can have. This has the advantage that it helps with the creation of models and helps reduce errors. For instance, if one tries to assign an illegal value to an object, the compiler will flag the error.
Four-to-one multiplexer of which each input is an 8-bit word. An example of the entity declaration vhel a D flip-flop with set and reset inputs is.
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The architecture body specifies how the circuit operates and how it is implemented. As discussed earlier, an entity or circuit can be specified in a variety of ways, such as behavioral, structural interconnected componentsor a combination of the above.
The architecture body for the example of Figure 2, described at the behavioral level, is given below. The header line of the architecture body defines the architecture name, e.
The architecture name can be any legal identifier. The vqn body of the architecture starts with the keyword begin and gives the Boolean expression of spkegel function. We will see later that a behavioral model can be described in several other ways. The architecture body ends with an end keyword followed by the architecture name.
A few other examples follow. The behavioral description of a two-input AND gate is shown below. The statements in the body of the architecture make use of logic operators.
Logic operators tutoriao are allowed are: In addition, other tutoriwl of operators including relational, shift, arithmetic are allowed as well see section on Operators. For more information on behavioral modeling see section on Behavioral Modeling. It is worth pointing out that the signal assignments in the above examples are concurrent statements. This implies tutofial the statements are executed when one or more of the signals on dder right hand side change their value i. For instance, when the input A changes, the internal signals X and Y change values that in turn causes the last statement to update the output Z.
There may be a propagation delay associated with this change. Digital systems are basically data-driven and an event which occurs on one signal will lead to an event on another signal, etc. The execution of the statements is determined by the flow of signal values.
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As a result, the order in which these statements are given does not matter i. This is in contrast to conventional, software programs that execute the statements in a sequential or va manner. The circuit of Figure 2 can also be described using a structural model that specifies what gates are used and how they are jann.
The following example illustrates it. Following the header is the declarative part that gives the components gates that are going to dfr used in the description of the circuits. These gates have to be defined first, i. These can be stored in one of the packages one refers to in the header of the file see Library and Packages below. The declarations for the components give the inputs e.
Next, one has to define internal nets signal names. Notice that one always has to declare the type of the signal. The statements after the begin keyword gives the instantiations of the components and describes how these are interconnected. A component instantiation statement creates a new level of hierarchy. Each line starts with an instance name e. U0 followed by a colon and a component name and the keyword port map.
This keyword defines how the components are connected.