1. EECS 473 Advanced Embedded Systems
Lecture 9:
Groups introduce their projects
Power integrity issues

2. Final proposal due today
I should have signed group agreement now.
I should have feedback by Tuesday night to all groups.
Expect sooner actually.

3. Where we are; where we are going
Labs 1-3 done, lab 4 due this week
PCB lab
Much less conceptual—it’s about learning a tool
Entering full-time project mode.
Have midterm and 2 homework assignments before project due.
Everything else is project (and lecture).
HW1 posted by the end of the day. Due 10/18 (13 days from now)
I expect it will take ~4 hours
It’s a nice prep for the midterm.
Also practice midterms are posted.
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)

4. Restart on electrical issues
Talked about PCBs last time.
Finished on sizing traces for power/ground
Talk today about power/ground issues.
Not melting the traces (pretty easy)
Keeping the voltage constant (or constant enough)
Pretty hard.
That idea is called “power integrity”
We’ll need to review some very basic electronics along the way.
215…

5. Trace width for a given current
Understanding Power and Energy
Trace width for a given current
Example #1
Say you have 1 amp current in an ambient max temp of 75 degrees
Your wire has a thickness of ” (fairly standard)
So you need a widith of around 40 mils.
What is Example #2?

6. Background issue #1: Inductance
EECS 215/Physics 240 “review”
Background issue #1: Inductance
An inductor “resists the change in the flow of electrons”
The light bulb is a resistor. The wire in the coil has much lower resistance (it's just wire)
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.

7. Background issue #2: Capacitance
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.

8. Background issue #3: Impendence
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.

9. EECS 215/Physics 240 “review”
A look at impedance (with capacitors, inductors and resistors vs. frequency)
Notice the log scales!

10. 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”
*and maximum. Don’t forget this is really a range though we usually talk about the minimum

11. So? We need the Vcc/Ground differential to be fairly constant.
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.

12. Power Integrity
Lots of frequencies
Even fairly slow devices these days are capable of switching at very high frequencies.
Basically we get drivers that have rise and fall times capable of going 1GHz or so.
This means we generally have to worry about frequencies from DC all the way to 1GHz.
Because our chip may be varying its draw at rates up to that fast.

13. Non-ideal devices. ESR is Effective Series Resistance
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.

14. Other things can add to ESR/ESL
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

15. Power Distribution Network
Power Integrity
Power Distribution Network
Talked a lot about keeping the power supply voltage constant.
Should think of situation as follows:
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?
Input
PDN
Processor
Output

16. 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?

17. Given the previous table..
Power Integrity
Given the previous table..

18. Power Integrity
Removing the PCB…

19. 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!
artcatid=0&clmid=65&artid=85396&pg=3&_pf_=1

20. 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.

21. Connecting ground poorly
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.

22. Consider the following
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”?
0.1A
0.02Ω
10A
3.3V
12V
Top figure from “The Circuit Designer’s Companion”. If you are going to do PCB design much, buy and read this book.

23. 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!

24. 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 low-parasitic cap out of the power/ground plane.
Finally we should consider anti-resonance*.
* provides a very nice overview of the topic and how to address it.

25. 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…

26. Additional “reading”
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!
Nice spice models of real capacitors.
A much more academic treatment of ESR.
Mildly amusing and useful (who doesn’t like magic smoke?)

27. Up next Next week: Week after: Week after that Batteries
Linear Regulators
Week after:
Fall break (HW due Wednesday after)
Exam review
Week after that
Milestone meetings
Introduction to special purpose processors (mostly DSPs)