Unmanned Aerial Vehicle

Unmanned Aerial Vehicle

Arab Academy for Science and Technology and
Maritime Transport

College of Engineering and Technology
Department of Computer Engineering

UNMANNED AERIAL VEHICLE
(UAV)

Presented by:
Alexander Mohamed Osman
Riyad Ahmed El-laithy
Ruyyan Ahmed El-laithy
Peter Raouf Zaky

Supervised by:
Dr. Ibrahim Imam

((July 2007))
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ACKNOWLEDGEMENTS

After thanking God the Merciful we would like to send our thanks to the following
people:

Firstly we would like to thank Dr. Ibrahim Imam for proposing the idea of an
Unmanned Aerial Vehicle and for accepting us to carry on that project.

Secondly we would like to thank Dr. Atallah Hashad for giving us a helping hand
whenever we needed one and for providing us with solutions for all the challenges we
faced.

We would like to thank Dr. Hassan Ibrahim for providing us with help with the
electrical problems we faced in our circuits.

We would also like to thank Dr. Gamal Selim for his encouragement, assistance and
understanding.

We would like to thank Dr. Yasser Galal for answering some questions we had
about DC motors.

We would also like to thank Eng. Ahmed Akl, Eng. Renad Kamal, Muhab Bahgat,
Ruyhan El-Laithy, Fady Mounier, Beshoy Helmy, Todd Elliot, and Sparkfun Electronics
for supporting us and/or making this possible.

Last but not least we would like to thank our parents & families for their love,
support, and understanding.

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ABSTRACT

Gathering information from locations which are inhabitable, hostile, or difficult to
reach is a crucial aspect for learning new information about unmarked territories and
activities and aids in human technological advancement. This project is concerned with
developing an agent for gathering visual information by holding a stationary position or
pursuing a dynamic target. The agent is a quadrotor VTOL (Vertical Take Off and
Landing) aircraft. This agent should have the capability to hover, fly and follow targets. It
should receive and transmit data wirelessly into a base station. It should move through a
predefined plan using a GPS receiver. It should also balance itself in the air through a
gyrometer and an accelerometer. In addition it would utilize four ultrasonic sensors for
obstacle avoidance and an extra one for landing assistance. The agent would also utilize a
wireless camera to transmit a bird s eye view to the base station.

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TABLE OF CONTENTS

1. INTRODUCTION .......................7

2. CONCEPTUAL DESIGN & PHYSICAL ASSEMBLY .. 12

3. ANALYSIS, COMPONENT-LEVEL DESIGN & SELECTION .18
3.1 Major Components ..18
3.2 PCB Design .. .......25
3.2.1 Interface Boards ...27
3.2.1.1 GPS interface board .27
3.2.1.2 Accelerometer / Gyrometer interface board ................28
3.2.1.3 RF Interface boards 24-G .. ............28
3.2.2 Motor Driver 29
3.2.3 The Brain.. ...31

4. CONTROL 33
4.1 Introduction .........33
4.2 SPI communication .35
4.3 Main PIC Implementation ................38
4.3.1 Pulse Width Modulation (Motors) ...38
4.3.2 ADC Operation ............43
4.3.2.1 Ultrasonic Sensors 50
4.3.2.2 Gyrometer .. ...52
4.3.2.3 Accelerometer . ..52
4.4 Secondary PIC Implementation ......55
4.4.1 GPS System .55
4.4.2 RF Transceiver .66
4.5 RC Unit ......75

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5. TESTING TROUBLESHOOTING AND REDESIGN 78
5.1 Testing ..78
5.1.1 LED Testing ..78
5.1.1.1 Accelerometer Testing ..78
5.1.1.2 Gyrometer Testing 78
5.1.1.3 SPI Testing 79
5.1.1.4 RC unit testing ..................79
5.1.2 LCD Testing ..80
5.1.2.1 Ultrasonic testing . ..81
5.1.2.2 Accelerometer Testing . ..81
5.1.2.3 RC unit Testing . ................82
5.1.2.4 GPS Testing . .83
5.1.3 RF Testing . . .83
5.1.3.1 Ultrasonic Testing . 83
5.1.3.2 Gyro Testing . 83
5.1.3.3 RC Unit . 85
5.2 Previous Chassis designs . ...88
5.3 RF Drivers . .....90
5.3.1 Laipac RF TX/RX ...90
5.4 Configuration 1 ......92
5.5 Configuration 2 ..96
5.6 Brain #3 .....99
5.7 Correcting Gyro Output 100

6. FUTURE IMPLEMENTATIONS ..102

7. CONCLUSION .......103

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APPENDIX A : COMPONENT DATABASE AND CHARACTERISTICS . .104

APPENDIX B : CONTROL CODE . 106

APPENDIX C : WEIGHT & THRUST CHARTS .. . 165

APPENDIX D : LITHIUM POLYMER BATTERY CARE .167

APPENDIX E : ICSP PROGRAMMING .169

APPENDIX F : REFERENCES .170

APPENDIX G : BIBLIOGRAPHY ..173

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INTRODUCTION

The rapid development of micro-processor technology and the continuous growth of
integration density of electronical and mechatronical components yields a significant cost
reduction of high tech products. Driven by this development it becomes feasible to embed
information processing and communicating devices in all sorts of appliances, toys,
production facilities, communication systems, traffic and transport systems etc.

With this integration and the aid of global positioning systems, there has been a
surge of development in Unmanned Vehicles (UV). The main benefits of UV s are that
they do not require human control and thus can be reduced in size and cost. They also limit
human error in several aspects, and reduce if not eliminate human endangerment.
Unmanned vehicles are developed for use in air, over land and under water by both private
and government agencies. Several unmanned systems exist such as Autonomous
Underwater Vehicles (AUV), Unmanned Ground Vehicle (UGV), and Unmanned Combat
Vehicles (UCV). NASA deploys USVs (Unmanned Space Vehicles) on rock gathering
missions from the Moon and Mars. The military advanced UAVs and renamed them to
UAVS (Unmanned Aerial Vehicle Systems) and are used in flight combat.

Government search and rescue departments find the UAVs helpful in inhabitable or
hazardous terrain such as earthquakes, floods or volcanoes, where no human lives have to
be risked. Institutions which have onsite geologists use UAVs for uncovering terrain and
rock identification, without having to deploy a whole crew working outside. Departments
of transportation can use this device to cover footage of inaccessible situations such as
dead-lock traffic jams or multiple car-crashes. Government law enforcement and
intelligence agencies can specifically find this device useful for reconnaissance and target
pursuance, where the UAV provides the advantages of cheap costs, stealth and a
diminished human risk factor.

The Unmanned Aerial Vehicle project has been an ongoing attempt to produce a
reliable autonomous hovering or flying vehicle. The project designed and implemented a
four-rotor hovering aerial vehicle. The advantages of a hovering vehicle over a fixed-wing
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flying vehicle include less complexity in design, minimal space for take-off and landing
(vertical take-off and landing (VTOL)), indoor flight, maneuverability in obstacle heavy
environments and of course the eye-catching ability of being able to maintain a static
position in mid-air.

The advantage of quadrotors over helicopters is that they do not require mechanical
linkages to vary rotor angle of attack as they spin, this simplifies design and control. The
use of four rotors allows each individual rotor to have a smaller diameter than the
equivalent helicopter rotor, for a given vehicle size, allowing them to store less kinetic
energy during flight. These smaller propellers reduce the damage caused should the rotors
hit any objects, this also makes the vehicles safer to interact with in close proximity.

The first RC application of a 4-rotor vehicle was the Roswell Flyer made by Area51
technologies. Now there are several commercially available quadrotor aerial vehicles, to list
a few, Atair aerospace quadcopter , Hammacher Schlemmer four rotor UFO , Keyence
Engager and gyrosaucer and the DraganFlyer V Ti . The team s design was inspired by
the DraganFlyer V, made by Draganfly Innovations Inc. where the four motors and props
are laid at the ends of an X Chassis, and in the center lay the majority of the circuit boards
and microprocessor dubbed by DraganFlyer Inc. as The Brain . (See figure below)

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System Block Diagram

A general control scheme can be seen in the diagram above. The controller block is
composed of two communicating MCU s (MicroController Units). The main MCU does
most of the calculations and decision making. The main MCU also receives inputs from the
proximity sensors and stability sensors, while the secondary MCU is responsible for
communicating with a GPS receiver for positioning and an RF module for wireless
communication. Both MCU s then drive the outputs for the four motors together.

The stability sensors block consists of a 3-axis Gyrometer for angular velocity
measurement and a 3-axis accelerometer for measuring acceleration. The proximity sensors
block consists of 5 ultrasonic sensors placed around the vehicle and under it, for obstacle
avoidance and assisted landing.

The GPS receiver block consists of a GPS module that provides position, velocity,
heading and altitude readings. The RF transceiver block consists of a 2.4GHz RF Module
that communicates bi-directionally with a remote control unit for sending and receiving
data.

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The Motor block consists of 4 high powered brushed motors with a gear ratio of
5.33:1 and 10x4.5 propellers. Both of these features provide a high thrust vehicle (As
opposed to high speed). These motors are controlled through switching transistor circuits
using PWM (Pulse Width Modulation).

The UAV works in three different modes, in the simplest mode a land based PC
sends out signals through an RF transceiver in order to steer the UAV in different
directions. In the second mode a land based PC receives images from an onboard camera,
then a pattern recognition system identifies a target object and sends signals to the UAV
through the RF transceiver to steer it toward the desired object. If the object is not found the
UAV rises in altitude quickly in order to find the object and re-track it. The third mode uses
an onboard GPS that gives the current position of the UAV and it compares that to its target
destination, and steers to its target destination then comes back to its initial point. In all
modes an accelerometer and gyrometer are used to provide stability, and ultrasonic sensors
are used to measure height and avoid obstacles and in turn to steer the UAV away from
them.

Because of the ambitious nature of the project, the team decided to build the UAV
from ground up. Development of our 4-rotor vehicle can be divided into four major
branches.

1. Conceptual Design and Physical Assembly.
2. Analysis, component-level design & selection.
3. Control.
4. Testing, Troubleshooting & Redesign.

Although these four stages overlapped and interfered with one another they can be
discussed independently, without much referencing to other sections.

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CONCEPTUAL DESIGN & PHYSICAL
ASSEMBLY

The conceptual design as stated previously was inspired by the DraganFlyer, and
the team s first step was to identify the design goals. These were the fundamental
requirements the team decided upon:

1. Ability to hover, in the sense of generating enough thrust and have enough
control in order to maintain a mid-air static position.
2. Maneuverability in all directions of a three-dimensional plane.
3. Sufficient endurance of no less than 10-15 minutes.
4. A very light-weight body, including a battery with the highest power to weight
ratio we could find since the battery is the heaviest single component of the
vehicle.
5. High residual thrust to hover thrust ratio, an acrobatic vehicle was desirable for
ability to demonstrate controllability and to perform difficult flight maneuvers.
6. Minimal size & complexity.

The team decided to stick very close to traditional designs of 4-rotor vehicles, where
four electric motors are placed on the corners of a rectangle, and drive four counter-rotating
propellers. These propellers would produce sufficient thrust for take-off, and according to
their different allocated power distributed on the four motors would provide
maneuverability. Any propeller spinning produces a torque on the body it is attached to. For
stability in flight the total resulting differential torque on the body should be zero. This is
demonstrated very clearly in helicopters. The main rotor on the roof of the helicopter
produces a large yaw torque on the body which is countered by the tail rotor on the rear of
the plane. Assuming the main rotor is on a constant rpm, the difference in power to the rear
propeller moves the helicopter around the z-axis.

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The proper rotation of the propellers, goes such as any two adjacent propellers
rotate in opposite directions, and any two diagonal propellers rotate in the same direction.
The sum of rotations of any two diagonal propellers should equal the sum of the remaining
two diagonal propellers. This makes the total differential torque on the body about the z-
axis zero. The figure below demonstrates the prop rotation direction.

At hover mode, all four propellers would be producing the same amount of torque
resulting in zero-net force on the vehicle about any-axis once gravity is taken into account.
To make the vehicle increase or decrease in altitude, the speed on all four propellers are
increased or decreased respectively. In order to move the vehicle in any direction of the x
or y axis, two propellers adjacent propellers are increased in thrust, this causes the vehicle
to pitch or roll in the desired direction, since the sum of the any two diagonal rotors is still
the same as their other diagonal pair, this prevents the vehicle from yawing in any direction
other than the desired course. Assuming the vehicle is in hover mode the following table
yields a summary of the vehicle control scheme. Use the previous figure for propeller
reference.

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Propeller 1 Propeller 2 Propeller 3 Propeller 4
Z+ (Up) + + + +
Z- (Down) - - - -
X+ (Left) + 0 0 +
X- (Right) 0 + + 0
Y+ (Forward) + + 0 0
Y- (Backward) 0 0 + +

As stated earlier, a lightweight body was a must in order to achieve maximum thrust
for ease of flight and acrobatic maneuvers. For the chassis of the plane carbon-fiber was
used, a very stiff and lightweight material, with a variety of practical uses commonly used
in racecars and RC planes for their unique characteristics. To save even more weight we
used the X-chassis design, where four motors would be placed on every end of the X-
chassis. This would also give a better chance for the high pressure to accumulate and
increase under the blade of the propellers to give higher lift than in a rectangular design. It
would also reduce the overall air resistance. The arms of the X-chassis were made from
hollow carbon-fiber tubes, and at the end of the tubes the motor mounts were placed. They
were welded together using a common adhesive known to the RC world as Epoxy .

On the bottom of the X-chassis the battery was mounted, keeping the battery on a
lower point would lower the center of gravity of the vehicle giving the vehicle smoother
pitching and rolling. On the four battery sides four ultrasonic sensors would be placed for
obstacle avoidance. On the bottom of the battery the fifth ultrasonic sensor was placed to
determine height, along with the wireless camera placed for surveillance purposes, video or
image capturing.

On the top of the X-chassis the UAV brain board was placed. It carries the
accelerometer, gyrometer, RF Transceiver, GPS, motor controllers, ultrasonic sensors
connections, and of course the Microcontrollers. The following figure below displays the
chassis.

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After going through the design and experimentation of three different prototypes
(found in 5.2 Previous Chassis designs). One of the most difficult tasks for us, that
absorbed most of our time was coming up with the chassis that can have completely
reduced air resistance, maximized technical output power when compared to theoretical
power of the DC Brushed Motors involved, uniform density, and as extremely lightweight
as possible with all the components that we have had to add on the UAV. The net weight on
the UAV including all added components added up 990g when measured on the scale,
which is almost 1 Kg. The theoretical output power given to us by the DC Brushed Motors
added to up to a maximum thrust of 390 grams per motor. (See APPENDIX C) Since we
have 4 motors on the UAV, the complete output power given by those motors is 1560
grams (1.56Kg). Technically, the team managed to output only around 350 grams per
motor, adding up to 1400 grams (1.40Kg) of thrust. The efficiency of our design brought us
89.74% of that power. The loss in power comes up to 10.26% due to friction forces, and
minimized air resistance.
It is made mostly out of lightweight Carbon Fiber and Balsa Wood for the base of
the electrical circuit. The total weight of the chassis without all the components comes to 43
grams. A CAD model was designed, shown in the following figures. An isometric view is
shown below, and the dimensions of the chassis design are shown in the next few pages.

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Top View:

Front View:

Right Side View:

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Calculations:

Motor Force:

Max OutputTheoretical/Ideal = 390 grams/Motor
Max OutputTechnical = 350 grams/Motor

Therefore, the Total Motor Output of 4 Motors at Full Power:
Max Output4Motors = 1400 grams/4 Motors
Maximum Payload = 1400 990 = 410 grams

Hence,
Max Output in Newtons = 1400 x 9.807 = 13.730 Newtons
Max Output per Motor = 13.730/4 = 3.432 Newtons

Net Force:

Therefore, Lift of Chassis at Full Power and when Differential Torque = 0.
Chassis mass = 990 grams = 0.99 Kg
Chassis weight = 0.99 Kg x 9.807 m/s2 = 9.709N
Lift = 13.730 9.709 = 4.021 Newtons

Acceleration:

Net Force = Lift - Gravity
= ma mg
4.021 = 13.730 9.709
0.99a = 1.4(9.807) 0.99(9.807)
a = (0.41(9.807)) / 0.99
acceleration = 4.061m/s2
Therefore, the Power to Weight Ratio: 1.5 : 1
Therefore, Lateral Thrust beyond Hover thrust = (4.061m/s2) / (9.807m/s2) = 0.4141g

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Torque:

= Acceleration / Distance to Center

= 4.061 m/s2 / 0.14m = 29.007 rad/sec2

= mass * radius2 * (angular velocity) = (0.495) x (0.14) 2 (29.007)
; where (0.99/2 Motors = 0.495 grams, since it takes 2 motors for the UAV to move front, back, left or right).

= 0.2814 Newtons

MAX = 4.061 x 0.14 = 0.56854 Newtons
A picture of the UAV with complete physical assembly can be seen below in the
following figure.

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ANALYSIS, COMPONENT-LEVEL DESIGN
& SELECTION

3.1 Major Components :

The selection of the motors were brushed motors the GWS EPS-350C with a
gearing ratio of 5.33:1, which peak out at 8.0V and 8.0A, each of these weigh 63g and are
projected to deliver 15.37oz (435.73g) of thrust at peak power. Four of these motors are
used, with one on each end of the X -chassis. A figure is placed below.

Counter-rotating propellers were selected as our default propellers, which are a must
in any quadrotor plane, because motors do not turn in the same direction. We selected
10*4.5 propellers which are large considered for our motor. Larger propellers are more
suitable for high thrust application, and smaller rotors are more suitable for high velocity
and aerodynamic capabilities. Our choice was the EPP1045 propeller. A figure of the
propeller is placed below.

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Heat syncs were also used to cool down the motors to increase durability and
efficiency as well as to dissipate the heat created by the motors for a longer, more durable
life. The team selected EHS300 an aluminum, multi-fin heat sync for good heat dissipation
and proper venting respectively. The heat sync has two large fins and 24 smaller fins. A
figure of the heat sync is placed below.

We needed a battery source that can provide more than 32A continuously,
considering each motor can consume 8A, the battery of choice was a Lithium-polymer
Thunder Power TP8000-2S4P two-cell 7.4V, 8AH battery. It can work continuously at 12C
(96A), and can burst at 18C (144A) which is more than sufficient to have all motors
working at full thrust. With a weight of 320 grams and dimensions of 128*50*29mm it had
a high power to weight ratio and size relative to its competitors. It would also give us about
a good 15 minutes of airtime if the UAV is flying at full power. A figure of the battery is
placed below.

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A compatible charger the Astro-flight 109D was selected. Charging rates from
50mA - 8A. Lithium polymer batteries can charge at a maximum of 1C of their rating, so
this charger can charge the battery in the fastest possible time which is 1 hour, for quick
practical testing. The battery is two cells, any battery with more than one cell requires a
balancer, so a blinky battery balancer was used which balances the cells before, after and
during recharge. A wattmeter was also required to measure the voltage and current of the
battery before and after recharge. A powerful and bulky power supply is required to
continuously deliver such current to the charger. The astro-flight power supply was used,
with an input of 110V/220V and an output of 13.5V, it delivers 12.5A. Figures of the
charger (top left), blinky battery balancer (top right), wattmeter (bottom left), and power
supply (bottom right) are placed below.

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The accelerometer used was the triple-axis ADXL-330. Works at 3.3V logic, and
consumes 0.32mA, it has three outputs for x, y and z axes. Minimum full scale range is
±3g, and a sensitivity of 300mV/g. The gyrometer used is the IDG-300 which also works at
3.0V logic and has a full scale range of 500°/sec, and consumes 9.5mA, but has only two
outputs, x and y. Because of this the team had to place two of these IC s onboard, to get
angular velocity about all three axes. Pictures of the accelerometer and gyrometer are
displayed below from left to right.

The IMU five degrees of freedom is an IMU (Inertia Measurement Unit) that
combines the IDG300 gyrometer and an ADXL330 accelerometer. This unit measures x
and y angular velocity and x, y, z accelerometer outputs, hence the name 5 degrees of
freedom . Its advantages over two separate units are firstly that the x and y outputs of both
have identical headings, and you only have one VCC and one GND connection.
Disadvantages are if this IC for any reason becomes defective you lose two IC s. A figure
of this IC is displayed below.

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Ultrasonic sensors used were the Max sonar LV-EZ1 which work at 5.0V logic and
have a maximum range of 255in (6.45m), which measures in increments of an inch, they
have analog, digital and pulse width modulated outputs. It consumes 2mA. Five of these are
placed onboard, four facing x and y axes, in order to detect obstacles around the vehicle,
and one on the bottom of the battery facing downwards to detect height and aid in landing.
We could not rely on the altitude reading of the GPS system for height because there is an
error tolerance of ±5m, this could result in hazardous landings. The extra ultrasonic sensor
on the bottom would virtually eliminate that error because its resolution is relatively quite
high.

For communication with ground, radio frequency IC s are used. The Laipac
TRF2.4-G transceiver was used. It operates at a high frequency, 2.4GHz. Data rate
transmission can work at either 250kbps or 1Mbps. It works at 3.0V logic consumes
10.5mA in TX mode and 18.5mA in RX mode. Maximum range is 280m. Each unit can
send and receive data interchangeably. One of the transceivers is placed onboard, and the
other is connected to a land-based PC, they send and receive data to and from each other.

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For unmanned guidance to different destinations a GPS system, the EM-406 was
used. Readings of latitude, longitude and altitude obtained serially are used to triangulate
the position of the IC. Power input is rated between 4.5V-6.5V and power consumption is
70mA, operating frequency is at 1.58GHz. A figure of the GPS is placed below.

jhnjh

For the surveillance system the WS-309AS system was used, the package comes
with 1.2GHz camera with a resolution of 628*582 and a horizontal definition of 380 lines.
The camera works at 9.0V, and consumes 85mA. A simple 9V battery operates the camera.
The package also comes with a receiver with audio out and video out. Linear transmission
distance ranges from 50m-100m. A picture of the camera and components are placed
below.

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The selected PIC programmer was the Olimex PIC-MCP-USB programmer. It is a
low cost PICSTART alternative, is MPLAB compatible and thus does not require a RS232
port. In addition it has an ICSP (In Circuit Serial Programming) connector (ICSP
programming explained in APPENDIX E). A figure of the programmer is displayed below.

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3.2 PCB Design

Required components for designing and etching a PCB are acetone, a laser printer,
glossy paper, a clothing iron, acid and a steel sponge. Firstly the surface of the brass board
is scrubbed with a steel sponge to remove any impurities and any oxidized brass. It is then
cleaned thoroughly with cotton drained in acetone. The team used the circuit designing
program called EAGLE 4.16r1 . Any circuit is printed on glossy paper, the printed glossy
paper is then well folded around the board to prevent any slip during ironing, then ironed
on the brass board. Ironing continues until the circuit becomes visible from the other side of
the printed glossy paper, or preferably when the white paper takes a yellowish/brownish
color indicating a slight burn. (Caution should be taken during ironing, if the brass board
becomes too hot, the brass actually deforms). After ironing, the paper should be removed
leaving the toner ink on the brass board. The brass board is then placed in the acid and left
until all brass surrounding the printed circuit is dissolved. After removing from acid and
rinsing in water, a steel sponge is gently scrubbed on the toner ink to leave the brass trace
under the toner ink while removing the ink. Holes are drilled into the circuit board in the
appropriate places where components are to be placed. After drilling is complete,
components are welded onto the board using solder and a soldering iron. All circuits used
for this project were designed in this manner. Pictures below (left to right) display this
procedure.
Before these boards were actually designed they were tested on bread boards first in
order to assure everything is working in order, because making an incorrect PCB means
much wasted time and raw materials. More of this can be referenced in Section 7, Testing
troubleshooting and redesign.

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3.2.1 Interface Boards

Learning from previous errors we found it would be more convenient to create
interface boards for individual IC s rather than integrate them into one large circuit. (Much
the way a desktop motherboard uses PCI cards instead of making one large board.) This is
because if any errors occur in the design, or redesigning is desired, the individual IC s
wouldn t need to be removed. Frequently exposing IC s to strong heat when welding can
damage these components.

3.2.1.1 GPS Interface Board

In his board the GPS cable is welded onto the left row of pins. The descending order
of these pins is; not used, GND, TX, RX, VIN, & GND, again. The first pin is ignored. The
second and last pins (both GND) connect to the right side second pin. The third pin TX
connects to the fourth pin on the right. The 4th pin on the left is RX that connects to the
third pin on the right.

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3.2.1.2 Accelerometer / Gyrometer Interface Board

This follows the same method as the GPS interface board. The E$1 row is the yaw gyro,
E$2 row is the roll/pitch gyro. E$3 row is the three axis accelerometer. E$4 row is the pin
headers that connect onto the main board.

3.2.1.3 RF boards

The TRW-24G is a very sensitive component therefore we designed this interface board
with a TRW-24G socket for plug and play action onto the board.

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3.2.2 Motor Drivers :

Designing a suitable motor controller circuit was a challenging task, especially due to
the lack of components here in Egypt. The controlled motors could take up to 64 Ampere
bursts for a startup current and up to 8 Amperes as a continuous current. In order to achieve
maximum power we needed to cause a minimal voltage drop in our circuit. We came up
with the following design objectives:

- Switching speed of up to 2KHz (for PWM control)
- Minimum Vce drop possible for more powerful motors
- High current Ic
- Low current Ib

Unfortunately the transistors fitting this description could not be found here in
Egypt, but we found a transistor 2SD1062. It is capable of running a current of up to 15A
and Vce of as low as 0.3V, but it needed a larger current for Ib than a PIC could provide,
therefore we added a TIP120 transistor as an interface between the PIC and the 2SD1062.
Since Vce of the 2SD1062 was a function of the Ic current we put 2 transistors in parallel to
drop the Vce as low as possible while at the same time assuring that it has enough capacity
to pass through the required current for the motor.

A main feature of this circuit is the PC817 optocoupler, an IC that interfaces
between the PIC circuit and the motor circuit. Isolating these circuits was necessary
because combining high current components with low current ones can damage the low
current components. The optocoupler in the following diagram is labeled as 2. The left
side of the optocoupler is connected to the PIC circuit and the right side is connected to the
motor circuit. The first rows of pins in order are GND (PIC circuit), Vcc (PIC circuit), GND
(Motor circuit) and Vcc (Motor circuit). Vcc from PIC (PWM output) circuit goes through a
1.5K resistor through optocouplers where the phototransistor is activated and returns to
the PIC ground. The signal in turn goes through the base of the TIP120 turning it on. The
emitter of the TIP120 connects to the base of the 2SD1062 transistors, whose collectors are

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connected to the motor and the motor is connected to the Li-Poly battery. A circuit
schematic is shown below. Resistors were placed to produce desired voltage drops.

In the final motor driver design, the optocoupler was removed from the motor driver
and put on the main brain. This was done in order to have smaller motor drivers, and to
have less connections between the main board and the motor driver. Also large motor
drivers facing upwards would make contact with revolving propellers, and if facing
downwards could cause noise with the ultrasonic sensors.

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3.2.3 The Brain
To avoid the mistakes that occurred in Configuration 2 mentioned in the Testing,
Troubleshooting & redesign section the team changed two things mainly. FirstlyTo avoid
the problem of circuit design or re-altering, it was decided that the IC s would be mounted
on separate boards that would mount on the main Brain board, much the way PCI slots
are mounted on a normal PC. In our previous design, should any circuit design errors occur,
a new board would have to be made, and all components would have to be welded off the
old board, and re-welded to the new brain. This takes a lot of time, and it is also potentially
damaging to the components to be frequently exposed to the welder. Secondly as for having
the problem of high power rated components alongside low power rated ones in one circuit,
optocouplers were used to interface between the Brain board and motor drivers, this is
more thoroughly explained in the previous section 4.6 Motor Drivers .

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This Main board was designed to accommodate two PIC16LF777s, 4 Motor
controller boards connected through 4 opto-couplers, a 3-axis accelerometer, 2 dual-axis
gyrometers, 5 ultrasonic sensors, a GPS receiver and a RF transceiver. To keep the circuit
as small as possible we used the internal 8MHz oscillators available in PIC16LF777 PICs
instead of adding more components to the circuit in the form of crystals and capacitors. The
circuit is powered by a 9V battery and has a 5V regulator as well as a 3V regulator for all
5V Logic components as well as the 3V Logic components to operate. We also added some
LEDs to simplify debugging. Later on we manually welded on some wires to two ICSP
connectors to program the two PICs without removing them from the circuit. (As seen in
the previous picture).

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CONTROL
4.1 Introduction

The Main PIC is responsible for reading and calculating the orientation of the plane,
and accordingly take a decision. The Main PIC has only 3 PWM modules, therefore we use
an extra PWM from the Secondary PIC. The Main PIC sends commands to the Secondary
PIC to increase or decrease the power of one PWM output, it also sends the orientation data
to be sent through the RF to the base computer station. The Secondary PIC takes the GPS
messages and extracts the required values and sends them to the Main PIC, as well as
through the RF to the base station. Regarding the control scheme, there are four separate
operation modes:

1. Hover Mode
2. RC Mode
3. GPS Mode
4. Tracking Mode

In Hover Mode:
Tries to keep the vehicle stable in position. The following pseudocode demonstrates
the operating algorithm.
Start up system
Read bias values from IMU sensors
Loop:
Read sensors
Calculate Angles & Height
If(Height<Required Meters)
Increase PWM
if(Height>Required Meters)
Decrease PWM
if tilted left
Tilt right
If tilted right
Tilt left
If tilted forwards
Tilt backwards
If tilted backwards
Tilt forwards
Repeat loop

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In RC Mode:

The Secondary PIC is the one that receives the RC commands through the RF, then
forwards them to the Main PIC to execute.

In GPS Mode:

The Secondary PIC takes the GPS messages and extracts the required values and
sends it to the Main PIC, and it sends all other useful data through the RF to the base
station. The Main PIC takes decisions according to its coordinates achieved from the GPS
from the Secondary PIC.

In Tracking Mode:
The base station receives the Video Feed from the Wireless Camera on board the
vehicle and searches for a blue target in view, if it is not found the vehicle will gain altitude
and search again. Once a target is found the plane will descend quickly and hover above the
target and keep following it. The Secondary PIC receives the commands from the base
station through RF and forwards the commands to the Main PIC which performs the
required actions.

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4.2 SPI communication
SPI communication enables quick communication between two PIC s. One is set as
a Master PIC and the other as a slave. Originally one PIC was intended to be used, but
failed. (refer to Testing, Troubleshooting & Design : Configuration 2). The connection is as
follows on the diagram below. The left block represents the Master PIC and the left block is
the slave. A bit is released from the Master SSPSR to SD0, and the slave PIC releases a bit
through it s SD0 also. The clocks SCK of both PICS are connected together. When a clock
pulse rises and falls from the master PIC a bit is transferred. Every consecutive clock
transfers a bit. Once the shift registers reach 8-bits (1 byte) the byte is transferred to the
serial input buffer and the shift register is ready to receive data again. Three connections are
required, CLK to CLK (C3-C3), Master data out to slave data in (C5 C4), and master data
in to slave data out (C4 C5).

Two registers must be set in both PIC s in order to enable this mode; SSPSTAT and
SSPCON. (Actual settings for these registers can be found in APPENDIX B : CONTROL
CODE)

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SSPSTAT (Status Register)

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SSPSCON (MSSP Control Register)

For desired interrupts bits 6 and 7 of INTCON (Global and peripheral interrupts)
should be set. Bit7 of PIE1(SSPIE) should be set. When interrupt occurs bit7 of
PIR1(SSPIR) is set. This occurs if either a byte is successfully transferred, also in case of
collision occurs or overflow occurs.

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4.3 Main PIC Implementation

Generally as aforementioned, this PIC uses PWM, SPI, and ADC, it decides the
orientation and heading of the plane. The following sections divide these tasks and explain
each of these elements independently.

4.3.1 Pulse Width Modulation
After we have finally tested all our sensors, GPS device and RF devices for correct
processed data, we can now begin to implement the results as output on the propellers
through motor control. This is achieved by the use of PWM. In the PIC 16LF777, it has
three pins for PWM. The control registers used to enable PWM on this PIC are CCP1CON,
CCP2CON, CCP3CON, PR2 and most importantly T2CON, since PWM is controlled by
Timer 2 in the microcontroller. These three CCPXCON registers let us enable capture
modes, compare modes or PWM. Of course here, we will enable the PWM.

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Bit 7: Unimplemented.
Bit 5: Should be set as 0. Second Least Significant bit in PWM mode. (10-bit Resolution).
Bit 4: Should be set as 0. First Least Significant bit in PWM mode. (10-bit Resolution).
Bit 3: Should be set as 1. (To enable PWM mode).
Bit 2: Should be set as 1. (To enable PWM mode).
Bit 1: Don t care in PWM. (To enable PWM mode).
Bit 0: Don t care in PWM. (To enable PWM mode).

The CCPXCON registers will be all set as following:
CCP1CON: 0x0F = 0b00001111;
CCP2CON: 0x0F = 0b00001111;
CCP3CON: 0x0F = 0b00001111;
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After setting the CCPXCON registers, we must now set the T2CON register where
most importantly we must enable TIMER2 of the microcontroller and then set the period
we need to control our DC Brushed Motors in an optimum way using the PR2 register and
setting it with a fixed value. By means of research and supervision, it was decided to
control our motors at a frequency of 750Hz (750 times per second).

For T2CON, we place the following settings:

After setting the CCPXCON registers, we need to now set the T2CON register which
enables TIMER2 in the microcontroller that will then control over the frequency or period
we need on the Pulse Width Modulation. In order do this we must set the following bits as
follows.

Bit 6: Should be set as 0. (Postscaling will not be needed).
Bit 2: Should be set as 1 in order to enable and turn on Timer 2.
Bit 1: Should be set as 1. (Since prescale with a value of 16 is required).
Bit 0: Should be set as 1. (Since prescale with a value of 16 is required).

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Our goal to control our motors at around 750Hz. Now since the microcontroller can
execute 2 million instructions per second (500 nanoseconds). Speed should be reduced by
prescaling. When you prescale your instructions per second over 16 which is our
maximum, then we have reduced the frequency to 125 KHz (125000Hz). This is where the
PR2 register comes in handy to further reduce frequency to 750Hz.

For PR2, we place the following settings:

PR2 is an 8-bit register made available in order to control the frequency output
needed on the DC Brushed Motors. After using the T2CON register for prescaling to
reduce frequency to 125 KHz, PR2 register is used to enter a decimal value that will control
and limit our frequency to 750Hz. The value to be placed in the PR2 register is calculated
as follows. We divide the 125000 Hz obtained by 750Hz which is what is needed.
125000/750 = 166.666667. Since the value to be placed in the PR2 register should be an
integer value and is an 8-bit register and carries no space for a floating point number, 167
should be entered after subtracting 1 from it.
Therefore,
PR2 = 166
The equation for PR2 is: round (Fosc / (4 x 16 x Period Required)) - 1
Hence,
Fosc = 8 x 10^6
PR2 = round(8 x10^6 / ( 4 x 16 x 750)) - 1
PR2 = round(8 x 10^6/ (48000)) - 1
PR2 = round(166.66666667)) - 1
PR2 = 167 - 1
PR2 = 166

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Setting outputs on the Tri-State Buffers on all ports of the Microcontroller:

Since the PWM pins are driving the motors they need to be se as output pins. This is
done by setting the registers TRISB and TRISC.

TRISC = 0x00 Hex = 0b00000000.
TRISB = 0x00 Hex = 0b00000000.

The diagram of the PIC 16LF777 can be used as a reference below for the output pins
CCP1 on Port C2, CCP2 on Port C1, and CCP3 on Port B5.

*NOTE: Please see APPENDIX B for the sample code of Pulse Width Modulation and how
to control it.

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4.3.2 ADC Operation

Here using the Analog - to - Digital converters is most crucial in order to automate
our Unmanned Aerial Vehicle (UAV). For the most part, most or all of our sensors,
ultrasonic, gyrometer and accelerometer give us feedback on our control system. The
Ultrasonic provides us with a way for collision detection and obstacle avoidance. The
accelerometer and gyrometer provide us with crucial data to help us stabilize our UAV in
mid-air and maintain a static hovering position. It can also help the UAV to auto-level after
traveling in a certain direction, like a co-pilot.

The outputs of those sensors are analog voltages. The Analog - to - digital converter
here helps with converting those outputs into useful data ready to be used and processed by
the microcontroller. In this project we use the 16LF777 PIC by Microchip. It contains an
abundant 14 channel 10-bit ADC.

We have 11 inputs from those sensors. Five alone for the ultrasonic sensors, placed on
the front, back, left, right, and bottom sides of our UAV for height accuracy. The
ultrasonic s range is far as 6.45m (254 inches) and as small as 15cm (6 inches) to aid the
UAV in landing due to its blind spot. Six channels are used for 2 Gyrometers and an
accelerometer. Each gyrometer outputs the rate of angular velocity in the X and Y planes,
so we need three channels since we have 2 gyrometers. One input/channel will be ignored
from the second gyrometer. The accelerometer needs 3 channels since it measures
acceleration in the X, Y, and Z directions. This makes a total of 11 channels. Therefore, 3
channels on our 16LF777 microcontroller will not be used out of the 14 channels.

In order to set this up in our PIC we must enable certain bits in our control registers of
the 16LF777 microchip. These control registers are the ADCON0, ADCON1, ADCON2,
PIE1, and PIR1 and last but not least the INTCON register to enable our interrupts
especially when the ADIF (AD Interrupt Flag) is set after every conversion in the PIR
register.
The result of the Analog-to-Digital Converter is placed in the ADRES (AD Result)
register. It consists of 2 8-bit registers, ADRESL (AD Result LOW) and ADRESH (AD
Result HIGH).
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For ADCON0, we place the following settings:

Bit 7: <ADCS1> Must be set as 1 since we are using the Internal Oscillator.
Bit 5: <CHS2> Analog Channel Select bit.
Bit 2: <GO/DONE> A bit that controls the start of conversion or end of conversion.
Bit 0: <ADON> Turns on the ADC module in the microcontroller.

Bits 5,4,3,1 are used to select the channels we need to take our inputs from. Therefore, you
need to toggle through them as we read our values over the output interval time. We start
out by reading through channel 0, then 1, then 2, until we reach channel 10 (11 Channels)
then go back to Channel 0 to take new readings to process for our new interval.

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Bit 7: <ADFM> Must be set as 1 for Right Justification in the ADRES
register. In reading our result from the ADRES register,
we read all the 8 bits from ADRESL and the least significant bits
of ADRESH and multiply it by 256.


Bit 5: <VCFG1> Must be set as 0 since our Vref+ is normally the VDD of the PIC.
Bit 4: <VCFG0> Must be set as 0 since our Vref- is normally the VDD of the PIC.

Bit 3: <PCFG3> Must be set as 0 since we need to enable 11 Channels.

The bits 3,2,1,0 of PCFG(X) remain fixed since we are enabling only 11 Channels for
the ADC to read from. The pins where pins AN11, AN12 and AN13 of the microcontroller
16LF777 remain digital I/O pins depending on the settings of the Tri-State Buffers for the
ports.


Bit 5: Must be set as 1, since we wish the conversion to take 12TAD (24 sec).

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The reason why 12TAD is necessary here is simply because one TAD is equivalent to
2 sec. The acquisition time must not exceed the minimum of 19.72 s which is how long
the ADC before the ADC starts conversion automatically.

Therefore, 2 s * 12 = 24 sec, which is how long the ADC needs to acquire our data
from one input channel.

In order to keep the microcontroller working efficiently and processing data without
having it constantly polling and wasting processing power on all kinds of data coming in
through the Sensors, GPS device or RF transceivers, we use interrupts. Concerning our
sensors we set the PIE1 control register in our microcontroller. The Analog-to-Digital
Interrupt Enable (ADIE) is bit number 6. We set it to 1. Whenever the ADC finishes a
conversion, it will set the Analog-to-Digital Interrupt Flag in (ADIF) to 1 in register PIR1,
interrupting the PIC. After we take our reading for the ADC, we must clear the ADIF in the
PIR1 register in our software or else the PIC will keep itself running in a loop. Then we
must change our channel through the bits 5, 4, 3, and 1 in the ADCON0 register. When this
is done, we start a new conversion by the setting the bit number 2 (GO/DONE) as 1 in the
ADCON0 register until the end of conversion is complete and the ADIF is set again calling
the interrupt function in our microcontroller.

Setting our inputs on the Tri-State Buffers on all ports of the Microcontroller:

Since we have already set our control registers of the ADC module most importantly,
we need to set the tri-state buffers on our ports in order to receive our inputs from the
sensors. This is done by setting the registers TRISA, TRISB, and TRISE.

TRISA = 0xFF Hex = 0b11111111.
TRISB = 0x0E Hex = 0b00001110.
TRISE = 0x07 Hex = 0b00000111.

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In the summary of registers shown above, we must be very careful when setting the
TRISE register because only the three least significant bits here control the PORTE Data
Direction Bus, unlike TRISA where the complete register is used for only 6 pins. If we set
the TRISE = 0xFF, it will cause the PIC to set two interrupt flags IBF and OBF and
enable PSP Mode , which will cause PORTD to engage in parallel communication. This
will cause the PIC to enter in an infinite loop of interrupts and if the flags are not cleared in
the software. It almost causes the microcontroller to seem to Halt in a sense.

Page 49

4.3.2.1 Ultrasonic Sensors
The ultrasonic sensors used on the UAV can detect up to 254 inches 6.45 (meters) and
the minimum distance it can detect due to its blind spot is 6 inches (15 cm). The sensor
generates a new reading every 49 milliseconds. Since the microcontroller can take readings
much faster than the ultrasonic sensor s output, if we take the readings at that speed, it will
cause a lot of noise in our program for the UAV, so it is best we take our readings every
49 milliseconds to avoid the noise and make sure we have a new reading every time to be
put to good use.

Every 0.01 Volts on our Ultrasonic sensor represents 1 inch of distance. Therefore, if
the voltage on the output pin of the ultrasonic sensor is 0.20 Volts, then the distance it reads
is 20 inches, therefore it is very simple to use.

In order to calculate the distance we need in our PIC 16LF777 we use a very simple
equation which is:

Distance (in Hexadecimal) = (Vin/Vref) X (2N) ;
where Vin : is the Voltage input coming from the Ultrasonic Sensor.
Vref : is the reference voltage from our circuit which is 3.30V
N : is the number of bits of the ADC which is 10, therefore is
1024
For example,
If Vin = 0.50V (which is equivalent to 50 inches read).
Vref = 3.30V

Then, 0.50/3.30 X 1024 = 155.1515 Hexadecimal

In the ADC of the PIC 16LF777, the ADRES (AD Result) register will read 155 and
will truncate the 0.1515.

If we take the reading 155 from the ADC and try to convert it back, it will be as
follows:

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Vin = (Reading from ADRES Register in HEX / 2N) X Vref

Vin = (155/1024) X 3.30V = 0.4995 Volts.

Therefore the error is: (1 (0.4995/0.5)) X 100 = 0.1 %
which is quite accurate.

*NOTE: Please see APPENDIX B for the sample code of the Ultrasonic Sensors.

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4.3.2.2 Gyrometer
The gyrometer used was the IDG300. This IC gives accurate readings of angular velocity.
All three angles were needed for control of the UAV, on the x, y and z axes; traditionally in
flight labeled as roll, pitch and yaw angles. Angular velocity is measured accurately with a
sensitivity of 2 mV/ º/s. So every degree of rotation would indicate 0.002V electronically.
The first thing to do was to interpret the signals into degrees, 0º - 360º.
This IC operates so that if the IC is rotated suddenly then stopped, you would get a
change in reading only when the IC is moving, only when there is angular velocity. Thus an
adder function is needed to constantly integrate the tilt intervally through the selected
frequency, as general equation is as follows:

SUM SUM new t
Where SUM is initially set to 0. new is the latest reading from the gyro output and T is the
sampling period.
After the electric signal would be received on the ADC ports of the PIC it would be
multiplied by the following equation to give degrees:

1024 t
AngleNew AngleOld Vin * *
3.3 0.62
Also any negative value for tilt had 360 added to it, since simple sin and cos
functions behave differently to negative values.

4.3.2.3 Accelerometer
The accelerometer used was the ADXL330. This gives accurate measures of
acceleration about all three axes. Typical sensitivity of this IC is 300mV/g, so every 1m/s2
of acceleration would indicate 30.58mV electronically. Primarily this IC has two main
functions. The first is to indicate the initial angles of x and y in reference to the xy plane
perpendicular to the gravity vector, so that the UAV can take off from any angled surface,
if an accelerometer was not used, the system would always assume that the plane it was
taking off from was always perpendicular to the vector of gravity, causing flight to be
unstable. To use the accelerometer as an inclinometer, assuming X and Y are the
acceleration values obtained from the corresponding axes on the accelerometer then, simply
Page 52


X
-1
X=sin
g
Y
-1
Y=sin g

The second use is to produce accurate estimations of acceleration, velocity and
position, for use in the simulation. A fixed reference point is taken, more accurately the
fixed axes at the point of takeoff. Acceleration and velocity in reference to that point are
calculated. Distance from that point is calculated, and distance traveled around that point is
also calculated. Considering the accelerometer uses the angles supplied from the gyrometer,
a traditional 3D rotational matrix is used to rotate the constantly generated acceleration
vectors around the reference axes, so that every value from the accelerometer has a X, Y
and Z component on the reference axes.

Rotation around the x-axis is defined as :

1 0 0
RX( X) = 0 cos X sin X where X is the roll angle
0 sin X cos X

Rotation around the y-axis is defined as :

cos Y 0 sin Y

RY( Y) = 0 1 0 where Y is the pitch angle
sin Y 0 cos Y

Rotation around the z-axis is defined as :

cos Z sin Z 0
RZ( Z) = sin Z cos Z 0 where Z is the pitch angle
0 0 1

Multiplying all these matrices together would give the following matrix:

Page 53


cos Y cos Z cos Y sin Z sin Y
1
A sin Y sin X cos Z cos X sin Z sin Y sin X sin Z cos X cos Z sin X cos Y

sin Y cos X cos Z sin X sin Z sin Y cos X sin Z sin X cos Z cos X cos Y

if X,Y and Z are the acceleration values obtained from the corresponding axes on the
accelerometer then,
ReferenceX = Xcos Xcos Y + Y(sin Ycos Xsin Z-sin Xcos Z) +
Z(sin Ycos Xcos Z+sin Xsin Z)

ReferenceY = Xsin Xcos Y + Y(sin Ysin Xsin Z+cos Xcos Z) +
Z(sin Ysin Xcos Z-cos Xsin Z)

ReferenceZ = - Xsin Y + Ycos Ysin Z + Zcos Ycos Z

Integrating with respect to time once gives velocity, and integrating twice gives
position. Adder functions are used for velocity and position for each reference axes.
Another adder function is created taking the absolute value of every acceleration reading,
then multiplying them by time twice in order to calculate the distance traveled. All adder
functions for total rigid body acceleration, velocity, distance from origin and distance
traveled, this simple equation is used.
2 2
V alue X Y Z2

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4.4 Secondary PIC Implementation

Generally as aforementioned, this PIC uses PWM, SPI, USART, and communicates
with an RF module. It handles communication tasks for the Main PIC. It also acts as a
secondary PWM module. The following sections divide these tasks and explain each of
these elements independently.

4.4.1 GPS System

GPS has become a widely used aid to navigation worldwide, and a useful tool for
map-making, land surveying, commerce, and scientific uses. GPS also provides a precise
time reference used in many applications including scientific study of earthquakes, and
synchronization of telecommunications networks. There is a constellation of 30 (earth
orbiting satellites as of April 2007) that transmit precise radio signals. Their orbits are set
up so that at any given point and time on the earth s surface there are at least six of these
satellites in reach. A figure below demonstrates the constellation of NAVSTAR GPS
satellites.

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A GPS receiver calculates its position by measuring the distance between itself and
three or more GPS satellites, using trilateration. Measuring the time delay between
transmission and reception of each GPS radio signal gives the distance to each satellite,
since the signal travels at a known speed. The signals also carry information about the
satellites' location. By determining the position of, and distance to, at least three satellites,
the receiver can compute its position using trilateration. Receivers typically do not have
perfectly accurate clocks and therefore track one or more additional satellites to correct the
receiver's clock error.

The figures below briefly explain trilateration, where at the center of each sphere
there is a satellite. When two spheres intersect they create lines. When the third sphere
intersects it creates a point revealing the location of the receiver.

The coordinates are calculated according to the World Geodetic System WGS84
coordinate system. Position is determined by latitude and longitude which are basically
angles, latitude ranges from 0-90 north and south, and longitude ranges from 0-180 west
and east. The figures below display latitude and longitude.

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To calculate its position, a receiver needs to know the precise time. The satellites
are equipped with extremely accurate atomic clocks, and the receiver uses an internal
crystal oscillator-based clock that is continually updated using the signals from the
satellites.

GPS satellites continuously transmit almanac and ephemeris at 50bps. The almanac
consists of coarse time information and orbital data (speed and path). The ephemeris gives
the satellites precise orbit. The almanac assists in the acquisition of other satellites. A
complete almanac transmission is a 37,500 bit navigation message that takes 12.5 minutes
to download. This long delay occurs when a new receiver is first turned on. Each satellite
transmits its navigation message with at least two distinct spread spectrum codes: the
Coarse / Acquisition (C/A) code, which is freely available to the public, and the Precise (P)
code, which is usually encrypted and reserved for military applications. The C/A code is a
1,023 bit long pseudo-random code broadcast at 1.023 MHz, repeating every millisecond.
Each satellite sends a distinct C/A code, which allows it to be uniquely identified.
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The receiver identifies each satellite's signal by its distinct C/A code pattern, then
measures the time delay for each satellite. To do this, the receiver produces an identical
C/A sequence using the same seed number as the satellite (two or more systems using
matching seeds can generate matching sequences of non-repeating numbers which can be
used to synchronize remote systems). By lining up the two sequences, the receiver can
measure the delay and calculate the distance to the satellite, called the pseudorange. The
pseudoranges are then the time the signal has taken from there to the receiver, multiplied by
the speed of light. The orbital position data from the Navigation Message is then used to
calculate the satellite's precise position. Knowing the position and the distance of a satellite
indicates that the receiver is located somewhere on the surface of an imaginary sphere
centered on that satellite and whose radius is the distance to it. When four satellites are
measured simultaneously, the intersection of the four imaginary spheres reveals the location
of the receiver. The orbital position data from the Navigation Message is then used to
calculate the satellite's precise position. Knowing the position and the distance of a satellite
indicates that the receiver is located somewhere on the surface of an imaginary sphere
centered on that satellite and whose radius is the distance to it. When four satellites are
measured simultaneously, the intersections of all four imaginary spheres reveal the location
of the receiver.

Often, these spheres will overlap slightly instead of meeting at one point. The
receiver then moves the overlapping pseudoranges with the same amount (regardless of
distance of receiver to satellite) till an intersection point is created this point is usually the
most probable position. This scenario is shown in the following figure.

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An overlapping pseudorange occurs here. Instead of having one intersection point, a
room is created by all three points of B . All distances are subtracted by the same amount,
in this case 0.5, in order to receive an intersection point at A . Point A is considered the
most probable point of the receiver.

Regarding GPS time as opposed to the conventional second, minute and hour; you
only have seconds, more precisely seconds of the week. In a normal clock when the
seconds reach 60 it starts a new minute. In GPS time when the seconds reach 604,800 it
starts a new week, this is calculated by 7(days)*24(hours)*60(minutes)*60(seconds).

As for GPS date as opposed to the year, month, and day format of the Julian
calendar, the GPS date is expressed as a week number and a day-of-week number. The
week number is transmitted as a ten-bit field, and so it becomes zero again every 1,024
weeks (19.6 years). GPS week zero started at (00:00:19 TAI) on January 6, 1980 and the
week number became zero again for the first time at on August 21, 1999. This event is
known as a rollover.

After a GPS does a full almanac download, GPS systems boot in 3 different modes.
Those would be cold start, warm start and hot start. In cold start, time and position are
known within some limits, the almanac is known and the ephemeris is unknown. In warm
start, time and position are known within some limits, the almanac is known, and at least
three satellite ephemeris are known from the previous operation. In a hot start all ephemeris
for all satellites are known so a hot start occurs. The GPS receiver chooses how to start
based on the time between last turn off and current turn on. If this time was a few minutes
the GPS chooses hot start which takes 1 second, if it was a few hours the choice is warm
start which takes 38 seconds, anything longer than that produces a cold start which takes 42
seconds.

Most GPS systems have two protocols SirF protocol and NMEA protocol. In our
case the NMEA protocol was used. NMEA protocol simply contains input messages and
output messages. (Refer to the NMEA reference manual)

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Input messages selected to initialize the GPS are:
$PSRF100,1,4800,8,1,0*0Ern
$PSRF103,04,00,02,01*22rn
$PSRF105,0*3Frn
$PSRF is used for input messages. The star means the following two characters are
checksum, NMEA checksum operates by 16-bit XOR a checksum calculator code is shown
in the APPENDIX B : CONTROL CODE, and /r/n represent carriage return and line feed,
whose HEX code are 0D 0A. All other fields in between them are data fields for different
settings.
$PSRF100,1,4800,8,1,0*0Ern
This message was used for setting the serial port. 100 in the first field represents serial port
settings. 1 in the second field is for NMEA protocol, 4800 is for baud rate, 8 is for 8 data
bits, 1 is for 1 stop bit, and 0 is for no parity bit.
$PSRF103,04,00,02,01*22rn
This line is used for enabling and disabling output messages, 103 is used for query/control
mode. 04 is used for RMC mode, 02 is used for releasing the message at 2Hz, 01 is used for
enabling checksum.
$PSRF105,0*3Frn
105 is used for development data. The 0 represents debug off should any error occur, so
that our PIC does not receive any unnecessary input. When a GPS is turned off, it s last
settings before being switched off will be saved in it s battery powered RAM. When turned
on, these settings resume. These input messages were considered necessary in order to set
the serial port correctly for USART communication, RMC mode was chosen because this
one single message had all the necessary information required. The third message is for
turning off debug to avoid unnecessary input to the MCU.

A 16-bit XOR CRC creator was necessary to give input messages. A JAVA code is
displayed in the APPENDIX B: CONTROL CODE.

Page 60

Later on a program called SiRF Demo PC GPS Utility v3.83 was found very helpful
for obtaining latitude and longitude coordinates for our tested range area. It can also be
used as an initialization alternative. To initialize your GPS time, you simply click setup
then click GPS Time PC Time as shown in the figure below. Usually the demo starts
in SiRF protocol. To switch it to NMEA protocol you simply click action then Switch to
NMEA Protocol , to open NMEA Setup. In this window as shown in the figure below, you
can select each message and it s frequency per second. Highlighting checksum is preferred
for message validation. For NMEA, baud rate should be set at 4800bps. After powering off
the GPS receiver, GPS time, message type and frequency are saved.

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Output messages received from RMC mode (in order) are UTC time, data validity,
latitude, north/south indicator, longitude, west/east indicator, velocity over ground in knots,
heading measured clockwise from north in degrees, and date A sample output message is
shown below:
$GPRMC,161229.487,A,3723.2475,N,12158.3416,W,0.13,309.62,120507, ,*10
The first two letters following the $ represent the device in use. The GP stands for GPS.
There are other devices such as:
LC Loran-C
TR Transit SATNAV
AP Autopilot (magnetic)
HC Magnetic heading compass
RA Radar
SD Depth sounder
VW Mechanical speed log

Latitude and longitude are displayed in degrees and minutes, At a latitude of 30° N
(Cairo, Egypt), the latitude minute = 1847.54m and longitude minute = 1608.1m (distances
change because the circumference of parallel of latitude changes, Earth is not a cylinder,
please refer to http://home.online.no/~sigurdhu/Grid_1deg.htm ), velocity is multiplied by
1.852 to change from knots to km/hr. Then course heading in degrees ranging from 0°-
360° moving clockwise from north. The final field before the checksum is date. The only
fields needed were data validity, latitude, longitude, velocity and heading. VTG mode was
desired to attain height, but during testing, height in MSL (Mean Sea Level) was quite
inaccurate. At a change of height of about 4 meters, the GPS detected a change of height of
10 meters which is an error of over 150%. Latitude, longitude velocity and heading are
transmitted via RF to the simulation.

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A destination in GPS mode is set by pre-inputting a target destination, in the PIC
program, in latitude and longitude. The following steps are taken:

1. Y = Target Latitude Present Altitude
2. X = Target longitude Present Latitude

3. Distance = X2 Y2

1 Y
4. TempAngle is obtained by tan
X
Desired course heading is obtained by the following scheme:
Y X Course
+ + TempAngle + 0°
- - TempAngle + 180°
+ - TempAngle + 360° (TempAngle is negative)
- + TempAngle + 180° (TempAngle is negative)

Resgister Settings

Communication between the PIC and GPS system is acheived by the Universal
Synchronous Asynchronous Receiver Transmitter (USART). In this case Asynchronous
mode is used. (refer to the PIC 16F777 pdf file, section 11.0 for more detailed information)
To enable this serial mode three registers must be set; TXSTA, RCSTA and SPBRG.
TXSTA is set in the following manner:

Bit 7: 0 Don t care (for Asynchrous mdoe)
Bit 6: 0 for 8-bit transmission
Bit 5: 0 for transmission enabled
Bit 4: 0 for Asynchronous mode
Bit 3: 0 this bit is unimplemented
Bit 2: 0 for High speed
Bit 1: 1 for TSR empty (TRMT)
Bit 0: 0 not used in 8-bit transmission

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SPBRG is the simplest where only a value is entered into the register. Considering
asynchronous mode is used and the system is low speed, the following equation is used,
where X is the value entered in SPBRG:

FOSC
X 1
64* BaudRate

When a baud rate of 4800bps with a frequency of 8MHz is entered into the equation
the resulting X value is 25.04, so 25 is the value used in SPBRG.
The RCSTA register is set in the following manner:

Bit 7: 1 for Serial port enabled
Bit 6: 0 for Enables 8-bit reception
Bit 5: 0 Don t care for Asynchronous mode
Bit 4: 1 to Enable continuous receive (called CREN)
Bit 3: 0 Don t care for 8-bit mode
Bit 2: 0 for no Overrun error(OERR)
Bit 1: 0 for no Framing error(FERR)
Bit 0: 0 Don t care for 8-bit mode

When transmitting input messages to the GPS system to initialize data, the data
message had to be inserted in the PIC s EEPROM via MPLAB before programming to PIC
(an .ECH file can be created with your EEPROM input by exporting a file (MPLAB), this
file is easier to load than re-inputting every time), because it consumed too much RAM.
Data is transmitted bit by bit via the TXREG register, the TSR register must be polled to
see whether the bit was sent out or not when TSR is empty only can u fill in the next bit.
Interrupts are undesired in this mode.

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For Universal Asynchronous reception, the 6th and 7th bit of the INTCON register
must be set, to enable interrupts, along with the 5th bit of register PIE1. An interrupt occurs
(bit5 of register PIR is set) under three cases, when a byte is received successfully, when an
OERR (Overrun error) or when a FERR (Framing error) occurs. If a FERR occurs the
message is discarded. The message is valid if; the message starts with a $ and ends with
0D 0A, the GPS sends an A in the 19th byte, no FERR error occurs, and the CRC check is
correct. If the message is valid, SPI communication transmits the latitude, longitude and
heading to the Main PIC, Also the RF transmits function is called to send this data (for use
in the simulator). This code in detail can be seen in APPENDIX B: CONTROL CODE.

Page 65


4.4.2 Radio Transceiver

This RF device is called a transceiver in the sense that the same unit can send and
receive, to and from another identical unit. Operating frequency is 2.4GHz, and data
transmission rate can be selected at either 250Kbps or 1Mbps. 250kbps works at a longer
range of 280m but after testing, range proved to be approximately 180m. Also 250kbps
improves receiver sensitivity. There are two modes direct mode and shock burst mode.
Shock burst works at a lower current and relaxed PIC operation. Low current consumption
occurs by using an onboard FIFO to transmit data at a low rate then transmit at a high rate.
PIC resources are saved by having an onboard CRC creator/checker for
transmitting/receiving respectively. Pre-amble, address, and CRC are stored on a buffer on
the RF then transmitted out, instead of letting the PIC do all this work. The transceiver can
receive simultaneously on two different channels. Only one channel was used in this
project.

Pins used

Used pins were CE (Chip Enable), CS (Chip Select), CLK (Clock), DR1 (Data
Ready1), DATA1, Vcc, and GND (1 represents pins pertaining to Channel1). The
transceiver requires a configuration word of up to 15 bytes. This is done through three pins;
CS, CLK and DATA1. Generally CE is turned off, CS is turned on, a delay is done to allow
onboard processing, and then data is fed in bit by bit as the clock toggles. The Shock burst
configuration word is as follows:

Shock Burst configuration Word:

The section bit[119:16] contains the segments of the configuration register dedicated to
Shock Burst operational protocol. After VDD is turned on Shock Burst configuration is done
once and remains set whilst VDD is present. During operation only the first byte for
frequency channel and RX/TX switching need to be changed.

Page 66

PLL_CTRL

Bit 121-120:

Controls the setting of the PLL for test purposes. With closed PLL in TX no
deviation will be present. For normal operational mode these two bits must both be low.

DATAx_W

Bit 119 112:
DATA2_W: Length of RF package payload section for receive-channel 2.
Bit 111 104:
DATA1_W: Length of RF package payload section for receive-channel 1.
NOTE:
The total number of bits in a Shock Burst RF package may not exceed 256!
Maximum length of payload section is hence given by:

DATAx_W(bits) = 256 (ADDR_W+ CRC)
ADDRx

Bit 103 64:
ADDR2: Receiver address channel 2, up to 40 bit.
Bit 63 24: ADDR1
*NOTE:
Bits in ADDRx exceeding the address width set in ADDR_W are redundant and can
be set to logic 0.
Page 67

ADDR_W & CRC

Bit 103 64:
Bit 63 24: ADDR1

NOTE:
Bits in ADDRx exceeding the address width set in ADDR_W are redundant and can
be set to logic 0.

ADDR_W & CRC

Bit 23 18:
ADDR_W: Number of bits reserved for RX address in Shock Burst packages.
NOTE:
Maximum number of address bits is 40 (5 bytes). Values over 40 in ADDR_W are
not valid.
Bit 17:
CRC_L: CRC length to be calculated by nRF2401 in Shock Burst.
Logic 0: 8 bit CRC
Logic 1: 16 bit CRC
Bit: 16:
CRC_EN: Enables on-chip CRC generation (TX) and verification (RX).
Logic 0: On-chip CRC generation/checking disabled
Logic 1: On-chip CRC generation/checking enabled

Page 68

This section of the configuration word handles RF and device related parameters.

Modes:
General device configuration:

Bit 15:
RX2_EN:
Logic 0: One channel receive
Logic 1: Two channels receive

NOTE:
In two channel receive, the nRF2401 receives on two, separate frequency
channels simultaneously. The frequency of receive channel 1 is set in the configuration
word bit[7-1], receive channel 2 is always 8 channels (8 MHz) above receive channel 1.
Bit 14:

Communication Mode:
Logic 0: nRF2401 operates in direct mode.
Logic 1: nRF2401 operates in Shock Burst mode

Bit 13:
RF Data Rate:
Logic 0: 250 kbps
Logic 1: 1 Mbps

*NOTE:
Utilizing 250 kbps instead of 1Mbps will improve the receiver sensitivity by 10 dB.
1Mbps requires 16MHz crystal.

Page 69


Bit 12-10:
XO_F: Selects the nRF2401 crystal frequency to be used:

Bit 9-8:
RF_PWR: Sets nRF2401 RF output power in transmit mode:

RF channel & direction

Bit 7 1:
RF_CH#: Sets the frequency channel the nRF2401 operates on.
The channel frequency in transmit is given by:
ChannelRF =2400MHz + RF_CH# * 1.0MHz
RF_CH #: between 2400MHz and 2527MHz may be set. The channel frequency in data
channel 1 is given by:
ChannelRF =2400MHz + RF_CH# * 1.0MHz
(Receive at PIN#8)
RF_CH #: between 2400MHz and 2524MHz may be set.

The channel frequency in data channel 2 is given by:
ChannelRF =2400MHz + RF_CH# * 1.0MHz + 8MHz
(Receive at PIN#4)
RF_CH #: between 2408MHz and 2524MHz may be set.

Page 70

Bit 0:
Set active mode:
Logic 0: transmit mode
Logic 1: receive mode

For more intricate details about the configuration word refer to the nRF2401 datasheet page
19.

Within Shockburst mode there are four different modes. They are displayed in the
following table.

ACTIVE MODE
There are two different options in Active mode, Transmit and Receive.

Transmit
1. When the application MCU has data to send, set CE high. This activates nRF2401
onboard data processing.
2. The address of the receiving node (RX address) and payload data is clocked into the
nRF2401. The application protocol or MCU sets the speed <1Mbps (ex: 10kbps).
3. MCU sets the CE to low, this activates a nRF2401 Shock Burst transmission.
4. nRF2401 Shock Burst:
RF front end is powered up
RF package is completed (preamble added, CRC calculated
Data is transmitted at high speed (250 kbps or 1 Mbps configured by user).
nRF2401 return to stand-by when finished

Page 71

Receive
1. Correct address and size of payload of incoming RF packages are set when nRF2401 is
configured to Shock Burst RX.
2. To activate RX, set CE high.
3. After 200ms settling, nRF2401 is monitoring the air for incoming communication.
4. When a valid package has been received (correct address and CRC found), nRF2401
removes the preamble, address and CRC bits.
5. nRF2401 then notifies (interrupts) the MCU by setting the DR1 pin high.
6. MCU may (or may not) set the CE low to disable the RF front end (low current mode).
7. The MCU will clock out just the payload data at a suitable rate (ex. 10kbps).
8. When all payload data is retrieved nRF2401 sets DR1 low again, and is ready for new
incoming data package if CE is kept high during data download. If the CE was set
low, a new start up sequence can begin.

The following flowchart displays the processes of receiving and transmitting.

Page 72

CONFIGURATION MODE
Similar to active mode Configuration mode has two options, Configure Transmitter, and
Configure Receiver.

Configure Transmitter
1. In configure transmitter, CE is turned off, and CS is turned on.
2. DATA1 with CLK send the configuration word to the RF.
3. A delay of (1ms) is issued to allow ample time for onboard processing.
4. Both CE and CS are turned off.

Configure Receiver
1. In configure receiver, CE is turned off and CS is turned on.
2. A delay (1ms) is issued. The configuration is then sent through DATA1 from the PIC
as the clock toggles.
3. CE and CS are then turned off and a delay (1ms) is used also for onboard processing.
4. CE is then left on as to enable receiving.

STAND-BY MODE
Stand by mode is used to minimize average current consumption while maintaining
short start up times. In this mode, part of the crystal oscillator is active. Current
consumption is dependent on crystal frequency.

POWER DOWN MODE
In power down the nRF2401 is disabled with minimal current consumption,
typically less than 1µA. Entering this mode when the device is not active minimizes
average current consumption, maximizing battery lifetime.

Page 73

DATA PACKAGE DESCRIPTION

Data packages contain four main sections, in MSB order Pre-amble, address,
payload and CRC. Pre-amble is either 4 or 8 bits and is added to the data packet. Address is
between 8 and 40 bits. Payload is the desired data being transmitted or received. CRC is
either 8 or 16 bits and used for validating message. More detail about the data package can
be seen in the table below.

For information regarding delays, (please refer to the RF-24G datasheet page22).

Recommendations:
1. Delays should be taken very carefully, ample time is required for onboard
processing.
2. Sequence of turning on CE and CS should be very accurate, or the transceiver will
not operate as desired.
3. Configuration word should be set very carefully.
4. Configuration word entry starts from the MSB to the LSB.
5. This IC is unlike other ICs, it is very sensitive to physical shock and short circuits,
three of these units were irreversibly damaged, which in our case cost much time.

Page 74


4.5 RC UNIT

The main purpose of this RC Unit besides enabling RC mode, is that the user can
interfere manually should any errors occur, such as vehicle misguidance. This can save the
vehicle from possible crashes.

1. KEYPAD TESTING

Required components are the keypad encoder MM74C923, 0.1uF capacitor, 1uF
capacitor, and a 16 key keypad. The 1uF capacitor determines the debounce key mask. This
is done by creating a debounce period of 0.01s (delay) of on the encoder. The 0.1uF
determines the scanning frequency at 400Hz. The encoder has an output enable as which
should be set at active low.

These CMOS key encoders provide all the necessary logic to fully encode an array
of SPST switches. The keyboard scan can be implemented by either an external clock or
external capacitor. These encoders also have on-chip pullup devices which permit switches
with up to 50 KHz on resistance to be used. No diodes in the switch array are needed to
eliminate ghost switches. The internal debounce circuit needs only a single external

Page 75

capacitor and can be defeated by omitting the capacitor. A Data Available output goes to a
high level when a valid keyboard entry has been made. The Data Available output returns
to a low level when the entered key is released, even if another key is pressed. The Data
Available will return high to indicate acceptance of the new key after a normal debounce
period; this two-key rollover is provided between any two switches. An internal register
remembers the last key pressed even after the key is released. The TRI-STATEÉ outputs
provide for easy expansion and bus operation and are LPTTL compatible.

Both the keyboard scan rate and the key debounce period by altering the oscillator
capacitor, COSE, and the key debounce mask capacitor, CMSK. Thus, the MM74C923's
performance can be optimized for many keyboards. The keyboard encoders connect to a
switch matrix that is 4 rows by 4 columns or 5 rows by 4 columns (MM74C923). When no
keys are pressed, the row inputs are pulled high by internal pull-ups and the column outputs
sequentially output a logic 0 . These outputs are open drain and are therefore low for 25%
of the time and otherwise off. The column scan rate is controlled by the oscillator input,
which consists of a Schmitt trigger oscillator, a 2-bit counter, and a 2±4-bit decoder. When
a key is pressed, key 0, for example, nothing will happen when the X1 input is off, since Y1
will remain high. When the X1 column is scanned, X1 goes low and Y1 will go low. This
disables the counter and keeps X1 low. Y1 going low also initiates the key debounce circuit
timing and locks out the other Y inputs. The key code to be output is a combination of the
frozen counter value and the decoded Y inputs. Once the key debounce circuit times out,
the data is latched, and the Data Available (DAV) output goes high. If, during the key
closure the switch debounces, Y1 input will go high again, restarting the scan and resetting
the key debounce circuitry. The key may debounce several times, but as soon as the switch
stays low for a debounce period, the closure is assumed valid and the data is latched. A key
Page 76

may also debounce when it is released. To ensure that the encoder does not recognize this
debounce as another key closure, the debounce circuit must time out before another closure
is recognized. The two-key roll-over feature can be illustrated by assuming a key is pressed,
and then a second key is pressed. Since all scanning has stopped, and all other Y inputs are
disabled, the second key is not recognized until the first key is lifted and the key debounce
circuitry has reset. The output latches feed TRI-STATE, which is enabled when the Output
Enable (OE) input is taken low.

The following circuit schematic was
used to connect the keypad to the
encoder.

For testing refer to the section 5.1 Testing.

Page 77

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