Input/output interface unit design factors

Importance of an interface unit

An interface unit connects the processor/controller to the outside world (sensors, keyboards, displays, actuators, other systems). It is needed because the processor and external devices differ in voltage and current levels, speed, signal type (analog vs digital) and data format. The interface unit:

• Adapts signal levels and drive capability (buffers, level shifters, drivers).
• Converts between analog and digital (ADC/DAC) and conditions signals (amplify, filter).
• Synchronises different speeds (latches, handshaking, buffers).
• Provides isolation and protection (opto-isolators, TVS) from noise and faults.
• Provides addressing/control (decoders, ports) so that the processor can select devices.

Factors in design

Input interface: signal type and range, source impedance, bandwidth, resolution and accuracy, noise and common-mode voltage, amplification, filtering and anti-aliasing, input protection (overvoltage, ESD), isolation, sampling rate and ADC choice, conversion time and triggering. Output interface: load voltage/current/power (driver capability), load type (inductive loads need flyback diodes), isolation (opto-coupler/relay), switching speed, fail-safe state at reset, latching of data, output protection and heat. Common: compatibility of logic levels (TTL/CMOS), timing and handshaking, power consumption, cost, EMI, and reliability, testability.