Resource: LCE12 Name: How to measure SoC power Date: 01-11-2012 Speaker: Andy Green

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How to measure SoC power
Andy Green, TI Landing Team lead, Linaro

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Introduction
I lead Linaro TI Landing Team at Linaro
Mainly been working on hardware the last 25 years
TI very interested in power optimization
SoCs have a lot of schemes needing software support
So there's a lot of things we work with that might, or should
impact system power
We need to observe and measure those expected impacts
Many people working in different areas can benefit from
looking at exact detail of where power is going in the
system...

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Sections
1: Minimum Electronics Primer
2: Measuring voltage and current
3: Arm Energy Probe hardware
4: Practical Board instrumentation
5: Error Sources
6: Commandline Linux AEP app
7: aepd and HTML5 UI
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}Hardware
Software

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Section 1: Minimum Electronics primer

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[1/7] What is voltage?
Voltage is like the “pressure” electrons have
to go somewhere
They might not be going anywhere at any
particular time, but the pressure is still there
In a battery, electrons are chemically
separated out to the – side and prevented
from leaking on to the + side through the
inside
So there's “pressure” built up for electrons
on the – side to want to go to the + side

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[2/7] What is voltage?
Voltage is measured in Volts / V
In SoCs we usually deal with 0.9 – 5V
range
Typical transistors want to switch around 700mV
USB = 5V
SoC IO = 1.8V
LED = 1.2V
DDR = 1.29V
ARM Vcore = 0.9 – 1.6V (DVFS)
L-ion battery cell = 3.7V
We can measure voltage using a
voltmeter of some kind

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[3/7] What is load?
Load is the thing we put between a voltage to make current
flow and get work done. Your dev board is a “load”.
It has a resistance, measured in Ohms / R

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[4/7] What is current?
Current is a measure of electrons flowing
through the circuit
It's measured in Ampere / A
Sometimes the symbol I is used to talk about current
How much current flows is a function of
voltage and load resistance (I = V / R)
A “short circuit” is ~0R load, max current flows
Examples
USB2 device Max current = 500mA, USB3 = 900mA
Typical max 5V adapter current = 2A
LED current = 1 – 10mA
L-ion battery cell = 2.5A (cf Capacity 2.5A/h)

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[5/7] So...what is power?
Power = current x voltage... P=IV
Voltage or current tends to increase or decrease == power
is increasing or decreasing accordingly
P=IV == V2
/R so V / 2 --> P / 4
This is why DVFS is so interesting...
Measured in Watts / W
Examples:
USB device max power 5V x 500mA = 2.5W
Typical AC adapter: 5V x 2A = 10W
LED: 1.2V x 1 – 10mA = 1.2 – 12mW
L-ion battery cell: 3.7V x 2.5A = 9.25W
Cf Capacity 9.25W/h

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[6/7] Why talk about power and not current?
There are usually many different power domains in a
system, each supplied by its own “power rail” at different
voltages
By converting voltage & current measurements to power,
you can compare power from different voltages or parts of
the system
For that reason, although V and A may be interesting all
power reporting should be done in W, so the numbers
are comparable
In the end you always want to compare individual rail power
to overall system power, which is definitely at a different
voltage... Watts only!

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[7/7] Summary
Voltage (V) is “pressure” for electricty to flow
Current (A) measures the flow of electricity
Load (R) is what the current flows through
Power (W) is the Voltage x the Current, or pressure x flow
We need to measure rail voltage and current, to end up
with a power figure in W

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Section 2: Measuring voltage and current

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Common-mode voltage measurement
Voltage is relative between two points
(it's also known as “potential
difference”)
One side of the input voltage is
decreed the “0V” reference to which all
the other system voltages are
compared (it's the – side of the DC
input by convention).
Common-mode voltage is the voltage
difference between your rail and “0V”

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Measuring current with an Ammeter
An Ammeter (current meter) can be
implemented as a sensitive voltmeter
measuring the difference in voltage
across an internal series shunt resistor
You interrupt the circuit to add the meter
in series, so the current passes through
the meter
This is how your multimeter works in A
range

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Measuring current with a voltmeter
Actually you could just add the shunt
resistor yourself and use a sensitive
voltmeter
That has advantages that the high
current path does not go via the meter
cables any more, just straight through
the shunt resistor
When you're not measuring, the shunt
resistor stays there and things work as
normal

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So to measure power in a rail
You have to break or interrupt the rail
Often there's a convenient way to do that without cutting
Insert a shunt resistor in series
Wire up one voltmeter to measure rail voltage vs 0V
Wire up another voltmeter to measure the voltage across the
shunt to find the current (knowing the shunt resistance,
I=V/R)
Multiply the common-mode voltage and current together to
get power

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Shunt Resistance Selection
The main criteria is voltmeter sensitivity
For AEP, its sensitive voltmeter for shunt
measurements tops out at 165mV, so that
is the voltage drop over the shunt we want
for worst case current
For larger currents, we also have to take
care that the shunt can cope with the power
it dissipates as heat
Next is the shunt selection chart for 165mV
drop at various maximum currents

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Shunt power dissipation selection for 165mV
Eg, 1.5A max --> 100mR shunt, 1/4W rated

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Rule of thumb for shunts
For AEP usage (165mV shunt range)
Many Watts like DC jack, or high power rails
0.22R or 0.1R, 1W or more
For normal rails
0.47R 0.25W - 0.5W

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Choosing lower value shunt than necessary
For some rails, like CPU DVFS rails, shunt loss at full scale
of voltmeter may be too much to lose
Eg, 165mV of 900mV rail is 18% loss
You can select a shunt that drops less than full scale at the
highest expected current
Eg, only drop 50mV of 900mV rail worst case
It works, but because you only use part of the range the
measurement ADC step size is larger, noise goes up and
precision goes down
Trick... if you didn't plan on lower value shunt needed, you
can double-up two or more in parallel to get R/n

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Unipolar vs Bipolar voltmeters
Unipolar or “1-quadrant” can just measure positive voltage
or current
Bipolar or “4-quadrant” can measure +/- common-mode
voltage and +/- current
+/- current very useful for battery, +/- voltage less so
Bipolar may have problems determining what “zero” looks
like
Unipolar should do better with “zero” but doesn't always

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How regulators work (or not)
Regulators operate in a loop
They allow more or less input to the output according to what
it senses at the output
They have an internal or external “sense” pin which lets them
watch the output and then act to make sure it stays at the right
voltage

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Difficulties with regulation
Wiring has “inductance” which affects the signal or power as
the load changes
Additional inductance of meter and wiring mean the regulator
cannot sense load changes properly
SoC will crash

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Modern Regulation strategies help
For high power, dynamic loads, SoCs provides a separate
“sense” pin for the regulator
connected right at the load, on the die
Regulator does what's needed to keep that regulated

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Input-side measurements
The extra impedence is one problem
Another is the voltage drop from the shunt, 150mV is not
much but on a 900mV DVFS rail, it will stop the SoC
working
Both problems can be avoided by measuring the input side
of the regulator instead
You don't see output voltage then though
Your power measurement include regulator losses

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Regulator efficiency
Switching regulators are designed for
peak efficiency (up to 95%) at a
particular load
For different loads, efficiency may decline
below 50%
Linear regulators are simpler but can
be grossly inefficient
Current at input side same as the output side
If input side voltage is much higher than
output, large power losses as heat result
Input – output delta is called “dropout voltage”
Only useful at low dropout voltage and low
currents
Used in sensitive analogue circuits though,
since they do not introduce switching noise

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Measurement bandwidth
How many samples per second do you need to analyze
current used by an SoC regulator?
Modern regulator switching frequency tends towards a few
MHz, if you measure input of regulator this is its “clock
frequency”
Higher switching frequency allows use of smaller inductors
Load only visible at input for part of switching cycle --> load is
typically modulated at a few MHz. So sample rate > ~5Msps not
useful
Spread spectrum and duty-cycle modulation also in use
Smoothing and reservoir capacitors hide short load bursts,
but all output load must appear at input, perhaps delayed
You can see plenty at 10ksps

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Power tree
The DC input jack and each regulator supplies a “rail”
Some designs still use a metal “rail” or “bus bar” to distribute the
power around the circuit, hence the name
One or more device connects to the rail to consume power
from it
Sometimes it is other regulators to produce a new rail supplied by the
parent one, ie, a power tree

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Power tree vs regulator efficiency
Ideally you add shunts and measure power on all rails
If rails go to several interesting places, you would want to
add individual shunts for power to each device
You can also measure the input side of the regulator that
supplies the rails
If you have power measurements on both sides of the regulator, you
can easily see its efficiency

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Four measurement strategies
Here are four common measurement strategies
DC IN
Indirect output-side regulator measurements
Shunts on all SoC rails
Shunts on all board assets

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1/4: DC input
Easiest rail to instrument
Doesn't need changes to board itself
Ultimately everything that consumed power got it from
the power input, by one route or another
Every activity on the board consumes power
But it can be hard to interpret “why” or where the power
was used if all you can see is one total number
If multiple activities are ongoing, you can't tell between them
But it's an easy and quick way to start
You can use it for “delta” comparison, same setup
before and after a change you want to test
There's uncertainty any changes are down to your delta
Take note of the error sources section later

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2/4: Indirect output-side measurements
Some SoC rails under DVFS control are interesting, but it
might not be possible to instrument them directly
Instead you can instrument the input side of their regulator
as discussed, and find the current (including regulator
overhead)
You can use a second channel then to monitor just the
common-mode voltage on the output side (not current)
In that way you can infer output-side current and voltage
information without changing the output side

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3/4: shunts on all SoC rails (input side)
SoC vendor primarily interested in SoC power consumption
Insight into exact path for extra power consumption (eg
DDR)

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4/4: shunts on all board assets
Most ideal scenario, shunts on all power consumption in
exact detail, using many channels
Samsung Origen breaks power tree into 14 shunts... perfect
Individual shunts for GPS unit, for example
This lets you answer questions like, “exactly how much
power is consumed by xxx?”, where XXX might be
GPS in standby
3G module when connected to tower but not in use
WLAN module over various power modes
HDMI PHY in use and standby
USB PHY, USB 5V generator etc etc
The more rails you have numbers for, the less uncertainty in
the remaining unknown power

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Pandaboard ES with 9 rails instrumented

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Section 3: Arm Energy Probe hardware

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Scaleable USB connectivity
3 channels per probe
As many probes as you need
Appear as ttyACM serial ports automatically
No built-in serial number or other way to tell them apart... care needed to make
sure the right probe maps to the right ttyACM number
Has a known serial protocol

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Arm Energy Probe architecture
Three channels of 2 voltmeters each
0 – 30V range
Common-mode voltage
0 – 165mV range
Amplified (x20) to increase sensitivity
cross-shunt voltage measurement
To turn into current value, need to know Rshunt

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Section 4: Practical board instrumentation

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Steps to instrument a board
Study schematic to find where to place shunts
Estimate correct shunt size and resistance
Place one 7-pin header per AEP
Wire up ground pins
Place shunts
Wire up shunts to header pins

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Reading Schematics
Schematics are drawings of how the components of the
electronic design are connected
Boxy things might be a chip, or part of a chip
Boxy things have a name, like U5 (IC 5) or R7 (Resistor 7)
On the PCB, the device will have the matching name nearby
Lines are connections, made into copper lines on the PCB
Lines can be named, wherever the same name is seen will
be connected together on the PCB
If you provide the board schematic to an EE, he can tell you
where it's possible to place your shunts

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Input-side inductors
Usually possible to replace these with shunt without cutting
Used to attenuate regulator RF switching emissions leaking
on to power cable, not critical for operation

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AEP connector
7-pin: 3 x 2 sense leads, 1 x 0V reference lead
Orientation of sense leads on shunt matters
Doesn't break if it's wrong, but numbers will be wrong (near 0)
White lead needs to go on “before” side of shunt

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Adding shunt to DC jack externally

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Adding shunt to DC jack internally
Usually your DC jack exposes the + power
You can cut it and bridge the cut with your shunt

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Preparing your board for measurements
Use a 7-way 0.1” header for each probe (3 channels)

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Fix headers to your board
Superglue the back to a spare region of the board

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Wire headers to your shunts and 0V
Add 0V connection
Add twisted pairs back to the shunt

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Adding SMT shunts
Remove old inductor, solder the shunt in place
Add the other end of the wires to the shunt

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Adding high power shunt on high current path
Same deal just bigger resistor

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Section 4: Error sources

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Error sources
All measured numbers are just bullshit until:
You know what they are supposed to measure / mean
You can repeat the meaurements
Different day / test channel / board / temperature
They are sane
If you measure total system power and one rail measurement is higher,
something is broken
If you know zero power is in use, does it say 0.000W?
You have a rough error budget for how the numbers were arrived at
Even when the numbers have a pedigree, +/- 10 - 20% vs absolute accuracy
is a reasonable assumption
If you have access to lab-calibrated, absolutely accurate equipment to
compare them with, you can trust them more

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Concept of repeatability
Numbers that cannot be repeated under similar test
conditions contain error
Need to understand why and how much, can still be usable
Ultimately if it's not repeatable, it's not measuring what you
thought it was

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Zero offset
Bits of the circuit, the shunt, the measuring device drift with
age and temperature
Here's what happened to “0mA” when I turned the air
conditioning on, over 15 minutes

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Effect of temperature on measuring device
Shunt resistance is sensitive to temperature
Take care about self-heating at high currents too
Different resistor chemistries act different, SMT ones seem better
Adc
Shunt voltage amplifier
Resistor reaction to temperature
Ntc / ptc networks

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Measurement bandwidth
The rate of sampling can also impact results
At 10Ksps, events significantly faster than 100uS may be
missed or under-reported
If you're unlucky load changes at a rate that is a multiple of
the sample rate can lead to aliasing
When measuring at the input side of the regulator, these
load changes are themselves sampled by the switching
action of the regulator
Generally even fast load changes are averaged fairly well

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Linearity of ADC + shunt amp
Supposed to ignore common-mode voltage
Measure the same 2mA for 2mA at 1V, as 2mA at 5V
But this is “0mA” measured at different common-mode
voltages...

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Silicon power consumption over temperature
Silicon switching efficiency is a function of temperature
23% (1.9W – 2.34W) increase in consumption for same
cpuburn just from die temperature rise from 30°C – 60°C
Repeatability...

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Differences between channels
Uncorrected (except zero offset) measurement of same
3.8mV shunt voltage (red line) using 12 channels in turn
Huge variation of reported numbers by channel

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Channel correlation correction effectiveness
After software correction (blue X)
Error less than -13%/+9% at 3.8mV

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AEP-specific problems with low voltage range
I mapped out the measured vs real results for different
common-mode voltages and currents in “2D”
These correction tables are applied in the GPL software

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Error budget estimation
Actual resistance of shunt is not what is marked on the
resistor due to manufacturing process spread
Typ +/- 5%, +/- 1% available
...and it varies by temperature...
Typical SMT (Yaego RL0805) 470mR changes by +300ppm / °C
10°C change is +1.4mR (0.3%)
... so conversion of shunt voltage to current has ~+/- 6%
(2% for 1% resistors) uncertainty
Channel current differences are around +/- 10% (at 10mA)
even after correction
Unknown ADC offset and linearity errors
In AEP case, numbers below a few mA are almost certainly
wrong due to linearity problems...

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Section 5: Commandline Linux AEP app

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Arm-probe architecture

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Architecture
Synchronizes sampling across multiple energy probes
Has a text config file defining information about each probe
channel (Rshunt etc)
Outputs ascii numbers in column format
Appends columns to stdin if present
Gnuplot-friendly output
Autozero support
Trigger support to synchronize capture to interesting events
Probe sample capture happens in forked process decoupled
from sample consumer code

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Section 6: aepd and HTML5 UI

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aepd and HTML5 UI

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Aepd architecture

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Websockets advantages
Realtime, immediate graphical update from aepd link
600px, up to 24fps == see all 10Ksps samples
“power scope”
Local or remote network link for browser

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Generic sample interface
Allows integration of alternate measurement hardware
Floating-point voltage and current
Posix semaphore and shared memory sample buffer
architecture
Arm Energy Probe support provided

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All samples captured
All samples of all channels captured to a spool file
120s buffer by default
Lot of storage involved
Browser views individually view the spool buffer at different
timescales and positions in time
Different from oscilloscope etc with only short display buffers
Doesn't matter if you “miss” an event, you can just scroll
back

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Averaging scheme in the storage ringbuffer
Spool buffer is arranged so it's possible to get an
instantaneous average for any range of samples
Every sample always contributes to the average
Allows immediate changes to “time zoom” in the UI

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Smart comparing of power tree
Understands parent / child (supply / consumer rail)
relationship of monitored rails
By default, “top parent” supply shown “in the background”,
and the child rails on top of each other in the foreground
At high zoom, you can see noise on top of noise...
This lets you see any delta between the known total power
and the known child supply power
Stuff you are not capturing
Regulator efficiency loss
Can also see individual rails by clicking on the key
This is useful to see minimal noise on the rail

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Calipers
At the top of the canvas are two slideable “calipers”
What's between these calipers is averaged and reported in
the stats table at the right
Updated at 10Hz
Accurate time measurement of power events
Accurate averages across selected power events

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“Search”
(This is not finished yet...)
Different paradigm than oscilloscope, since operates on deep,
complete buffer post-capture
“fades out” different parts of dataset based on power level
search criteria
Determine duty cycle of matching / non-matching regions
Get power averages just for matching regions

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Uses of deep buffer 100% capture
True analysis of boot timing from first power to various
activities in boot, in addition to power involved
Can't be replicated by “from inside” measurements

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All software GPL'd and available today
git://git.linaro.org/tools/arm-probe.git
Need AEP
Instrument your board with shunts
Linux box to host AEPs on USB and aepd
Google Chrome (best HTML5 implementation)
Works on Android Chrome as well
Expected to be useful as basis for vendor in-house power
monitoring tools in future
GPL project...

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