The functional specification is a contractual document describing what you are building. It will be used not only to guide the design, but also to provide the baseline for testing the product. Because the specification is contractual, it cannot be altered without sign-off from all affected parties.

Structure

There are four main parts to the functional specification:

1. Introduction and overview
2. User Interface
3. Technical Interfaces
4. Specifications

Introduction and Overview (2-3 pages)

The overview should be a brief introduction to your product. Describe what purpose it is serving and give a high-level description of how it will work. The overview should briefly mention all of the basic functions of your product and provide a basic sketch of your product. Remember that this is not marketing material - do not hype your product here.

1. Introduction
2. Basic functions
3. Illustration (sketch) of your product

Specifications (3-5 pages)

The specifications document is a contractual document.

In your design matrix, several design goals and criteria were stated informally. In this section you will describe these formally in terms of specifications.

First, include the following sentence at the top of your specifications for clarity:

"The User Interface and Technical Interface descriptions included with this specification document are considered contractual specifications."

For each of the design goals:

1. Express the design goal in specific criteria. There generally will be more than one specification for each design goal.
2. Describe each specification as specifically as possible. Use numbers unless it is not possible.
3. Each specification must be testable - the details of the test will be left for later, but you should immediately be able to think of several ways of testing each specification.

Your project will typically have 10-20 specifications. You do not have to specify issues related to user or technical interfaces in the specification section since they are included below.

User Interface (2-3 pages per basic function)

The UI specification is a contractual document.

Specify how a user will operate your product. For each basic function listed in the overview, include the following:

1. Interfaces used (pushbuttons, displays, ports, etc.). You may want to refer to your illustration in the overview.
2. Procedural description of the user's operation of the function. List the steps that a user must take to accomplish this function. Remember, this is a specification, so you should be concise in your descriptions.
3. Diagrams that show samples of any display screens used for this function.

Technical Interfaces (1-3 pages per interface)

The Technical Interface specification is a contractual document.

Describe the analog, digital and mechanical interfaces for your product.

Kevin Bolding October 24, 2006