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1、DESIGNING A DIGITAL SYSTEM WITH VHDL Valentina Stoyanova KukenskaAbstract: In this paper a digital system designing with VHDL is presented. Here are exposed sequentially all the phases of the very digital systems designing. The main methods are also on show here. The project descriptions types are p
2、resented. The stress is put on the use of VHDL for synthesis of structural and behavioral models.For creating the project of the chosen digital system an integrated system WebPack was used, as well as ModelSIm XE II for the models simulation. Keywords: Design, VHDL, digital systems, model, WebPack 1
3、. INTRODUCTIONThe digital systems are complex ones, consisting of lots of components. As far as the automated design of such systems is concerned, methods for designing time reducing and limiting the complexity of the task are sought out and applied. A method of the kind is connected with the decomp
4、osition and hierarchy principles. The decomposition of the systems is realized in a way, which differentiates functionally independent modules. A digital system can be described as a module with inputs and/or outputs. The electrical values on the outputs are some function of the values on the inputs
5、. One way of describing the function of a module is to describe how it is composed of sub-modules. Each of the sub-modules is an instance of some entity, and the ports of the instances are connected using signals. This kind of description is called a structural description.In many cases, it is not a
6、ppropriate to describe a module structurally. One such case is a module, which is at the bottom of the hierarchy of some other structural description. For example, if you are designing a system using IC packages bought from an IC shop, you do not need to describe the internal structure of an IC. In
7、such cases, a description of the function performed by the module is required, without reference to its actual internal structure. Such a description is called a functional or behavioral description.Usually, for structural and behavioral description, either Verilog or VHDL is used. In this paper a d
8、esigning with VHDL is presented. Here are exposed sequentially all the phases of the very digital systems designing. The main methods are also on show here. The project descriptions types are presented. The stress is put on the use of VHDL for synthesis of structural and behavioral models. Here are
9、presented several VHDL models of computer systems components. 2. METHODS AND STAGES IN DIGITAL SYSTEMS DESIGNIn digital systems design, as well as design of complex systems, a couple of methods are in use: top - down designing; up - down designing.In top - down designing the building up of the syste
10、m is usually started from below in upright direction through elaborating the element blocks schemes, assembled later to form the whole product.An advantage of this method is the use of representation on functional block level and the lower, the structural level, is addressed only during the error ch
11、eck simulations within the project.The up-down designing starts with a specification on the highest level. After that, the project is being decomposed into functional blocks and the requirements for the income and outcome time proportions are specified. The functional models are described through be
12、havioral models or by models on register levels and are subsequently simulated.Some of the advantages of the methods are: n easier execution of the tasks specifications; t allows a projects check on system level, without tackling the structural details; The projects check is done, with no regard to
13、the technology of its realization. That allows that the choice of technology be made on a later stage of the designing project.The most effective up-down designing method is the use of an abstract description of the scheme and the sequential details specifying of the different hierarchy levels descr
14、iption. The digital systems design goes through the next stages: Specification; Functional (electrical) designing; Physical designing; Manufacturing; Testing.Through specification the product parameters, necessary for its proper destination, are determined.Through the functional (electrical) designi
15、ng, the electrical scheme, responsible for the functions and parameters of the product, in terms of the specification, is elaborated.The behavioral stage serves as a description for the scheme as a system, and its entries and exits are marked out. In most of the cases, VHDL models are used.The Funct
16、ional (electrical) designing deals with main functional blocks elaboration. Usually a detailed VHDL description of the functional block is made and being checked by a VHDL simulation.With the increasing complexity of the projects, for the elaboration on structural level, the technique of synthesis i
17、s applied. It allows that the scheme with logical elements be synthesized from a VHDL description. Through logical description details such as charging, elements delay, are specified and crucial methods and problems with time scattering of signals are defined.The Physical designing stages strongly d
18、epend on technology. The common task is concerned with the deploying of the logical elements and defining (tracing) their interrelations. Provided that for the product realization PLD, CPLD or FPGA chips are used, then the result of the physical designing represents a configuration file for designin
19、g the chosen devices resources. The testing of the project represents a number of procedures, used by designers, to provide: adequacy between project and specification; the execution of the project in terms of the chosen technology. The designing process is usually iterative, including pre-designing
20、 of given parts, until the intended indicators are obtained.For the tasks of testing in electrical designing (the functionality of the product and its electrical parameters), simulations are used.The simulation on behavioral level defines how the product will run, before its actual compounding block
21、s are chosen. For working out of the behavioral models, the hardware description languages are used (VHDL, Verilog and others).Through simulation, on a logical primitives level, the schemes are built up with basic logical elements “AND-NO”, “OR-NO”, invertors and triggers and are being simulated in
22、order to find out irrelevances with their expected acting.In functional testing, the delays are not concerned or they are supposed similar for all logical elements.Error identification after the physical designAfter topologys final elaboration are made the next procedures: check out of the tech norm
23、s throughout manufacturing; check out for the projects authenticity.The tech norms for manufacturing are specific for each technological process. The authenticity verification of the project aims to guarantee the products proper working. It includes: finding out the interconnection of the scheme; fi
24、nding out the parasite components of the topology.3. TYPES OF DESIGN DESCRIPTIONSThrough the designing process, three types of design description are in use: behavioral; structural; physical.The behavioral description tackles the system as if it were a kind of “black box” with its entrances and exit
25、s, with no regard to its structure. The aim is to ignore the redundant details and to concentrate on the specification of the necessary for the functions, which are to be done by the product. On this stage, languages for the apparatus part are used HDL (Hardware Description Languages) - VHDL, Verilo
26、g and others. The structural description defines the way that the system is to be built up. Here, the systems structure, made of blocks and their interrelations, is tackled. The subsystems, which are to provide its functional execution, as well as their detailed description for analysis of the opera
27、tional speed, charging and so on, are defined. The structural description can be presented by languages for the description of the hardware, as well as by electrical schemes.The design process is connected with the transformations of the systems descriptions and their sequential details specificatio
28、n. Decomposition from behavioral to structural description can be realized on a number of levels in a hierarchy. From the highest to the lowest, these levels can be outlined as it follows: system level; functional level; logical level; scheme level.On the highest system level, the systems behavior i
29、s represented by algorithms that describe its functions. In order that these functions be executed, the architecture of the system is worked out, including microprocessors, memories, main boards and other structural components. On the lower level, the systems behavior is described by Bolivia equatio
30、ns. For their execution, logical elements and triggers are used.4. USE OF VHDL FOR SYNTHESIS OF STRUCTURAL AND BEHAVIORAL MODELSVHDL is a Hardware Description Language for describing digital system 2.VHDL is designed to full a number of needs in the design process. VHDL contains a number of faciliti
31、es for modifying the state of objects and controlling the flow of execution of modules.In VHDL, an entity is such a module which may be used as a component in a design, or which may be the top-level module of the design. The entity declarative part may be used to declare items, which are to be used
32、in the implementation of the entity. Once an entity has had its interface specified in an entity declaration, one or more implementations of the entity can be described in architecture bodies. Each architecture body can describe a different view of the entity.The declarations in the architecture bod
33、y define items that will be used to construct the design description.Signals are used to connect sub modules in a design. The sub modules in an architecture body can be described as blocks. A block is a unit of module structure, with its own interface, connected to other blocks or ports by signals.
34、A signal assignment schedules one or more transactions to a signal (or port).The primary unit of behavioral description in VHDL is the process. When more than one process is activated at the same time, they execute concurrently.A process statement which can be used in an architecture body or block.
35、The declarations define items which can be used locally within the process.A process may contain a number of signal assignment statements for a given signal, which together form a driver for the signal.VHDL descriptions write them in a design file. After then invoke a compiler to analyze them and in
36、sert them into a design library. A number of VHDL constructs may be separately analyzed for inclusion in a design library. These constructs are called library units. A design file may contain a number of library units.The behavioral model represents a functional interpretation of the designed digita
37、l system. The hardware of the digital device is regarded as a kind of a discreet system. Its behavior is described as a number of operations. These operations are applied within the systems database. Within the creation of behavioral VHDL models, operations are described by processes and their inter
38、connections-by signals. On fig 1 is presented a VHDL model of a linear decipherer.library IEEE; use IEEE.std_logic_1164.all;entity DESHIF is port (x1,x2,x3 in: std_logic; J: out std_logic_vector(0 to 7);end DESHIF;arhitecture STRUCTURAL of DESHIF is component AND3 port (I1,I2,I3: in std_logic; O1: o
39、ut std_logic); end component; component NOT1 port (I1: in std_logic; O1: out std_logic); end component; signal a,b,c: std_logic;begin U1: NOT1 port map (I1=x1,O1=a); U2: NOT1 port map (I1=x2,O1=b); U3: NOT1 port map (I1=x3,O1=c); U4: AND3 port map (I1=a,I2=b,I3=c,O1=J(0); U5: AND3 port map (I1=a,I2=
40、b,I3=x3,O1=J(1); U6: AND3 port map (I1=a,I2=x2,I3=c,O1=J(2); U7: AND3 port map (I1=a,I2=x2,I3=x3,O1=J(3); U8: AND3 port map (I1=x1,I2=b,I3=c,O1=J(4); U9: AND3 port map (I1=x1,I2=b,I3=x3,O1=J(5); U10: AND3 port map (I1=x1,I2=x2,I3=c,O1=J(6); U11: AND3 port map (I1=x1,I2=x2,I3=x3,O1=J(7);end STRUCTURA
41、L; architecture DATA_FLOW of DESHIF issignal T1,T2,T3: bit;beginT1= not x1;T2= not x2;T3= not x3;F1=T1 and T2 and T3;F2=T1 and T2 and x3;F3=T1 and x2 and T3;F4=T1 and x2 and x3;F5=x1 and T2 and T3;F6=x1 and T2 and x3;F1=x1 and x2 and T3;F1=x1 and x2 and x3;end DATA_FLOW; fig.1 VHDL code of a linear
42、deciphererStructural VHDL models are means for reflecting the projects hierarchy. They are built up by decomposition of digital systems of functionally interconnected parts. These parts are presented as components, and their interconnections are realized through signals. These signals enter and exit
43、 the components via ports.For example, within the designing of the digital module, presented on 2, its structural model has three main components-counter, decoder and light-diode seven-segment display. On fig.2 is shown a part of the structural model, synthesized in WebPack. Fig.2 . VHDL structural
44、descriptionThe simulation starts with an initialization phase, and then proceeds by repeating a two-stage simulation cycle. In the initialization phase, all signals are given initial values, the simulation time is set to zero, and each modules behavior program is executed. This usually results in tr
45、ansactions being scheduled on output signals for some later time.The purpose of the simulation is to gather information about the changes in system state over time. This can be done by running the simulation under the control of a simulation monitor. The monitor allows signals and other state inform
46、ation to be viewed or stored in a trace file for later analysis. It may also allow interactive stepping of the simulation process, much like an interactive program debugger.The computer-synthesized models of the structure and behavior of the digital systems are used for the elaboration of project un
47、its. 5. CONCLUSIONWith the increasing complexity of the projects, structural presentation on a logical elements level, becomes a hard, even impossible. Therefore, a higher abstraction level description would allow optimal results to be reached, such as consummation, characteristics, size and price. The hardware description language VHDL is quite suitable for purposes of that kind. It can be used for a high-level behavioral description, as well as for detailed structural description.This language provides: a standard