Project
A good design is one that solves a particular problem in the most appropriate and efficient manner. To achieve this the designer requires not only a good understanding of the problem but also a wide ranging knowledge of available techniques and technologies. Inevitably this means that design ability increases with experience, but this should not be seen as reducing the importance of a systematic and methodical approach.
The top-level design results in a block-diagram form of description of the system and a specification of each block of hardware and software. Based on this information, work can move to progressively greater levels of detail.
· For Monday, March 19th, you should describe the following for your Project Proposal:
Customer requirements
The customer requirements represent the problem which the system is to solve. In some circumstances the customer and designer may be one and the same person, but the principle remains the same. The requirements of the system are usually, and correctly, expressed in terms of the problem rather than in terms of the solution.
Top-level specification
The top-level specification is an attempt to define a system that will satisfy the customer’s requirements. The definition is usually in the form of a written description in a natural language (the term ‘natural’ is used here to distinguish it from a programming or other computer language), but may include appropriate mathematical equations or expressions. We shall see later, when we come to look at electronic design tools, that there are other more precise methods of defining a system. The problem with a written specification in a natural language is that it is extremely difficult to write in a way that is not open to misinterpretation.
It is important to ensure that the specification defines what the system is to do, not how it is to do it. Such topics as the appropriate device family, or the use of analogue or digital techniques, do not fall within the realms of the specification.
· For Monday March 26th, you should complete the following for your Progress Report:
Top-level design
Once the specification of the system is complete, the design stage may be commenced in a top-down manner. For large projects the first task is usually to divide the system into a number of manageable modules. A specification is then produced for each module, enabling it to be designed and tested independently.
One of the earliest design decisions to be made concerns the choice of technology. Invariably it will be possible to implement a given function in a number of ways, using, for example, analogue, digital or software techniques. Often large systems will include elements of each of these methods. Until these decisions have been made it is usually impossible to progress to more detailed aspects of the design.
In addition to providing a specification for the system, it is common at this stage to define a series of tests which the resultant system must perform to prove its suitability for the task in hand. These tests will then form the basis of system testing and the ultimate demonstration of the system.
Detailed design
If the top-level design has been performed efficiently, the detailed design stage of the project should be relatively straightforward. Each hardware section will consist of a series of functions which can usually be assembled from standard circuit building blocks. The design should be drafted in 5Spice; the SPICE models of specialized components are often available on the manufacturer’s websites.
Shopping List
Make a list that specifies what (and how many) components you’ll need for the project. For diodes and integrated circuits, give as much information as you can; we’ll likely need to make some substitutions, so we’ll need to know the specifications to look for.
Data Sheets
Manufacturers of integrated circuits provide Data Sheets that tell you how their chips get wired up, quantify their performance (speed, leakage currents, …), give some sample circuits, and give some test data. These are invaluable when you’re building and testing a circuit. You can often find these by googling the part numbers. www.datasheetcatalog.org has many available.
Here is an example Project Proposal.
· From Monday, April 2nd to the end of the semester, you will be work on constructing your project. You should use the following approach. Be sure to keep notes in you lab notebook on what testing you have done!
Module testing
When the design of a system is complete, the various modules must be constructed and tested to ensure that they perform their required functions correctly.
Unlike design which is performed in a top-down direction, testing is usually performed using a bottom-up approach. This involves first verifying the operation of each individual circuit element, and then investigating the functioning of progressively larger subsystems. Testing is performed in this way since both error detection and fault location are easier when dealing with small sections of circuitry rather than a complete system. Since the theoretical expectations for a complex circuit can be difficult to derive, any experimental tests should be accompanied by computational ones, i.e., simulations in 5Spice. Any faults found at this stage must be rectified before continuing to system testing.
System testing
When each module has been tested and any corrective work performed, the complete system may be assembled and tested. Only at this stage is it possible to see whether the system meets its top-level specification, and to confirm that it does indeed fulfill the customer’s requirements. It will normally be necessary to demonstrate the system to the customer and to perform any prescribed system proving tests.