Again, as a start point, we will look at the development phase as a whole and see how it is organized.
Globally, the development phase looks like a complex loop with multiple return path branches:
As already stated, it is one of the goals of the TOP-DOWN design method to secure the development steps so as to minimize the number of iterations, speeding up the development process and creating better products.
Specification in the development phase is somewhat equivalent to definition in the product life. The goal is to define what has to be done. The difference between definition and specification lies in the standpoint. Definition, sometimes called “Requirement Specification” defines what is required from the application, from the user's standpoint. Specification, sometimes called “Design Specification” defines the product from the implementation, from the designer's standpoint. Creating the Design Specification from the Requirement specification is the very first actual design step. Just like the requirement specification, the design specification is a list of product functions and performances. Examples will be given later on for design specifications.
As this book's title states, design is out primary goal. Design will be detailed along the next chapters, so it will not be here.
Manufacturing in the development phase is one part of production in the product life. During the development phase, manufacturing is normally limited to prototypes or at least low volumes and may not use exactly the volume production equipment or the same tooling even though sometimes it does. In any case, the required tooling must be developed.
Validation consists in checking that the product complies with its definition, but also in checking that the production equipment can manufacture it in good conditions for both functions, performances and cost. The product must be checked extensively, on a significant number of parts and reliability must be evaluated. Definition correctness with respect to customer need must be checked as well. Some products are just fine, they comply to their specs, but the users don't like them...
The validation phase has to validate everything, from the product definition with respect to the actual customer needs to the product functions and performances, to the product matching with the production equipment and with the economic aspects.
Starting from here, we will focus more and more on one particular product: The analog integrated circuit. IC development differs from other electronic products development mainly for the design and manufacturing phases. More precisely, it is because manufacturing is very specific that design has to take into account this specificity.
These characteristics impact the design since the “try and fix” method is not really possible or is very expensive and time consuming. On the other hand, IC design is not limited by available components or building blocks as these can be designed on request. IC design is not really limited by complexity. IC development is one of the few domains of human activity where it would be possible to manufacture something more complex than what can be designed!
At the difference of board design, IC design is not limited by available building blocks as these are created as required. From experience, this is a huge difference and it can be felt when a board designer or an electronic subsystem designer is involved in specifying an ASIC. Usually board and subsystem designer have real difficulties in defining their needs as they are used to choose available ICs and try to build their application with that. Usually, they can explain what they did, which components they used, but they have hard times explaining what the target was and what the design process was.
When it comes to specify an IC, the first questions are really:
What functions and performances do I need for the IC for the global application to work fine? Answering these questions in detail will define the “Requirement Specification” that describes the circuit from the application requirement standpoint.
And then, from functions and performance as described in the requirement specification, the designer can start the design process by wondering:
How can I build that? What blocks do I need? With what performances? How should I organize these blocks to get the job done? The answers to these questions are the basement of the future design. Altogether, they define the “Design Specification” that describes the circuit from the implementation standpoint.
After the design is completed, a “Data-sheet” is issued. This document describes the circuit from the user standpoint with the goal of helping him to develop the application.
A significant difference between board and IC manufacturing lies in tooling costs prototype costs and manufacturing lead time. Depending on the process, mask costs lie in the 50k to 1M+ dollars range.
The development loop forward path can last a significant time. During all that time, the development loop is open, there is no feedback. Does that mean that the development is out of control? That would be unacceptable as too risky and too expensive but in fact it is not the case. Each of the big steps has its own internal control loop:
In the next chapter, we will finally focus on design, the primary goal of this book, as the title suggests.
The overall development loop should ideally be walked through only once. But if something goes wrong in the process some rework is required. If this happens, it means that the Top-Down method has not been applied properly.