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Lecture 1: Embedded Systems Overview, AVR

Hardware/Software Introduction

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Embedded Systems Overview

• Computing systems are everywhere• Most of us think of “desktop” computers

– PC’s– Laptops– Mainframes– Servers

• But there’s another type of computing system– Far more common...

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Embedded Systems Overview

• Embedded computing systems– Computing systems embedded

within electronic devices– Hard to define. Nearly any

computing system other than a desktop computer

– Billions of units produced yearly, versus millions of desktop units

– Perhaps 50 per household and per automobile

Computers are in here...

and here...

and even here...

Lots more of these, though they cost a lot

less each.

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A “short list” of embedded systems

And the list goes on and on

Anti-lock brakesAuto-focus camerasAutomatic teller machinesAutomatic toll systemsAutomatic transmissionAvionic systemsBattery chargersCamcordersCell phonesCell-phone base stationsCordless phonesCruise controlCurbside check-in systemsDigital camerasDisk drivesElectronic card readersElectronic instrumentsElectronic toys/gamesFactory controlFax machinesFingerprint identifiersHome security systemsLife-support systemsMedical testing systems

ModemsMPEG decodersNetwork cardsNetwork switches/routersOn-board navigationPagersPhotocopiersPoint-of-sale systemsPortable video gamesPrintersSatellite phonesScannersSmart ovens/dishwashersSpeech recognizersStereo systemsTeleconferencing systemsTelevisionsTemperature controllersTheft tracking systemsTV set-top boxesVCR’s, DVD playersVideo game consolesVideo phonesWashers and dryers

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What is an embedded system?

• What makes a microcontroller:– Self Contained

• CPU• Memory• I/O

– Application or Task Specific• Not a general-purpose computer• Appropriately scaled for the job

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What is an embedded system?

• Embedded PCs?

• “Soft” Processors on PLDs?

• Systems On A Chip?

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Designing Embedded Systems

• Microcontrollers– Don’t have keyboard and monitor jacks– Must use ports to perform I/O

• Inputs – to sense things• Outputs – to control things• Related Component Topics

– Common Interfaces– Part Packages

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What you will do:

• Labs– Lab 1: Introduction to AVR STK500

Hardware/Software, a couple of simple c programs– Lab 2: A/D converter– Lab 3: Optical Sensors– Lab 4: 4 bits D/A converter– Lab 5: Controls and Feedback– Lab 6: Motor Control - open loop– Lab 7: Motor Control - simple feedback control– Lab 8: Motor Control - proportional feedback control

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What you will do:

• Final Project – your proposed project

• Final Project– Hardware– Software– Presentation– Report

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Introduction to AVR

• CodeVision AVR C Compiler Professional version – Installed in 20 PCs in room EN229– Compile programs with more than a thousand

instructions.– Provides many useful assembly programs used by

your C programs. You write your programs in C completely. AVR C compiler will integrate all required programs together

– More about AVR C compiler when presenting Lab 1

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The Atmel AVRTM is a family of 8-bit RISC microcontrollers produced by Atmel.

The AVR architecture was conceived by two students at the Norwegian Institute of Technology (NTH) and further refined and developed at Atmel Norway, the Atmel daughter company founded by the two chip architects.

History of AVR

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Introduction to AVR

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Used in Lab

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PB2 PB3 also used as Analog Input 0 (AIN0) and Analog Input 1 (AIN1)

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The Analog Comparator comparesthe input values on the positive pin AIN0 and negative pinAIN1.

When the voltage on the positive pin AIN0 is higher than the voltage on the negative pin AIN1, the Analog Comparator Output, ACO, is set. ACO is kept in bit 5 of Analog Comparator Control and Status Register

The comparator’s output can be set to trigger the Timer/Counter1 Input Capture function. In addition, the comparator can

trigger a separate interrupt, exclusive to the Analog Comparator.

The user can select Interrupt triggering on comparatoroutput rise, fall or toggle

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Interesting Instruction Examples:

• NOP – Do nothing for 1 cycle

• SLEEP – Sleep until reset or interrupted

• WDR – Watch Dog Reset

AVR Instruction set manual available in the course website

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Timers: Why we need them

• Provide accurately timed delays or actions independent of code execution time

• How are Timers used?– Accurate delay

• Read the timer, store value as K. Loop until timer reaches K+100.

– Schedule important events• Setup an Output Compare to trigger an interrupt at a

precise time When the value in the Output Compare Register matches

the value in timer/counter register -- triggers an interrupt

-- Measure time between events• When event#1 happens, store timer value as K• When event#2 happens, read timer value and subtract K• The difference is the time elapsed between the two events

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AVR Timer/Counter 0

• 8 Bit

• Wrap-Around

Up Counter

• Interrupt on

overflow

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AVR Timer/Counter 0

• 8 Bit Up Counter

– counts from 0 to 255 (0xFF), then loops to 0

– Internal or External Clock source• Prescaler• Output compare match output through OC0 for

timer/counter 0 compare match, i.e. PB3, pin 4

• Interrupt on Overflow

– Transition from 255 to 0 can trigger interrupt if

desired

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OC0, Output Compare Match output:

Whenever TCNT0 equals OCR0 (Output Compare Register 0), the comparator signals a match

The PB3 pin can serve as an external output for the Timer/Counter0 Compare Match. The PB3 pin has to be configured as an output

AVR Timer/Counter 0

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AVR Timer/Counter 1

– 16 Bit– Dual Comparators A,B (output captures)– Up Counter– Interrupt on:

• Overflow• Compare A/B• Input Capture of external event on ICP pin.

– Can also act as an 8, 9 or 10 bit PWM Up-Down Counter.

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The Input Capture unit of Timer/Counter captures external events and gives them a time-stamp indicating time of occurrence.

The external signal indicating an event, or multipleevents, can be applied via the ICP1 pin or alternatively, via the Analog Comparator unit.

The time-stamps can then be used to calculate frequency, duty-cycle, and other features of the signal applied.

Alternatively the time-stamps can be used for creating a log of the events.

AVR Timer/Counter 1

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Timer 1 and Output Compare• The AVR has two output compares (OCR1A/B)

– OCR1A/B are 16-bit registers– When the value of OCR1A/OCR1B matches that of Timer1:

• A user-defined action can take place on the OC1A/OC1B pin (set/clear/inv) i.e.,(OC1A )PD5 / (OC1B) PD4 need to set as output• An interrupt can be triggered• Timer1 can be cleared to zero

– Once set up, output compares operate continuously without software intervention– Great for:

• Precise recurring timing• Frequency/Tone generation (maybe sound effects)• All kinds of digital signal generation – Infrared communications – Software-driven serial ports

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Timer 1 and PWM• Pulse-Width Modulation

– Useful for using digital circuits to achieve analog- like control of motors, LEDs, etc

– Timer 1 has two channels of PWM output on OCR1A and OCR1B

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Timer Control: I/O space

• Timer 0:– Control Register (TCCR0) for clock selection, external clock or internal clock, prescaler etc.– Timer/Counter0 (TCNT0) holding counter value

• Timer 1:– Control Register A & B (TCCR1A/B)– Input Capture Register (ICR1)– Timer/Counter1 Output Compare Register A and B (OCR1A/B)– Timer/Counter1 (TCNT1)

• Timer Interrupt Registers (Mask and Flag Registers) are Common to Both Timers

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AVR Timer/Counter Sources• Shut Off

• CPU frequency divided by 1,8,64,256,1024

• At 8MHz that’s: 1/8us, 1us, 8us, 32us, 128us

• External Input (rising or falling).

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Interrupts

• Interrupts halt normal code execution in order to go do something more important or time sensitive

• Interrupt “Handlers”– Using the Interrupt Vectors

• Interrupts are used for:– RESET– Timers and Time-Critical Code– Hardware signaling

• “I’m done”• “Something’s happened that you want to know about”• “I have something for you”

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Watchdog Timer: reset the MCUThe Watchdog Timer is clocked from a separate On-chip Oscillator which runs at 1 MHz

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Reading Assignment: Chapter 1 of Embedded C Programming and the Atmel AVR