Chapter 4

Interface Devices

This book is primarily concerned with embedded control. That is, a microprocessor-based system which is dedicated to a limited range of tasks, such as washing machine control, traffic light operation, credit card verification, etc. This chapter looks at the electrical characteristics of typical I/O devices.

Single Line Input

Switches come in two varieties: toggle (latching) and momentary (non-latching). Most microprocessor systems use momentary inputs. This is intentional so that upon a power failure, the system returns to a known state prior to the next power-up.

The switch consists of a pair of contacts, one of which has either a pull-up resistor or a pull-down resistor as illustrated in Figure 4.1. The signal ...

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