Embedded systems. Exercise session 1. Introduction and project presentation

Transcription

1 Embedded systems Exercise session 1 Introduction and project presentation

2 Introduction Contact Mail : michael.fonder@ulg.ac.be Office : 1.82a, Montefiore Website for the exercise sessions and the project : Today Project presentation Overview of technologies available for your project Basic reminders in electricity 2

3 Embedded Systems Project Aim Design, realisation and programming of an embedded circuit. The precise subject (the type of circuit, its purpose,...) is left as a choice for each group but some requirements have to be fulfilled. Those requirements are : The necessity to have real-time constraints and concurrent tasks; The necessity to design and implement the electronic circuit needed to make your project meets its goal in practice; The necessity to use a real-time computing platform (e.g. a microcontroller) to implement the logical part of the project. Concretely Group of 3 or 4 students Several intermediate mandatory deadlines (see website for detailed schedule) 3 preparation labs 3

4 Embedded Systems Project First Deadlines Groups formation: For the embedded systems project 3-4 students/group For the major project in electronics 4-5 students/group Ideally today at the end of the session Or before next week after the exercise session by mail Project ideas presentation For the embedded systems project Fri Oct 13 after the theoretical course For the major project in electronics Wed Oct 11 9am Present a few slides which describe your project and how you plan to implement it. The aim of this presentation is to assess the feasibility of your idea based on our experience and to guide you toward something more adapted to the awaited level if necessary. 4

5 Embedded Systems Project Typical Workflow 1) System specification and constraints What your circuit should achieve Your project definition 2) Define a global solution Find ways to achieve the specifications Choice of sensors, actuators and other important components 3) Conceive an electronic schematic which meets specifications Find all the components required for your circuit to work and interconnect them properly 4) Test and validate each critical component individually 5) Complete circuit prototype realisation and tests (return to step 2 if needed) 6) Design and realisation of a printed of a Printed Circuit Board (PCB) (not 5 mandatory)

6 Practical Notes for the Project Material We will lend you a toolbox containing the cables needed to use the devices of the r100 lab and some basic components required to start the project immediately. All the content of this toolbox will have to be handled back at the end of the year for your project to be evaluated. Missing parts will have to be repaid! You will have a small budget to buy additional components to complete your circuit. Advice The project combines electronics, informatics and a small part of signal processing. All of those parts are important for the good completion of your project. So take care of each of them. Practical advice will be given during the exercises sessions and in feedbacks. Ignoring them will certainly make you lose time and maybe points at the end of your project. So, for your own good, pay attention and listen to what is said! 6

7 Microcontrollers : First insight So, the project revolves around microcontrollers But what are they? Microcontrollers (MCU) are small on-chip computers with a series of input and output pins. They contain at least all the components required to run a basic embedded program. They often include additional modules which ease their interfacing with the outer world. 7

8 Microcontroller example : PIC16F1789 Microchip PIC16F1789 Core Features 8-bit words, 16-bit instructions Only 49 instructions set Up to 8 MIPS (Millions of Instruction Per Second) Free programming tool Program memory : 16ko Data EEPROM : 256 Bytes RAM : 2048 Bytes Clock frequency up to 32MHz 36 general purpose Input/Output pins Note The program memory holds the binary data to be executed by the microcontroller. The EEPROM can be used to write and store data between different power cycles. The RAM can be used to write and store data during a single power cycle. 8

9 Microcontroller Peripheral Modules I Additionally to the core computing unit and its digital IO pins, microcontrollers generally embed several independent peripheral modules which ease the interfacing of the MCU with the external world. Example of Analog Peripherals Analog to Digital Converters (ADC) ADC are used to convert an analog voltage into binary data Digital to Analog Converters (DAC) DAC are used to generate an analog voltage from digital data Comparator Modules Comparators are used to interface analog circuits by comparing two analog voltages and providing a digital indication of their relative magnitude. Operational Amplifiers Operational amplifiers are use to process (e.g. amplify and or filter) analog signals. 9

10 Microcontroller Peripheral Modules II Additionally to the core computing unit and its digital IO pins, microcontrollers generally embed several independent peripheral modules which ease the interfacing of the MC with the external world. Example of Digital Peripherals Timers Timers are used to precisely measure elapsed time. Pulse Width Modulation (PWM) Signal Generators PWM signals are periodic binary signals which stay high during a given proportion of their period and are low everywhere else. Digital busses and communication lines interfaces (e.g. I2C, EUSART,...) Digital busses and communication lines are used to transfer binary data between two digital devices according to some given protocols. 10

11 General considerations Microcontrollers are computers. In order to interact with the outer world, they need both inputs (e.g. sensors) and outputs (e.g. actuators). Inputs capture information from the world (e.g. microphones, cameras,...) Outputs interact with the world (e.g. motors, pumps, LEDs,...) Analog vs digital Microcontrollers are digital devices which means they work with binary data (0s and 1s).The interface between the microcontroller and the sensors or the actuators is either analog or digital: Analog devices transmit or receive their information thanks to a continuous current or voltage modulation => need to translate this information from or to binary data The microcontroller communicates with other digital devices via busses or communication lines 11

12 Sensor ideas Distance Ultrasound Infra-red Positioning Accelerometers Gyroscopes GPS Input Push button Keyboard Joystick Other Microphones Pressure Serial bus Note Each sensor measures a particular information with one particular design => 2 sensors measuring the same information will have a different behaviour depending on the design and the techniques they use 12

13 Output ideas Actuators DC motor Servo motor Valve Visual information LED or laser LCD screen VGA signal Note Microcontrollers are designed to process and transmit signals, not power => NEVER draw the power required for an output directly on a pin the MCU if their power requirements excess the power limit of the pin 13

14 Example : Turret of Doom Aim Detect movement within a given range and point at the moving points with a laser Real-time constraints Periodic mapping of the room by using a distance sensor mounted on a servo-motor Coordination between head and laser Embedded components 1 IR distance sensor 2 Servo-motors 1 laser 2 LEDs 1 reset button Other examples : LED cube Ball Balancing 14 Now you have all the bricks required for finding your own project subject.

15 About Exercise Sessions The exercise session will have two main objectives: Providing you with practical knowledges to achieve your project Making exercises about notions seen during the theoretical course Sessions about the project During the sessions about the project, we will cover the following points: Detailed review of all the important modules of the PIC16F1789; Presentation of useful common electronic components; Presentation of useful simple circuits blocks; Introduction to the rules of electronic circuit design. 15

16 Conclusion for Today s First Part Project Be creative! Keep it simple! Begin now! Send me an with your group Note Comments and critics on the exercise sessions are welcome 16

17 Conclusion for Today s First Part Project Be creative! Keep it simple! Begin now! Send me an with your group! Note Comments and critics on the exercise sessions are welcome 17

18 Basic Electricity Reminders In order to build consistent circuits, some fundamental principles have to be understood. Basic Intuitive Idea The basic principle in electricity is to move charges trough electrical loads in order to perform a work. The work to perform can take various forms such as mechanical movement (e.g. motors), electromagnetic waves emission (e.g. antennas, light bulbs), To formalize this, we need to introduce five fundamental notions which are: Voltage Amperage Impedance Power Energy 18

19 Basic Electricity Reminders Voltage Voltage, electric potential difference, electric pressure or electric tension (formally denoted V or U, but more often simply as V or U) is the difference in electric potential energy between two points of an electric circuit per unit electric charge. Unit : Volt [V] Measuring a voltage is done in parallel of a load with a voltmeter or an oscilloscope Note: Some points of the circuit have to be chosen as a reference for measuring voltages around the circuit. All points which have a higher (resp. lower) potential than this reference have a positive (resp. negative) voltage. This reference is referred as the ground and is highlighted by the following symbols on schematics: 19

20 Basic Electricity Reminders Kirchhoff s Voltage Law The principle of conservation of energy implies that the directed sum of the electrical potential differences (voltage) around any closed network is zero. Example: Loop 1 : V1 + V2 + V3 = 0 Loop 2 : V3 + V4 + V5 = 0 20

21 Basic Electricity Reminders Amperage Amperage or electric current (formally noted I) is a flow of electric charges through a medium. In electric circuits this charge is often carried by moving electrons in a wire. Unit : Ampere [A] Measuring an electric current is done in series with a load with an ammeter Note: Flow of electric charges means the number of electric charges which pass through a given area per unit of time. 21

22 Basic Electricity Reminders Kirchhoff s Current Law The principle of conservation of electric charge implies that at any node (junction) in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node. Example: At node : I1 + I2 - I3 = 0 => I3 = I1 + I2 22

23 Basic Electricity Reminders AC vs DC When voltages and currents remain constant over time we talk about a direct current (DC) circuit. At the opposite when they vary over time, the circuit is said to work with alternative current (AC). The difference between both situations is critical since AC circuits can develop some dynamic behaviours which depend on the frequency of the signals passing through it. Power and Energy Power is the amount of work produced (resp. consumed) by unit of time and is expressed in Watt [W]. In electricity the power produced or consumed by a component is given by : P=IV The energy produced (resp. consumed) by a component is simply the integration of the power produced (resp. consumed) over time. The energy can be expressed in Joule [J] or Watt-hour [Wh]. 23

24 Basic Electricity Reminders Impedance Electrical impedance (formally denoted Z) is the measure of the opposition that a circuit presents to a current when a voltage is applied. Impedance possesses both magnitude and phase (it s a complex number). When a circuit is driven with direct current, there is no distinction between impedance and resistance; the latter can be thought of as impedance with zero phase angle. Unit : Ohm [Ω] Note: Impedance extends the concept of resistance to AC circuits The notion of impedance is useful for performing AC analysis of electrical networks, because it allows relating sinusoidal voltages and currents by a simple linear law; Ohm s Law. 24

25 Basic Electricity Reminders Equivalent Impedance Circuits often contain complex networks of impedances. In order to understand it behaviour, it is often useful to simplify those networks into a simple equivalent impedance. This can be easily done with two simple rules: For impedances in series Zeq = Z1 + Z2 For impedances in parallel 1/Zeq = 1/Z 1 + 1/Z2 25

26 Basic Electricity Reminders The three notions introduced previously are obviously linked together. The relation between each of them is given by Ohm s law. Ohm s Law Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points: U=ZI If we only consider the magnitudes, we have: U = Z I Note: When Z is real, U = Z I 26

27 Common Basic Components Resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, divide voltages, bias active elements and terminate transmission lines, among other uses. A resistor is characterized by its resistance (R). Resistance unit : Ohm [Ω] Impedance : Z = R => independent of f Symbol Note: Inductors have values that typically range from 1 Ω to 10 MΩ. 27

28 Common Basic Components Capacitor A capacitor is a passive two-terminal electrical component that stores electrical energy in an electric field. This is done by creating a difference in charges between two conductors. The typical physical form and construction contains at least two electrical conductors often in the form of metallic plates or surfaces separated by a dielectric medium. A capacitor is characterized by its capacitance (C), which is the ratio of the electric charge on each conductor to the potential difference between them. Capacitance unit : Farad [F] Impedance : Z = 1/(j 2πf C) => when f, Z and when f, Z Symbol Fixed Capacitor Polarized Capacitor Note: Inductors have values that typically range from 1 pf to 1 mf. 28

29 Common Basic Components Inductor An inductor, also called a coil, is a passive two-terminal electrical component that stores electrical energy in a magnetic field when electric current flows through it. An inductor typically consists of an electric conductor, such as a wire, that is wound into a coil around a core. An inductor is characterized by its inductance (L), which is the ratio of the voltage to the rate of change of current. Inductance unit : Henry [H] Impedance : Z = j 2πf L => when f, Z and when f, Z Symbol Note: Inductors have values that typically range from 1 µh (10 6H) to 1 H. 29

30 Common Basic Components Diode Diodes are two terminal components which have the two following properties: They let the current almost freely flow when the voltage between its cathode and its anode is positive (forward biased) and reach an given threshold V F; When reverse biased, they let only a very small amount of current flow (~high resistance) until the voltage reach a given threshold V BR at which the diode enters the breakdown mode. Symbol Note: Normal diodes usually get destroyed when entering the breakdown mode. 30

31 Common Basic Components Particular Types of Diodes Some diodes are designed in a way which exacerbate one of the particular properties of the semiconductors used in the diode. Among those special diodes, we can find: Zener diodes which are able to withstand high reverse currents in breakdown mode without burning; Light emitting diodes (LED) which emit light with a given wavelength; Photodiodes which create an non-negligeable reverse current when exposed to light. Symbols Zener diode LED Photodiode 31

32 Concrete Application Examples Low Pass Filter In the following circuit, we define Vin as a voltage applied to the circuit and Vout a voltage we want to measure. Express Vout as a function of Vin for the circuit below. What do you observe? Assuming that the cutoff frequency of a filter is the frequency at which the input voltage gets attenuated by a factor 2, what is the cutoff frequency of this filter? Note: This circuit is meant to carry signals only, not power! 32

33 Concrete Application Examples Voltage divider In the following circuit, we define Vin as a voltage applied to the circuit and Vout a voltage we want to measure. Express Vout as a function of Vin for the circuit below. What do you observe? Note: This circuit is meant to carry signals only, not power! 33

34 Concrete Application Examples Current Limitation We want to plug a LED on 5V. In order to limit the current which flows through it, we connect a resistor in series. Assuming the LED has no internal resistance and a V F of 0.7V, what should be the value of the resistance to limit the current passing through the LED to 3.5mA? 34