17.1

EE 109 Unit 17 - Pulse Width

Modulation

17.2

Power

• Recall (or learn) that Power is a measure of:

– _____________________________________

• In an electronic circuit, P = ______________

– Power = Current & Voltage (each may be ______________ w/ time)

• A circuit that draws a constant 2 mA of current at a constant 5V would

consume ____________

• Since voltage and current may change rapidly, it is often helpful to

calculate the average power

– Just sum the total power and divide by the total time

5V

0V

1 s

.5s

1 s

.3sI = 1A

Average Power

= _______________

17.3

Output Devices

• What do the following have in common?

– Servo motor that can rotate to any angle w/in 180 degrees

– ____________________

– ___________________________ with various power levels

• They are controlled by Pulse Width Modulation (PWM)

– Usually a 3-pin interface: Power (Vcc), GND, PWM Signal

17.4

Duty Cycle• A pulse is just a short window of time when a signal is

'on'

• We could repeat the pulse at some regular period, T

• We define the duty cycle as

Duty Cycle % = _________________________

5V

0V

T

T/2

5V

0V

T

T/4

T T T

Duty Cycle

= ____%

Duty Cycle

= ____%

17.5

Power & Duty Cycle• When we light up an LED we

often just turn a PORTxx output

'on' and leave it 'on'

– This supplies the ____________

power possible to the LED

• We could _________ the output

at some duty cycle (say 50%) at a

fast rate

– Fast so that the human eye can't

___________________

– Average power would be ½ the

original always 'on' power

– Result would be a _____________

__________________

R1

Vs

+ V1 -i

+ VLED-

uC

PORTXX

5V

0V

5V

0V

T

T/2

T

PORTxx 'on' constantly

PORTxx 'on' 50% of time

17.6

In-Class Activity

• Write a program with a loop that turns on the LED

(PORT B5) for x milliseconds and then turns it off for

100-x milliseconds

– Initially set x = 100

– Now set x = 50

– Now set x = 20

– Now set x = 10

– Now set x = 2

• Notice result may be non-linear

• A similar tactic is used in your _______________

when you want to cook something at __________

etc. power.

17.7

PWM

• Modulation refers to _____________ a value based on some

___________ (i.e. changing one signal based on another)

• Pulse width modulation refers to modifying the width of a

pulse based on another signal

• It can be used to _______________ one signal into another

– Example below of

sine wave represented

as pulses w/ different

widths

• Or it can just be used

to alter average power

as in the last activity

17.8

Simple Digital-To-Analog

• Connecting a PWM output to a resistor-capacitor circuit

as shown causes the voltage at Vc to "integrate" the

digital PWM signal (charge the capacitor)

– Analogy: Imagine you have a leaky bucket (i.e. capacitor) and

you want to produce a variable level (i.e. _______________) of

water by only turning the hose (___________ output) on or off

VPWM

5V

0V

Vc

5V

0V

17.9

Servo Motors

• Many embedded systems use servo

motors to move or rotate mechanical

devices

• Most servo motors use some form of

pulse width modulation to control the

direction and speed of their rotation

• 2 Kinds

– ____________ servo motors: can only rotate

through a certain _____ (usually 180

degrees)

– _______________: can keep spinning round

and round while pulses are provided

17.10

Standard Servo Motor

• Pulse width determines ________________ of servo motor

• Must continue to give pulses for the duration of time it takes

to ________________________________________

• No pulses = _____________

Pulse width = 750us

Full left

Centered

Full right

20 ms 20 ms

Pulse width = 1500us

20 ms 20 ms

Pulse width = 2250us

20 ms 20 ms

Do an Internet search for Standard Servo Motors & try to find

the appropriate pulse width for each

position

17.11

Continuous Servo Motors

• Pulse width determines ________________ of

rotation

• Controlled via PWM (Pulse Width Modulation)

– Short pulse = Rotate one direction

– Medium pulse = Stop

– Long pulse = Rotate other direction

20 ms20 ms 20 ms

Pulse Width = _______ us = Full Speed Clockwise

Pulse Width = ________ us = Stopped

Pulse Width = ________ us = Full Speed Counter-

Clockwise

17.12

Implementing PWM

• Can use delays or timers to make your own pulse

signals

• Most microcontrollers have hardware to

automatically generate PWM signals based on the

contents of some control registers

• Many microcontrollers use the Timers to also serve

as PWM signals

– Recall the timer module gave us a counter that would

increment until it hit some 'modulus' (MAX) count which

would cause it to restart and also generate an interrupt

17.13

Using Timers for PWM

• For PWM we can use that counter to just count 0 to some

MAX count making the:

– PWM output = '1' while the count < threshold (OCRxx) and

– PWM output = '0' when the count >= modulus (OCRxx)time

OCRxA

0

MAX (255)

time

OCRxB

0

MAX (255)

PWM Output 1 PWM Output 2

17.14

PWM Control Registers

• In this slide packet we will use the 8-bit Timer/Counter0 rather

than the 16-bit Timer/Counter1

• Refer to Timer Slides w/ following additions

• Set WGM0[2:0] bits for Fast PWM mode as opposed to CTC

• Timer/Counter0 can produce two PWM outputs on Arduino pins

D5 and D6, each with its own threshold value, so you need to pick

which one you want to use

– Bits COM0A[1:0] and threshold register OCRA control operation of output

D6 (PORTD6)

– Bits COM0B[1:0] and threshold register OCRB control operation of output

D5 (PORTD5)

See datasheet, textbook or other documentation for further explanation

17.15

PWM Control Registers

WGM01,

WGM00

WGM02=0 WGM02=1

(Ignore )

00 Normal

(Counter)

Unused

01 Phase

Correct PWM

Phase Correct

PWM

(Top=OCRA)

10 CTC (Timer) Unused

11 Fast PWM

(Top=255)

Fast PWM

(Top=OCRA)

COM0?1,

COM0?0

Output Compare pin

(assume WGM02=0)

00 Don't use Pin

01 Don't use Pin

10 Set Pin on CTR=0x00, Clear

pin on match=OCR?

11 Clear Pin on CTR=0x00, Set

pin on match=OCR?

CS0

[2:0]

Prescaler

010 Clk / 8

011 Clk / 64

100 Clk / 256

101 Clk / 1024

TCCR0A Reg.Timer/Counter0 Control Register

COM0A1

COM0A0

COM0B1

COM0B0

- - WGM00

WGM01

TCCR0B Reg.Timer/Counter0 Control Register

FOC0A

FOC0B

- - WGM02

CS02 CS00CS01

• Set WGM bits for PWM mode [usually

Fast PWM mode] as opposed to CTC

• Pick COM0?[1:0] for desired waveform

• Still need to pick a prescaler to slow

down the clock

• Set OCRA or OCRB to the desired

threshold which will effectively control

the duty cycle of the PWM output

17.16

Exercise

• Try to use PWM to make your LED glow at

various brightness levels similar to what you

did earlier with normal digital I/O

17.17

A BRIEF SUMMARY

Review of some key concepts from the first half of the semester and

revisit what CECS prepares you to do in the future….

17.18

A Few Big Ideas 1

• _________________________ bits in a register tells the

hardware what do and when (this is SW interacting with HW)

• ___________ matters

– Your software is executing ___________ compared to how fast a

human can do something

– You can use that to your advantage: blinking an LED at a fast rate can

give the illusion it's always on but just more dim

– Or it can work to your disadvantage: One button press may look like

__________ because a loop may see one press on multiple iterations.

– We must write our software with this in mind

17.19

A Few Big Ideas 2

• Clocking or enables are necessary to say ________

– Digital signals are always 1's and 0's so just looking at the

bits doesn't tell us how many we have

– We usually need ____________ (pulses) to tell the

hardware when we want it to grab the data

Just looking at this set of digital values, are we sending 0101 once, twice, three times, how many? Once because we use the clock/enable to indicate that.

But without the clock we'd have no clue how many times we are trying to write

0101

17.20

A Few Big Ideas 3

• External events happen __________________ with your

software (don't know "when" something has happened)

– Your software program is the brains for how to process

information but it doesn't magically know "when" something

has happened?

– We have to keep checking it (polling) or

– Hardware designers built "interrupt" mechanisms to help

• Many tasks can be done in ___________________; SW

may be easier to code/use but HW provides parallelism

– A 0.1 second timer can be done in SW using delays but then

software can't do much else

– Or in HW using timers allowing SW to do other tasks

17.21

Remember Day 1

http://www.cmu.edu/news/image-archive/Boss.jpg

http://prisonerofclass-5933.zippykid.netdna-cdn.com/wp-content/uploads/2013/05/iphone.jpg

http://firstcallappliance.com/wp-content/uploads/image/microwave.jpg

http://www.amazon.com/Fisher-Price-T-M-X-Tickle-Me-Elmo/dp/B000ETRE0Q

http://oeatech.net/wp-content/uploads/2011/03/RADARSAT2-satellite.jpg

• Computer engineering prepares

you for a broad set of fields

– You could work in the SW industry

– You could work in the HW industry

– You will be most qualified for jobs

that combine that knowledge

• We've been focused on the

software/hardware interaction

embodied in embedded systems

17.22

You Can Do That…

C / C++ / Java

Logic Gates

Transistors

HW

SW

Voltage / Currents

Assembly /

Machine Code

Applications

LibrariesOS

Processor / Memory / I/O

Functional Units

(Registers, Adders, Muxes)

Devices & Integrated Circuits(Semiconductors & Fabrication)

Architecture(Processor & Embedded HW)

Systems & Networking(Embedded Systems, Networks)

Applications(AI, Robotics, Graphics, Mobile)

Cloud & Distributed Computing(CyberPhysical, Databases, Data

Mining,etc.)

Scripting &

Interfaces Networked

Applications

What we've been focusing on thus far

17.23

Dive Into a SmartPhone

• Here's a picture of what's inside the

iPhoneTM 6

• Both sides of the circuit board are

populated with chips

http://www.techinsights.com/teardown.com/apple-iphone-6/

https://d3nevzfk7ii3be.cloudfront.net/igi/6MaZk5cEE2tm1uCj.hugeBattery

2-sided Circuit Board

17.24

What's Inside Your SmartPhone

• What's inside an iPhone 6?

• Microcontrollers/microprocessors

– Apple A8 APL1011 SoC + Elpida 1 GB LPDDR3 RAM

– SoC = __________________…Not just a processor but a

processor with custom hardware to do specialized tasks…on-

board graphics processor in this case

– NXP LPC18B1UK ARM Cortex-M3 Microcontrollers

– Similar on-board I/O modules as the _____________. Take a

look…

– http://www.nxp.com/products/microcontrollers/cortex_m3/

• Modem + Amplifiers + Transceivers for wireless

communication

– Qualcomm MDM9625M LTE Modem + many others

http://www.techinsights.com/teardown.com/apple-iphone-6/

17.25

What's Inside?

• A gyroscope, accelerometer, and touchscreen

– InvenSense MP67B 6-axis gyroscope and accelerometer

combo

– Broadcom BCM5976 Touchscreen Controller

– Both use some form of _____________________ to

sense motion or touch

• Memory Storage

– SK Hynix H2JTDG8UD1BMS 128 Gb (16 GB) NAND Flash

• Other specialized HW I/O modules

– Murata 339S0228 Wi-Fi Module

– Qualcomm PM8019 power management IC

– Cirrus Logic 338S1201 audio codec

http://www.techinsights.com/teardown.com/apple-iphone-6/

17.26

Computer Engineering & HW

• Computer engineering prepares you to

work in jobs that design these kinds of

systems by:

– Learning how to design digital circuits

using logic gates [AND, OR, NOT]

(EE 154 and EE 254 Digital System Design)

– Learning how to optimize processors to

execute software as efficiently as possible

(EE 457 Computer Architecture)

– Learn how to assemble many HW pieces

(processor cores, RAM, specialized HW) to

form systems-on-chip

(EE 454L – SoC Design)

– Learn some of the physics and science of

fabricating these designs on silicon

(EE 277L and EE 477L VLSI Design) http://www.anandtech.com/show/8562/chipworks-a8

Die Photo of the Apple A8 SoC Processor