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DEPARTMENT OF
ELECTRONICS AND COMMUNICATION ENGINEERING
SUBJECT: EMBEDDED SYSTEM DESIGN (ESD)
By
Y.Pradeep
Associate Professor
UNIT 2: TYPICAL EMBEDDED SYSTEM
Lecture-6-7:
Topic : “The I/O Subsystem”
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The I/O Subsystem:
The I/O subsystem of the embedded system facilitates the
interaction of the embedded system with external world
The interaction happens through the sensors and actuators
connected to the Input and output ports respectively of the
embedded system
The sensors may not be directly interfaced to the Input ports,
instead they may be interfaced through signal conditioning and
translating systems like ADC, Optocouplers etc
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I/O Devices: Light Emitting Diode (LED)
Light Emitting Diode (LED) is an output device for visual indication in
any embedded system
LED can be used as an indicator for the status of various signals or
situations.
Typical examples are indicating the presence of power conditions like
„Device ON‟, „Battery low‟ or “Charging of battery‟ for a battery
operated handheld embedded devices
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I/O Devices: Light Emitting Diode (LED)
LED is a p-n junction diode and it contains an anode and a cathode.
For proper functioning of the LED, the anode of it should be connected
to +ve terminal of the supply voltage and cathode to the –ve terminal of
supply voltage
The current flowing through the LED must limited to a value below the
maximum current that it can conduct.
A resister is used in series between the power supply and the resistor to
limit the current through the LED
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The 7 – segment LED display is an output device for displaying alpha
numeric characters
It contains 8 light-emitting diode (LED) segments arranged in a
special form. Out of the 8 LED segments, 7 are used for displaying
alpha numeric characters
The LED segments are named A to G and the decimal point LED
segment is named as DP
The LED Segments A to G and DP should be lit accordingly to
display numbers and characters
I/O Devices – 7-Segment LED Display
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The 7 – segment LED displays are available in two different
configurations, namely; Common anode and Common cathode
In the Common anode configuration, the anodes of the 8 segments
are connected commonly whereas in the Common cathode
configuration, the 8 LED segments share a common cathode line.
I/O Devices – 7-Segment LED Display
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Based on the configuration
of the 7 – segment LED unit,
the LED segment anode or
cathode is connected to the
Port of the
processor/controller in the
order “A” segment to the
Least significant port Pin and
DP segment to the most
significant Port Pin.
• The current flow through
each of the LED segments
should be limited to the
maximum value supported
by the LED display unit
I/O Devices – 7-Segment LED Display
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Optocoupler is a solid state device to isolate two parts of a circuit.
Optocoupler combines an LED and a photo-transistor in a single
housing (package)
In electronic circuits, optocoupler is used for suppressing interference in
data communication, circuit isolation, High voltage separation,
simultaneous separation and intensification signal etc
Optocouplers can be used in either input circuits or in output circuits
I/O Devices – Optocoupler
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Two parts are used in an optocoupler: an LED that emits infrared light
and a photosensitive device that detects light from the LED.
Both parts are contained within a black box with pins for connectivity.
The input circuit takes the incoming signal, whether the signal is AC or
DC, and uses the signal to turn on the LED.
The photosensor is the output circuit that detects the light and depending
on the type of output circuit, the output will be AC or DC.
I/O Devices – Optocoupler
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Stepper motor is an electro mechanical device which generates
discrete displacement (motion) in response to dc electrical signals
It differs from the normal dc motor in its operation. The dc motor
produces continuous rotation on applying dc voltage whereas a
stepper motor produces discrete rotation in response to the dc
voltage applied to it
Stepper motors are widely used in industrial embedded
applications, consumer electronic products and robotics control
systems
The paper feed mechanism of a printer/fax makes use of stepper
motors for its functioning.
I/O Devices – Stepper Motor:
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Based on the coil winding arrangements, a two phase stepper motor is
classified into Unipolar and Bipolar
Unipolar:
A unipolar stepper motor contains two windings per phase. The
direction of rotation (clockwise or anticlockwise) of a stepper motor
is controlled by changing the direction of current flow.
Current in one direction flows through one coil and in the opposite
direction flows through the other coil.
It is easy to shift the direction of rotation by just switching the
terminals to which the coils are connected
Bipolar:
A bipolar stepper motor contains single winding per phase. For
reversing the motor rotation the current flow through the windings
is reversed dynamically.
It requires complex circuitry for current flow reversal
–
I/O Devices – Stepper Motor:
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• An electro mechanical device which acts as dynamic path selectors for
signals and power.
• The “Relay” unit contains a relay coil made up of insulated wire on a
metal core and a metal armature with one or more contacts.
• “Relay” works on electromagnetic principle.
• When a voltage is applied to the relay coil, current flows through the
coil, which in turn generates a magnetic field.
I/O Devices – Relay:
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• The magnetic field attracts the armature core and moves the contact
point.
• The movement of the contact point changes the power/signal flow
path.
• The Relay is normally controlled using a relay driver circuit
connected to the port pin of the processor/controller
• A transistor can be used as the relay driver. The transistor can be
selected depending on the relay driving current requirements
I/O Devices – Relay:
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It is a piezoelectric device for generating audio indications in
embedded applications.
A Piezo buzzer contains a piezoelectric diaphragm which produces
audible sound in response to the voltage applied to it.
Piezoelectric buzzers are available in two types
1.Self-driving
2.External driving
I/O Devices -Piezo Buzzer:
Self-driving contains are the necessary components to generate sound at a
predefined tone.
External driving piezo Buzzers supports the generation of different tones.
The tone can be varied by applying a variable pulse train to the piezoelectric
buzzer.
A Piezo Buzzer can be directly interfaced to the port pin of the
processor/Controller.
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I/O Devices – Push button switch:
Push Button switch is an input device.
Push button switch comes in two configurations, namely “Push to Make”
and “Push to Break‟
The switch is normally in the open state and it makes a circuit contact
when it is pushed or pressed in the “Push to Make” configuration.
In the “Push to Break” configuration, the switch normally in the closed
state and it breaks the circuit contact when it is pushed or pressed
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The push button stays in the “closed” (For Push to Make type) or
“open”(For Push to Break type) state as long as it is kept in the pushed
state and it breaks/makes the circuit connection when it is released.
Push button is used for generating a momentary pulse
I/O Devices – Push button switch:
