Download EECS 498 Advanced Embedded Systems

EECS 473
Advanced Embedded Systems
Lecture 9:
Groups introduce their projects
Power integrity issues
Project groups
• Please give a 2-3 minute overview of your
project.
– Half the groups will do this each day.
• So each group goes (about) once a week.
Teams status updates for today
•
•
•
•
•
Team Alert (Home Alert)
Team Fitness (Fitness watch)
Team Glasses
Team Mouse (Control in hand)
Team WiFi (WiFi localization)
Final proposal due today
• Final proposal
– Today Thursday @10pm
– Mail to Yitian, Matt, and me
– Include “473 proposal” and your team name in
the subject.
– I should have signed group agreement now.
• I’ll try to have feedback by Sunday night to all
groups.
Where we are;
where we are going
• Labs 1-3 done, lab 4 due Sunday
• PCB tutorial left to go.
– Much less conceptual—it’s about learning a tool
• Entering full-time project mode.
– Have midterm and 3 homework assignments before
project due.
• Everything else is project (and lecture).
• HW1 posted, due 10/21
– You should be putting in ~15 hours/week into the project.
• The more you put in now, the less you have later.
– And some things (reorders, PCB redos) just take time—can’t cram.
• You really (really) want time to debug!
• Large part of project grade based upon fully working.
– More complex projects get a bit more slack, but…
– Project due a week before classes end (design expo)
Now…
• Do a review of basic circuits
– Physics 240/EECS 215
• Discuss power integrity
– Return paths
– RCL circuit issues
Some electrical issues related to
the building of PCBs
EE issues overview
High-speed PCB design issues
• There are a lot of electrical issues to deal with when
working with high-speed PCBs.
– Supplying power, storing energy and dissipating heat
• Power supplies, batteries, and heat sinks.
– Power Integrity (PI)
• We need to be sure that we keep the power and ground at
approximately constant values.
– Signal Integrity (SI)
• We need to make sure data on the wires gets there.
– Electro-magnetic interference/compatibility (EMI/EMC)
• We need to watch out for generating radio-frequency noise
– The FCC is a bit picky about this.
• We don’t want RF noise to interfere with us.
Outline
• Background
– Understanding power and energy
• EECS 215 “review/introduction”
– On capacitors, inductors, resistors and impedance
• Power integrity (PI)
Power!
• Electric power is the rate at which electric energy
is transferred by an electric circuit. The SI unit of
power is the Watt. (Wikipedia)
• Power (as opposed to energy), in-and-of itself is
important in embedded system design.
– For example there may be a cap on power draw from
a given set of batteries.
• That is, they can’t supply energy at more than a given rate.
– Melting issues are power issues
• Admittedly over time.
Understanding Power and Energy
Power vs. Energy
• Power consumption in Watts
– Determines battery life in hours
– Sets packaging limits
• Energy efficiency in Joules
– Rate at which power is consumed over time
– Energy = power * delay (Joules = Watts * seconds)
– Lower energy number means less power to
perform a computation at same frequency
Understanding Power and Energy
Power vs. Energy
EECS 215/Physics 240 “review”
Background issue #1:
Inductance
• An inductor “resists the • The light bulb is a resistor. The
wire in the coil has much lower
change in the flow of
resistance (it's just wire)
electrons”
– so what you would expect when
you turn on the switch is for the
bulb to glow very dimly.
• What happens instead is that
when you close the switch, the
bulb burns brightly and then
gets dimmer.
– And when you open the switch,
the bulb burns very brightly and
then quickly goes out.
http://electronics.howstuffworks.com/inductor1.htm
EECS 215/Physics 240 “review”
Background issue #2:
Capacitance
• A capacitor resists the
change of voltage
– When you first connect
the battery, bulb lights
up and then dims
– If you then remove the
battery and replace with
a wire the bulb will light
again and then go out.
http://electronics.howstuffworks.com/capacitor1.htm
EECS 215/Physics 240 “review”
Background issue #3:
Impendence
• Impedance (symbol Z) is a measure of the overall
opposition of a circuit to current, in other words:
how much the circuit impedes the flow of
current.
– It is like resistance, but it also takes into account the
effects of capacitance and inductance.
– Impedance is measured in Ohms.
– Impedance is more complex than resistance because
the effects of capacitance and inductance vary with
the frequency of the current passing through the
circuit and this means impedance varies with
frequency!
• The effect of resistance is constant regardless of frequency.
http://www.kpsec.freeuk.com/imped.htm
EECS 215/Physics 240 “review”
A look at impedance
(with capacitors, inductors and resistors vs. frequency)
1.000
Pure
inductor
Impedance
0.100
Cap/resistor
Pure cap.
0.010
0.001
1.E+03
1.E+04
1.E+05
1.E+06
Frequency
Notice the log scales!
1.E+07
1.E+08
1.E+09
Power Integrity
Power Integrity
• In order to get digital electronics to work correctly, they
need a minimum voltage differential.
– If we get below that, the devices might
•
•
•
•
Be slow (and thus not meet setup times)
Lose state
Reset or halt
Just plain not work.
• Even a very (very) short “power droop” can cause the
chip to die.
– In my experience, this is a really common problem.
• Keeping power/ground constant and noise/droop
free is “Power Integrity”
Power Integrity
So?
• We need the
Vcc/Ground differential
to be fairly constant.
– But rapid changes in the
amount of current
needed will cause the
voltage to spike or droop
due to inductance.
• We basically want a
“no-pass” filter.
– That is we don’t want to
see any signal on the
Vcc/Ground lines.
– The obvious thing?
• “Add a capacitor”
– That should keep the
voltage constant, right?
• The problem is we need
to worry about a lot of
frequencies AND
capacitors aren’t ideal.
Power Integrity
Lots of frequencies
• Even fairly slow devices • This means we
these days are capable
generally have to worry
of switching at very high
about frequencies from
frequencies.
DC all the way to 1GHz.
– Basically we get drivers
that have rise and fall
times capable of going
1GHz or so.
– Because our chip may be
varying its draw at rates
up to that fast.
Power Integrity
Non-ideal devices.
• ESR is Effective Series Resistance
• ESL is Effective Series Inductance
• Ceff is the effective capacitance.
– How does quantity effect these values?
• Obviously impendence will be varying by frequency.
Power Integrity
Other things can add to ESR/ESL
• Generally a bad solder job can make ESR/ESL
worse.
• Packaging has an impact
– wires have inductance so surface-mount packages
preferred
• Pads can have an impact
Power Integrity
Power Distribution Network
• Talked a lot about keeping the power supply
voltage constant.
– Should think of situation as follows:
Input
PDN
Processor
Output
PDN
– If the processor drops 3.3V and uses 100mA, what is
it’s effective resistance?
– If the power supply is 3.3V, the processor uses 100mA
and the total resistance of the PDN (Power
distribution network) is .01Ω, what voltage does the
processor really see?
Consider an FPGA with the following
characteristics
• Acceptable voltage range is from 2.65 to 2.75V
– Max current is 5A.
– What is the largest impedance we can see on the
PDN and still have this work?
Power Integrity
Given the previous table..
Decoupling Impedance vs Frequency
1.000
Z(pup)
0.100
Z(tant)
Impedance
Z(1uF)
Z(0.1uF)
Z(0.01uF)
Z(pcb)
ZT
0.010
0.001
1.E+03
Z(LICA)
1.E+04
1.E+05
1.E+06
Frequency
1.E+07
1.E+08
1.E+09
Power Integrity
Removing the PCB…
Decoupling Impedance vs Frequency
1.000
Z(pup)
0.100
Z(tant)
Impedance
Z(1uF)
Z(0.1uF)
Z(0.01uF)
Z(pcb)
ZT
0.010
Z(LICA)
0.001
1.E+03
1.E+04
1.E+05
1.E+06
Frequency
1.E+07
1.E+08
1.E+09
Power Integrity
What is the PCB part?
Power Integrity
But wait…
• VRM
– Voltage regulator module
• bulk bypass (tantalum) and
decoupling capacitors
(ceramic).
– These capacitors supply
instantaneous current (at
different frequencies) to the
drivers until the VRM can
respond.
• However sets of different
capacitors cause problems!
http://www.pcbdesign007.com/pages/columns.cgi?
artcatid=0&clmid=65&artid=85396&pg=3&_pf_=1
Power Integrity
Other power integrity issues
• Of course, one source of power integrity
problems is coming from the processor
– Power supply just can’t keep up with processor
varying (what we just did)
• But there are other problems.
– And these are issues introduced by the PCB
designer.
• Don’t be that guy/gal.
Power Integrity
Connecting ground poorly
• One big issue is that people think of ground as, well,
ground.
– It isn’t.
– Only one point is “0V”.
• Everything else has a higher voltage.
– Wires aren’t perfect.
• It’s really easy to make this mistake.
– Classes like EECS 215 basically encourage it.
– Better to think of things as “return path” not ground.
• And yes, you can make the same mistake with power, but people
do that a lot less often.
– Partly because we often have different “Vcc” levels on the board.
– But mostly because we just think of power and ground differently.
Power Integrity
Consider the following
• Consider the figure on the
right.
– Why is the top picture “wrong”?
• Let’s consider the case of “A”
being DC motor that runs at
120 Watts (12V 10A).
• B is processor drawing 100mA
– Wire from A to PSU return is
15cm long, 400mils wide.
– What is the voltage at the
“ground”?
3.3V
0.1A
12V
10A
0.02Ω
Top figure from “The Circuit Designer’s Companion”. If you are going to do PCB design much, buy and read this book.
Power Integrity
Review: Power integrity (1/2)
• Processors and other ICs
have varying current
demands
– Sometimes at frequencies
much greater than the device
itself runs at
• Why?
– So the power/ground inputs
need to be able to deal with
that.
• Basically we want those wires
to be ideal and just supply
how ever much or little
current we need.
– If the current can’t be
supplied correctly, we’ll get
voltage droops.
• How much power noise can
we accept?
– Depends on the part (read
the spec).
• If it can run from 3.5V to 5.5V
we just need to insure it stays
in that range.
– So we need to make sure that
given the current, we don’t
end up out of the voltage
range.
• Basically need to insure that
we don’t drop too much
voltage over the wires that
are supplying the power!
Power Integrity
Review: Power integrity (2/2)
•
So we need the impedance of the
wires to be low.
– Because the ICs operate at a wide
variety of frequencies, we need to
consider all of them.
– The wires themselves have a lot of
inductance, so a lot of impedance at
high frequencies.
•
•
Need to counter this by adding
capacitors.
Problem is that the caps have
parasitic inductance and resistance.
– So they don’t help as well as you’d like
– But more in parallel is good.
– Each cap will help with different
frequency ranges.
•
•
We also can get a small but lowparasitic cap out of the
power/ground plane.
Finally we should consider antiresonance*.
* http://www.n4iqt.com/BillRiley/multi/esr-and-bypass-caps.pdf provides a very nice overview of the topic and how to address it.
Power Integrity
Power Integrity (PI) summary
• Power integrity is about keeping the Vcc/ground
difference constant and at the value you want.
• Covered two issues:
– Many devices that sink power do so in “pulses”
• Due to internal clocks and time-varying behavior
• Need caps to keep value constant
– But parasitic ESR/ESL cause problems
– So lots of them==good
» Reduce ESR/ESL
» Increase capacitance.
• Anti-resonance can cause problems!
– Need Spice or other tools to model.
» Will do a bit of this next time
– Also, need to watch return paths
• Can easily bump up your ground level
– Cuts into your margin for the work above…
Additional “reading”
•
http://www.murata.com/en-us/products/emiconfun/capacitor/2013/02/14/en-20130214-p1
– Very nice coverage of ESR and impedance in a non-idea capacitor. Touches on the fact
that ESR varies by frequency! Very readable and short!
•
http://ksim.kemet.com/
– Nice spice models of real capacitors.
•
http://doc.utwente.nl/64874/1/tiggelman.pdf
– A much more academic treatment of ESR.
•
https://www.youtube.com/watch?v=sW0a9d_vWoc
– Mildly amusing and useful (who doesn’t like magic smoke?)