Systems & Devices – Part 1

Alexander Nelson

February 1, 2022

University of Arkansas - Department of Computer Science and Computer Engineering

What composes wearable, ubiquitous, or

invisible systems?

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Components of Systems

Ubicomp systems are composed of a subset of the following:

• Sensors

• Actuators

• Communications

• Displays

• Storage

• Computation

where the components & scale of each is defined by the application

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Sensors

Types of Sensors

Many different types of sensors

Wikipedia categorization (summarized):

• Acoustic

• Chemical

• Electric Current

• Environmental

• Proximity

• Inertial

• Optical

• Pressure

• Force

• Thermal

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Sensor Interfaces

Systems use many interfaces to obtain sensor values

These include:

• Binary Signals

• Analog to Digital

• Parallel Interfaces

• Serial Interfaces (SPI, I2C, UART)

• Pulse Width Modulation

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Binary Signals

Binary Signals – 0 or 1

Pushbuttons, Threshold values, counts, etc...

P2 is latched periodically

Will be 0 unless switch is engaged

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Debouncing

Imperfect contact can create “glitches”

Use delays to verify signal is steady

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Analog vs. Digital

Analog Digital

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Conversion

To work with sensor data, analog signals must be converted to

digital

Analog to Digital Conversion (A to D)7

Sampling/Conversion

Sampling – Converting Analog to Digital Values

Precision/Quantization Errors – Error introduced because digital

values have a defined precision

Aliasing Errors – Error introduced by periodic signals

Nyquist Rate – Minimum rate at which periodic signal can be

sampled without introducing aliasing errors

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Aliasing Error

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1http://www.bb-elec.com/Learning-Center/All-White-Papers/Data-

Aquisition-and-IO/The-Fine-Art-of-Analog-Signal-Sampling.aspx

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Measurement Circuits

Sensor manufacturers create elements that have a predictable

response to a stimulus

These responses modulate a quantity that can be sampled by a

digital system

Typical quantities:

• Resistance

• Capacitance

• Inductance

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Analog to Digital – Voltage Divider

Resistor Sensors – Thermometers, Photoresistors, Force sensors,

etc...

Measure resistance through two known quantities

Derived from Ohm’s Law (w/ assumptions)

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Analog to Digital – Wheatstone Bridge

Wheatstone Bridge – Voltage Measurement

Can allow differential measurement, non absolute references, and

can be extremely precise

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Analog to Digital – Capacitance Measurement

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Analog to Digital – Capacitance Measurement

Capacitance can be measured by the voltage change over time

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Parallel Interfaces

Parallel Interface – Many binary signals to represent word-length

values

D0-D7 represent a single byte to be transferred

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Serial Interfaces

Series – a number of things coming one after another

Serial Interface – Bits transferred one after another

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Serial Interfaces – UART

UART – Universal Asynchronous Receiver-Transmitter

Asynchronous – No shared clock

Data transfer is unidirectional

Transmission Speed – Usually given in baud (bits per second)

Receiver & transmitter have foreknowledge of transmission speed

Data frame – Data transferred byte-by-byte

Each frame is bordered by start & stop bit(s)

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Serial Interfaces – SPI

SPI – Serial Peripheral Interface

Synchronous unidirectional master–slave data transfer

Master device configures clock, clock polarity, and when

communication occurs

Data frame – Frame started by a logic 0 on the Source Select line

Determines which slave device is to listen

Data is transferred bit-by-bit, latched at either rising or falling

edges

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Serial Interfaces – SPI

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SPI Timing Diagram

2https://en.wikipedia.org/wiki/Serial Perip-

heral Interface Bus#/media/File:SPI timing diagram2.svg

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Serial Interfaces – I2C

I2C – Inter-Integrated Circuit (I-squared-C)

Address-based 2-wire communication interface

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Serial Interfaces – I2C – Data Transfer

1. Data transfer initiated(S) – SDA pulled low, SCL stays high.

2. SCL is pulled low, and SDA sets the first data bit level while

keeping SCL low (during blue bar time).

3. The data are sampled (received) when SCL rises for the first

bit (B1). For a bit to be valid, SDA must not change between

a rising edge of SCL and the subsequent falling edge.

4. This process repeats, SDA transitioning while SCL is low, and

the data being read while SCL is high (B2, ...Bn).

5. The final bit is followed by a clock pulse, during which SDA is

pulled low in preparation for the stop bit.

6. Stop bit (P) signaled when SCL rises, followed by SDA rising 21

Serial Interfaces – I2C – Addresses

I2C has only 2 data lines

Uses addresses to specify which chip listens

Address is 7 bits followed by a write bit

Write bit specifies if action is to read or write at register address

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Pulse Width Modulation

Pulse Width Modulation – Encoding message through pulsing

signal

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3By Zureks - Own work, CC BY-SA 3.0,

https://commons.wikimedia.org/w/index.php?curid=1835361

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Pulse Width Modulation

Applications of PWM:

• Sensor output

• Servo Input

• LED dimming

• Power Delivery

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