Page 18 - Embedded Microprocessor Systems Real World Design

Requirements Definition

The requirements definition (which, again, may actually be part of the product
specifications), describes what the product is to do. In a very large company, the
marketing department or a major customer may define the requirements. In a
smaller company, the hardware and software engineers may sketch out the require-
ments definition. For a small, oneengineer project, the requirements may be the
result of a momentary inspiration.
The requirements definition can take the form of a book-defining every inter-
action, interface, and error condition in the system-or a single-page list of what
the finished product must do. In either case, the requirements definition must
describe:

What the system is to do
What the real world 1/0 consists of
What the operator interface is (if any)

In a small embedded control system, defining the requirements is crucial, as it
prevents problems later when you find out that there is insufficient RAM or that
the microprocessor you have chosen is too slow for the job. A simple example of
this is the following system definition for a swimming pool pump timer. (Appendix
A contains the complete requirements definition and specifications.)

System description: A swimming pool timer that cycles the alternating current
(AC) pump motor on a swimming pool.
Power input: 9 to 12V DC from a wall-mount transformer.
Pump is a 1/2-hp, single-phase, AC motor, controlled by mechanical relay.
Provision is to be made for a switch closure input that inhibits pump
operation if the water level is low.
User can set the length of time the pump is on and off. An override is
available to permit turning off the pump when it is on for maintenance and
turning on the pump when it is off so that chemicals can be added.
On/off/override time is to be adjustable in 30-minute increments from 1/2
hour to 23 hours.
A display will indicate the on/off condition of the pump, the time remaining,
and whether the pump is in override mode. The display also will indicate the
condition of the water-low monitor.
Minimum switches and knobs.
In addition to a list of requirements and functions like this, a system that is
intended to be a commercial product might also include requirements for EMI/

System Design 3