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15th November 2012, 08:55 AM |#15576  
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This is long INFO post from real chip designer that help to create CPU/GPU and other chips for the living for 14 years now, so respect

He sent me PM, for now he cant post that by him self.

Vikas is monitoring our thread and want to say his professional stand about UV/OV and why it's works for some and why not for others.

I am calling Vikas(vikas.mishra) to the speech stand

Hello people.

Let me introduce myself - my name is Vikas Mishra and I am a chip designer by profession. .
I have worked on critical parts of design of TI OMAP4, OMAP5, Nvidia Tegra 3 etc and have been doing this for the last 14 years.
Of late - I have seen a lot of folks posting BUGS about undervolting of the GPU/CPU.
I think I can explain what are the possible issues with undervolting/overclocking in a laymans language.

It is a little long winded but I think the length is needed for providing the appropriate context.

* What is inside your Cellphone

Your cellphone is an amazing device. It is a full fledged computer
that fits into your pocket. They have all the standard components
that a computer has - except that they are all usually soldered on
the motherboard directly and are not meant to be user-servicable.

The chief components inside your cellphone are
1. Application Processor (AP)- this is the heart of a modern
cellphone. These are manufactured by many companies - the main
ones are Qualcomm, Nvidia, Samsung and Apple. The other not so
well known ones are made by Texas Instruments, ST Ericsson,
Marvell and Broadcom.

A modern AP has logic to control the camera and process the image
that it generates, to do video encoding (video recording) and
video decoding (movie watching), Audio processor etc. in addition
to the well known CPU and GPU.

2. Power Management Controller - This is the chip that is
responsible for generating and regulating the voltages that are
used by all the components on the board.

3. DRAM - not very different from the DRAM found on a PC (except
that it is lower voltage)

4. Flash - for storage

5. Touchscreen controller

6. Logic for microphone, speaker

7. Battery

One of the most complex piece of circuitry on the phone is the AP
and the power management controller.

* Circuit Basics

A modern AP has millions of circuit units called (Flip
Flops). These flip flops have two parameters associated with them
called Setup time and Hold time. More details on what a flip flop
can be found on the wikipedia at . This is a
nice bit of bedside reading if you are interested.

A setup time roughly indicates what frequency you can run a design
or an AP at before it becomes unstable.

A hold time roughly indicates the maximum voltage till which a
design is stable.

A fully technical analysis of what is involved in these timing
parameters requires a degree in electrical engineering but in broad
terms the problem is described below.

Chip designers diligently ensure that all of the millions of the
flip flops in a chip meet the setup and hold time across a broad
range of voltages and silicon parameters. They do a pessimistic
analysis to ensure that a chip will run reliably across a wide
range of voltage/frequency combinations.

However, contrary to the popular belief, chips vary widely in their
silicon parameters. Even chips on a the same wafer and different
flip-flops within the same chip can have widely different silicon
parameters. This is why what works on one particular chip will not
work on the other chip.

Your silicon manufacturer provides a range of voltages and
frequencies across which the device can work reliably. The phone
manufacturer will further narrow down the range depending on the
other components they choose within the phone board.

* How does voltage affect the design

Reducing voltage makes the design slower and increasing voltage
makes the design faster.

So can I keep on increasing the voltage for ever and make the
circuit faster and faster. The answer is no - a point will come when
the circuit will become unreliable. This becomes unreliable because
the "hold time" of one or more of the flops will start

As you reduce the voltage of the design, the circuit will start
becoming slower. However typically it will continue to work till at
apoint it starts failing - this failure occurs due to violation of
"setup time" of one or more flops in the design.

So what happens when the setup time or the hold time of a design
fails - the answer is that it is unpredictable. Meaning suddenly if
you ask the processor what is the value of 2+2, the answer it will
provide could be unreliable - in some cases it could be 3, in some
cases it could be 4 in some cases it could be -2349783297 (a random number).

I am of course oversimplifying but I hope you get the picture.

* How does undervolting affect your phone processor

The reason undervolting is so appealing to people because they
thing that undervolting will save power and improve battery
life. While this is true in theory, in practice there is a caveat.

It will reduce the power of the chip, but the power consumed by the
phone as a whole will not improve. In some cases in fact it can
deteriorate. Let me explain.

The most power hungry part in the phone is not the AP - it is the
LCD screen. All of these screens consume a ton of power. So even
though your AP is now consuming lesser power, the overall impact to
the phone as a whole is not that much.

If you accompany undervolting with a frequency reduction (which you
should), the total time taken for doing a web page rendering (for
example) would increase. During this time the screen is on and it
has more than compensated for the power that you saved in the

You could of course come up with examples where this wouldn't
happen - but on a whole, IMHO, you should leave the voltage of the
AP/GPU/CPU to the guys who know the system best - the guys who
designed the chip and people who manufactured it.

* How does overvolting/overclocking affect your phone processor

If you want that last drop of performance from your phone and you
over clock it, at a point some of the design flops will start
violating the hold time and the design will stop working reliably.

Again, in some anecdotal cases this would work - but this is not a
reliable means/mode of working. Just because your friend's or your
first cousin's girlfriend's phone works - doesn't mean yours will
work as well.

* What are the user observable impacts of undervolting/overclocking?

It is hard to say - simply because there are so many of flops in
the design.

In some cases - you wouldn't see anything wrong with the phone
until one day you do. In some cases it will result in a SOD
immediately. In some cases it will result in your phone not waking
up reliably.

IMHO the risks of issues with undervolting/overclocking far
outweighthe potential gains you may get out of it. Usually there
is no lasting damage to the phone/AP if you overlock/undervolt but
it is possible to do it. For example, You run the phone at such a
high frequency that the chip temperature becomes more than what it
was designed for and the Silicon just fails.

So "Just say No" . Don't overclock or undervolt your phone -
leave it to the guys who really understand what they are doing.

Last edited by dorimanx; 15th November 2012 at 08:59 AM.
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