1、DESIGNING A DIGITAL SYSTEM WITH VHDL Valentina Stoyanova Kukenska Abstract: 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
2、presented. 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
3、 1. INTRODUCTION The 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 dec
4、omposition 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 inp
5、uts. 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 n
6、ot appropriate 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.
7、 In 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 pape
8、r a 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 presented. The stress is put on the use of VHDL for synthesis of structural and behavioral models. Here
9、 are presented several VHDL models of computer systems components. 2. METHODS AND STAGES IN DIGITAL SYSTEMS DESIGN In 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 th
10、e system 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
11、error check 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 t
12、hrough behavioral 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
13、regard to 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 l
14、evels description. 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 (electr
15、ical) designing, 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
16、used. The Functional (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 is applied. It allows that the scheme with logical elements be synthesized from a VHDL description. Through logical description details
