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Interface Design
Omid Fatemi
University of Tehran 1
Interfaces and Interfacing
Definitions of “interface” from Webster’s Dictionary:
noun: the place at which independent systems meet
and act or communicate with each other.
e.g. human - machine interface
digital - analogue interface
terminal - network interface
TTL - CMOS interface
parallel or serial interface
verb 1: to connect by means of an interface.
verb 2: to become interfaced.
verb 3: to interact or co-ordinate harmoniously.
e.g. to interface a printer to a computer
to interface a PC to a mainframe
to interface TTL logic with ECL logic
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Why is computer interfacing
important ?
1) The human - machine interface determines the
ultimate success or failure of many computerbased systems.
2) Digital systems exist within and must successfully
interact with an analogue natural environment.
Digital - analogue interfaces are unavoidable.
3) Rather than designing digital systems from
elementary components, computer engineers
more typically assemble new systems from
existing subsystems.
Interfaces between digital hardware subsystems
and between software modules must be
thoroughly considered.
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Why is computer interfacing
important ?
4) With advances in technology, special attention
must increasingly be paid to analogue effects
affecting high-speed “analogue” interfaces.
5) Interfaces are a key area of standardization
activity. The choice of standards can have
great competitive implications.
6) Interfacing software is typically tricky and
time-consuming to design and test. Much
effort can be saved if proper interfacing
techniques are adopted (and poor interfacing
techniques avoided) and good design practices
and design strategies.
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System-Level Interfaces
Human
Users
DigitalAnalogue
Interface
HumanMachine
Interface
Analogue
Environment
Digital-Digital Interface
Other Digital Systems
Human-machine interface:
Input devices: keyboard, mouse, microphone, camera
Output devices: CRT, printer, light panel, audio amp.
Digital - Analogue Interface:
Input devices: A/D converters, modems, sensors
Output devices: D/A converters, modems, transducers,
actuators, stepper motors
Control devices: switches, multiplexers, amplifiers,
attenuators
Digital - Digital Interface:
Connectors: wires, ribbon cable, coax, twisted pair, PCB
I/O devices: buffers, level-shifters, synchronizers
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An Example – PC Computer
Lab Board
CPU
Cache
Main
Memory
Memory
Controller
Serial
Port
Controller
Keyboard
Mouse
Hard
Drive
Disc
Controller
Parallel
Port
Controller
Diskette
Drive
Diskette
Controller
Video
Controller
System Bus
Printer
CRT
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Typical Mechanisms at Interfaces
1) Data buffering and flow control
-- helps compensate for short-term mismatches in
data generation and consumption rates
-- groups data into larger, more efficient chunks
2) Synchronization, handshaking
-- compensates for asynchronous clocks, clock skew
affecting synchronous signals, devices with
different response times
-- bit stuffing to equalize bit rates (telecom technique)
3) Digital processing
-- framing, packet assembly and disassembly
-- data communication protocols
-- encoding, decoding, code conversion
-- error detection and error correction
-- data compression (e.g. using Huffman codes)
4) Analogue signal conversion and conditioning
-- restore correct voltage and current levels
-- restore correct signal rise and fall times
-- pulse-shaping and channel equalization
-- take precautions to reduce noise and reflections
-- carrier modulation and demodulation
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Typical Interface Design
Connect
Sense Reality
Touch Reality
Connect
Transform
Compute
Embedded Systems
Micros
Assembler, C
Real-Time
Memory
Peripherals
Timers
DMA
Convey
PC interfaces
HCI
Cooperate
Busses
Protocols
Standards
PCI
IEEE488
SCSI
USB & FireWire
CAN
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Sensors
more generally a transducer:
“a device that converts variations of one
quantity into those of another”
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Sensors for Box Counting
• Sensors connect the
digital world to the
analog real world
• light sensistive
resistivity
• voltage divider for
change in resistance
• data logged by a
computer
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Shaft Angle With Potentiometer
• shaft angle proportional
to voltage Vs
• voltage Vs changes
because of the change
in resistance
• simple application of
Ohm’s law
• current is constant
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Linear Variable Displacement
Transformer (LVDT)
• moving iron core
changes properties
of transformer
• iron core position
changes
primary/secondary
voltage ratio
• difference in phase is
measured and
tranformed to a
voltage
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Strain Guage
• resistance varies with the
ammount of stretching
(strain)
• flexure can be measured
with a strain guage
• force can also be
measured
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Thermocouple
• Seebeck effect: current will flow through a junction of
disimilar metals if there is a temperature difference
• millivolts and non linear
• varies from 6 uV/C to 90 uV/C
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Thermopile
• multiple thermocouples can be connected together to create
a thermopile where multiple measurements can be used to
find the arithmetic mean of several temperatures
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Phototransistor
• current through external
resistor varies with light
intensity
• can be used to detect
light levels or movement
• sensitive to different
colors or wavelengths of
light
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Hall Sensor
• material of thickness t has
current i flowing through it
• magnetic field H
perpendicular to i
• voltage developed on third
axis
• E = RHi/t
– R is hall constant
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Sensor Resolution Improvement
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Nonlinear Characteristics
•
•
•
•
•
Hysteresis is caused by memory
Saturation is when sensor input exceeds range
Breakdown is usually a damaged overdriven sensor
Bang-bang is a sudden large change in output
Dead zone is where input change makes no difference
University of Tehran 20
2 Dimensional Accelerometer
• ADXL202: Low-Cost ($25)
2g Dual Axis Accelerometer
with Digital Output
• dynamic acceleration (e.g.
vibration)
• static acceleration (e.g.
gravity)
• Low Power < 0.6 mA
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Ultrasonic
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Sensor Interfaces: summary
• voltage source
– directly measured
• variable resistance
– can be converted to a voltage and measured
– voltage divider for coarser measurements
– wheatstone bridge for finer measurements
• variable capacitance
• variable inductance
• variable signal
University of Tehran 24