This handbook targets designers so it is mainly focused on design. In this section, design is described from a very general standpoint. What is said here can apply with modulation to any kind of design, it is not restricted to analog IC design. Later on, we will focus on electronics and more precisely integrated electronics into a more detailed approach.
3.1 Design activity
What is design? The word itself has a number of possible meanings:
- Arrangement of parts
In this book, by the word “design”, we mean some sort of a combination of all these meanings. In addition, we can state that design activity also should also includes the required elements for manufacturing the product efficiently:
- Not only must the designer organize the product components together to meet the requirement, but also has he to think about and implement the various technical elements that will allow manufacturing and testing the product.
3.1.1 Designer’s work
What does a designer do all the day long at work?
A designer's work consists in only two things:
- Implementing and validating these choices.
But these two “simple” things keep designers busy all day (and sometimes night!) long.
What does a designer choose and why?
At this point, we have to make a difference between designing a simple object and designing a composite object created by combining simple objects.
3.2 Simple objects design
When designing a simple object, the designer has to choose only three parameters:
- Material. The material the object will be made from.
- Shape. The global object shape.
- Size. The object physical dimensions.
The characteristics of a simple object depend only on these three parameters:
- The material has intrinsic properties such as hardness, resistivity, or solder-ability. The object obviously inherits the properties of the material it is made from. Material is the most intrinsic characteristics of a simple object. A stainless steel object is stainless because of the intrinsic property of the stainless steel. Manufacturing process can modify material properties.
- The shape also exhibits properties. A circle has the largest possible area for a given perimeter for instance. The object also inherits the shape properties. The shape can modify some of the object properties with respect to the material properties but it cannot change everything. Shape is an extrinsic property. Whatever its shape, an ordinary steel object will not be stainless.
- Given material and shape, the size impacts the object properties. A large cup contains more liquid than a small cup. A large screw is stronger than a small one. Size is not a property in itself if modifies the intrinsic and extrinsic properties. It is a numeric property.
To summarize, material is an intrinsic parameter, the inner one, shape is an extrinsic parameter, the intermediate one, and size is a numeric parameter, the outer one.
3.3 Options and criteria to make a choice
Of course, when it comes to make a choice, at least two things are required:
- A list of options to choose from.
- A set of criteria to compare options and make a choice.
3.3.1 List of options
It is important to have at least two items to make a choice. That may seem obvious but there are so many implied choices where the designer chooses the first idea that comes to his mind that this has to be said again and again. Very often, having to find two options to choose from suggests the designer other, more relevant options. After a training period, this can become a real way of thinking and makes design an even more enjoyable activity.
Choosing between options requires that a comparison is possible. Comparison requires at least one criterion. When several criteria are considered, comparison is usually based on a weighted average. Among usual criteria are:
- Technical criteria like performance or reliability
- Economic criteria like development cost or production cost.
- Cultural criteria like aesthetics or environmental impact.
Imagine we have to design an object intended to cut food (this object is know as a knife in real life...)
- Material should be hard enough to cut almost everything and ensure long service. Steel for instance is harder than wood.
- Shape should be such that it minimizes cutting effort. A “V” shaped profile with an acute angle will be more efficient than a square profile. But the shape should also be such that it does not cut the user's fingers ! So, the actual object should have two different sections with different shapes. Finally, it appears not to be a simple object!
- Size should be such that it fits in ones hand. Blade section should be sufficient for enduring the cutting effort.
What about costs?
- Steel is more expensive than wood or plastic. A this point we have to define the acceptable compromise between cost and lifetime.
- For the handle, the material can be softer, cheaper than the blade material at the expense of a more complex assembly process. Again, it's a compromise.
- Manufacturing process impacts costs. Drop forged steel, machined steel or cast steel have different costs.
What about cultural criteria?
- With respect to these criteria, the handle material can be chosen for its look and feel or for its environmental impact, from wood to gold...
- Shape can be made attractive, in addition to be functional. But the definition of a “nice shape” is really a matter of culture, of context, of period.
3.4 Complex object design
For a complex object, resulting from a combination of simple objects, things change a bit with respect to simple objects.
- In addition to choosing and validating the choices, the designer has to divide, to split, to distribute. The problem to be solved has to be divided in simpler problems, the process has to be serialized.
- This is where design is actually a creative activity.
- The three parameters the designer has to choose change a bit:
- Nature of lower level objects stands for material: Our knife is not made from steel but from a blade and a handle.
- Topology of arrangement stands for shape: Our blade and handle have to be assembled together in the right way.
- Values of lower level object stand for size. Our blade has a size, but also a hardness for instance.
- These parameters are functionally equivalent to those of a simple object:
- Nature is the most intrinsic parameter. A capacitor is a capacitor and behaves as a capacitor whatever the way it is used and whatever it's value. This is why nature as the intrinsic property is equivalent to material.
- Topology is the extrinsic parameter. Connecting capacitors in series changes the resulting value, not the capacitive behavior. This is why topology as the extrinsic property is equivalent to shape.
- Value is the numeric parameter. It is not a property in itself, it defines the value of a property. This is why it is equivalent to size. A complex object may have more than one characteristics so it may have more than one value.
For complex systems, objects can be complex as well. In this case, nature can be a functional definition such as “amplifier” of “DSP”. Topology cannot be described easily and often requires a schematic or a formal language description. Value can be a set of parameters since the more complex the object, the more properties it exhibits.
3.4.1 Options and criteria
The same principles as for simple objects apply for selecting from a list and having criteria for selection.
Making a choice means selecting an item within a set. Keeping the first idea that comes to mind is not making a choice. At least two options should exist for a choice to take place. Very often, when a first choice is performed between the initial options, new options come to mind because of the reasons that make some options fail.
As an example again, imagine our goal is to design an electronic circuit:
- Components can be a resistor or a MOS or another electronic component. This nature is logically equivalent to the material like steel or wood for a simple object.
- Components such as resistors can be connected for instance in series or in parallel. This topology choice is logically equivalent to the shape like square or circular of a simple object.
- A capacitor, for instance, has a value in Farads which is logically equivalent to the size of a simple object in meters or inches.
3.5 Architecture and Sizing
Choosing topology and nature of sub-blocks is called“ARCHITECTURE”. Choosing values for sub-blocks characteristics is called “SIZING”.
Making and implementing choices is an error prone process:
- To make choices, the designer uses his brain and he may make mistakes.
- To implement the choices, the designer uses his hands and he may make errors.
For these reasons, validating choices is mandatory. Many issues during product design result from lack of validation.
The simple examples above have introduced the idea of hierarchy for addressing complexity. Complex designs can be managed only if complex objects are divided recursively into less complex objects. This methods also allows to share the workload between several designers to get the job done quicker.
In a hierarchical electronic design, the various blocks are usually called “cells”. One cell differs from all the other ones: The Top-cell. As its name states it, this cell contains all the other ones and no other cell contains the Top-cell. Some cells contain only electronic components but no other cells. These cells are usually called leaf-cells. This refers to a tree. It has a trunk, branches and leaves. Some branches hold leafs and other branches. Some branches hold only leafs.
- Trunk is called Top-cell
- Branches are called Cells
- Leaves are called Leaf-cells
3.8 Design levels
Designing can be thought a bit like cooking.
- You may use a recipe and execute it without any change.
- You may change quantities to make the recipe suitable for a larger or a smaller group.
- You may decide to change one element by another.
- You may merge two recipes
- You may create a new recipe
This depends on the situation and on your skill in this field. It is the same for design and these options can be called design levels.
The design level can be defined by a number ranging from 0 to 4. Ranking is based on “depth” of what the designer deals with. The deeper the parameter, the higher the score. In this scale, nature is the heaviest parameter, topology is second and value is the lightest parameter.
- Level 0: Copy. Change nothing.
- Level 1: Re-size. Change values.
- Level 2: Modify. Change topology, natures and values.
- Level 3: Combine. Merge topologies and change natures and values.
- Level 4: Create. Imagine new topologies, choose natures and values.
This design level details the kind of design activity:
Many classic design problems have some standard solutions that have proved to be efficient and reliable. Using these solutions “out of the box” results in the fastest and safest design route. This is the base of the so called IP business.
- The only requirement is that the copied cell meets or exceeds the target specification.
- One limitation is that sometimes a simpler solution might exist and would result in a smaller silicon area. A compromise must be found in this case between a shorter and safer design route that reduces NRE costs and a smaller and simpler solution that reduces production costs.
If no “ready to use” solution is available, resizing an existing solution is a reasonably fast and safe design route. Resizing is changing sub-blocks values. Basically, resizing changes a cell performance without changing its functionality.
- Requirements are that the origin cell functionality meets or exceeds the requirements and that re-sized cell performance can meet the target specification.
- The same limitation as for copy exists: A simpler solution may exist but would require more design work. Again a compromise is often required.
If functionality has to be changed, modifying an existing solution is faster and safer than more creative design routes. Modifying is changing both topology, cells nature and characteristics.
- The approach being more flexible and creative than lower levels, limitations in functionality and performance is less critical.
- The limitation that exists in copy and re-size approaches is not as critical here: If a simpler solution exists it can be implemented in the modification process eventually at the expense of more design work. Again a compromise is often required.
If nothing is available to re-size or modify, combining existing solutions brings a new solution while keeping some safety and limiting effort. In this approach, combining existing or modified topologies extends the functionality and performance domain further.
- This approach suffers only few limitations and is one of the most powerful. It can bring outstanding performance at a reasonable design effort.
If everything else has failed, a new solution must be created. This is a longest and most difficult design route. It is strongly recommended to always use the lowest possible design level. The boundary between combining and creating is not very clear. Creating something new is generally combining existing functions. Creation might be defined as bringing a new functionality that did not exist before.
3.8.6 Design route
It is a good practice to begin with level 0 and then increase level until a solution is found.
- The time spent trying the lowest levels is short and anyway shorter that starting at a too high level.
- But, in addition, it is also a good way of reviewing the specification and finding missing items at an early stage. And part of the work, like setting up the design validation can be reused.