I like mazes and puzzles.  And I like algorithms.  When I was young, I discovered that most mazes could be solved almost trivially by applying a simple divide-and-conquer approach: solve the problem from both ends.  I would start with a pencil in each hand, one at the beginning and one at the end of the maze.  Then all you really have to do is draw until the lines cross.  I didn’t realize it at the time, but the effect was to halve the depth of the search tree – two trees of depth N/2 generally works out better (way better!) than one tree of depth N, especially when you can apply some high-level guidance and heuristics, like a kid staring at a maze.

As we all have been thinking in recent years about the problems of power dissipation in devices, chips, SoC’s and systems, I thought it would be interesting to start to look at the problem at the highest level – that of the end product.  Beyond the system level, above the system software, and even above the application software, the ultimate challenge is to be able to analyze and optimize power at the product level.

You must be thinking, “Is he nuts?  We’re not even close to the point of completely understanding power behavior through the system level, let alone interactions and dependencies through the software stack.”  So what is this all about?  Simple.  I’m just suggesting to tackle the problem from the finish of the maze.  I’d like to know, starting on the finished product end, how much power is consumed in particular modules, for particular operations. 

To get started, I’ve lined up my trusty set of iOS devices: iPad wifi, iPhone 4, iPhone 3GS, and iPhone 3G.  I’ve nixed my iPod Touch because it doesn’t have enough free memory to load our test vehicle.  As a measuring stick, I’m using a digital copy of the movie Star Trek (hey, if you’re gonna watch a movie 100 times or so, it might as well be a good one).  To develop a baseline, I fully charged all of the devices, and then played Star Trek as many times as I could until each device shut down.  This was done in a “max battery” configuration: airplane mode, with wifi, bluetooth, and location services all off, brightness and sound at minimum.  I suppose you could use this setup as a movie player in a very dark airplane (if you knew how to read lips)!  It might not be realistic, but the experiment yielded very interesting results.  All of the devices performed much better than I expected.  The iPad played Star Trek (2 hours, 6 minutes, 46 seconds) 7 and ¾ times before draining its battery completely!  That’s over 16 hours of video playback!  Similarly, the iPhone 4 had a Star Trek score of 4.75, and the 3GS around 4.1.  Both were over 8 hours of video playback.  The iPhone 3G only lasted through 1.75 Star Treks, but it’s also the oldest device (over 2 years), so its battery capacity is probably down by at least 30-40% (no I don’t know how many “charge cycles” it’s been through).

After all of those millions of recall petitions, email campaigns, blogs, and general nasty comments about battery life, is it possible that we’ve all got an 8+ hour portable movie player in our pockets and purses?  Yep.  So why is it that my original iPhone couldn’t last through the morning at work, with subsequent versions only marginally better?  What’s using all that juice?  And wouldn’t it be cool if we could tell during the design phase of the phone that this would be a problem?

We’ll answer these questions starting next time.  Let’s start walking back from the end of the maze.

By Cary Chin

07 July 2010

“Leakage Power – the big picture”

We’ve all been working on power engineering for a while now, and leakage power is always one of those generally-confusing topics.  Sub-threshold leakage, field-effect transistors, MOSFET weak-inversion regions – these are all things that cause even experienced engineers’ and computer scientists’ eyes to glaze over.  It all falls under the general topic of static power, or “standby power” in more macro terms.

We all know of standby power as the power your television and computer (or other appliances) consume when they are in standby mode, waiting to be turned on by you.  Generally, it’s energy that is consumed that doesn’t serve any useful purpose, other than standing by for input.  To stretch the definition, it might even include all of those blinking “12:00” clocks on our VCR’s over the years – thank goodness we finally got rid of those!

The recommended practice for wasting as little energy as possible on your home electronics is to unplug them, or switch off the power strip when they are not in use.  But did you know that it makes a difference which end you unplug?  Even with the latest EPA Energy Star guidelines, power adapters are limited to ½ watt under NO LOAD – that’s right, just the adapter plugged in without the appliance draws some energy.  I don’t know about you, but we must have at least 40-50 adapters plugged in 24/7 in our house – those things are going to add up.

In low power engineering terms, this is exactly the idea of “power gating” or “power shutdown” – unplug those transistors when they’re not needed, and you lower the leakage power to zero.  Of course, unplugging your computer and plugging it back in every time you need it is a little inconvenient, so in our real life example we must make the same tradeoffs that we make when engineering our advanced low power chips – how long will it be off versus how long it takes to start back up.

And finally, I heard this morning that with the current heat wave on the east coast, energy demand was soaring, and the power grid was strained, but holding its own.  I immediately thought of power rail analysis.  And when the news story added that some utility companies were lowering their output voltage to save energy, I could only think of three letters, “DVS”!  Maybe I’ve been at this for too long…

This blog originally posted on the Low Power Engineering Community 6/10/10.  http://chipdesignmag.com/lpd/absolute-power

If you’ve been reading this blog for a while you know that I’m kind of an Apple bigot – from the original Macintosh (remember that “1984” commercial?) through those clunky Mac clones, cool (and sometimes HOT!) laptops, Newtons, and now iPods, iPhones, and iPads, I’ve pretty much tried ’em all. So no surprise that I was carefully tuned into this week’s announcement of the iPhone 4. Here are my impressions from a low-power engineering standpoint.

Bigger battery: Hallelujah! From what I can tell looking at the somewhat-confusing details, battery capacity of the iPhone 4 is probably somewhere around 1,600 mAh, compared to around 1,200 mAh for the 3GS. Steve made a point to mention that new iPhone 4 case design allowed more room inside, specifically used to fit an enlarged battery. While this isn’t “low-power engineering” per se, increasing battery capacity is just as effective as decreasing energy consumption from the standpoint of usability, so this is a big plus.

New A4 chip: Another winner. Same chip as in the iPad, providing zippy performance with many advanced low power features. Check out my April blog for impressions on the iPad. With similar graphics resolution to an iPad, this phone (uh, I meant Mobile Internet Device) should have plenty of horsepower.

Higher-resolution display: On the downside (from the power standpoint), four times as many pixels will require more power to drive and update. It’s difficult to predict how much of an impact the new display will have, but based on the published info it appears that the new display hasn’t hurt battery life too much, at least from what we can see down at the bottom line. And the pictures looked great.

Multitasking: As much a feature of the phone as the OS, but the biggest question I have is regarding the need for multitasking on a phone. I understand the competitive pressures of Android and WebOS, and the “Hold on, I’ll find that presentation and e-mail it to you right now” commercial is pretty compelling, but on this kind of a device multitasking seems like too much power (literally) to put into the hands of hundreds of thousands of App developers out there from age 3 to 93, with an even wider variance in training and skill. The currency of a mobile device is power, and “using power wisely” is taking on a whole new meaning. It’s taken us this long to get up to 1,600 mAh, and I for one don’t want to use any of it on harvesting my virtual farm in the background or a runaway beer-drinking app!

Can’t wait to get my hands on one! See you all next month.

This blog originally posted on the Low Power Engineering Community 5/13/10.  http://chipdesignmag.com/lpd/absolute-power

We all think of mobile electronics today as smart phones, laptops, cameras, video equipment, gaming machines, and a whole host of devices designed to untether us from the dreaded power cord. Yet we don’t usually associate our most mobile and, recently, most electronic device in the same category. Let’s take a look…

Back in the year 2000, when I was working on processor design at Sun Microsystems, Scott McNealy was fond of pointing out that the value of the electronics in the cars back then had caught up with and overtaken the value of the steel in those same cars. “You’re driving a computer to work, not a car,” he quipped. While today’s cars don’t look any more like computers than their brethren of 2000 on the outside, we’ve added GPS navigation, satellite radio, integrated MP3 storage, voice recognition, Internet connectivity, WiFi, Bluetooth, heads-up display technology, automatic lane tracking, automatic parallel parking, solar charging, and a host of other features in addition to the electronics actually used to build the car.

And there’s more to come. There’s no doubt in my mind that a “few” years from now, “mass transit” will mean electronic highways that can accommodate a much higher density of cars with many fewer traffic accidents and injuries. Put those same cars in manual control mode, and you’ll be able to zip along a country road with the wind in your hair, adjusting the exhaust noise to your pleasure, from “hot rod” to “silent” (they’ll just be recordings). The only downside is that you’ll also very likely be getting an automated speeding ticket in your e-mail.

We’re not there yet, but at some point in the future the power usage of electronics built into cars might even exceed the power needed to drive the wheels. Wouldn’t it be funny to ask the car salesman of the future, “I like the super-deluxe 3D heads-up display with sense-around technology, but how will it affect my mileage?”

Things are definitely getting interesting.

This blog originally posted on the Low Power Engineering Community 4/08/10.  http://chipdesignmag.com/lpd/absolute-power

My iPad arrived as promised last Saturday morning at 11:00am. Woohoo! And it’s been ON for probably 80 of the 100 hours between then and now. While my parents (now in their mid-80s) think I’m nuts, my kids are having a great time! Here are my top five initial impressions.

1. It’s a more personal experience. I haven’t thought much before about how restricting it is to use a mouse and keyboard to interact with everything on a computer. My iPhone changed that somewhat, but on a device that small you lose more than you think by having to scroll around all the time. This thing is great for browsing the Internet, reading news and books, even watching a movie or listening to music, all from your favorite armchair or (I can’t wait for summer) lounging by the pool.
2. It’s a more social experience. I brought my fresh-out-of-the-box iPad to a family gathering on Saturday and it was the center of attention. The optimal placement for one of these things is NOT on a desk or on a bookshelf, but on a coffee table or a dining room table, where people can see and interact from all sides – like the old PacMan table-top arcade game! Suddenly a laptop, with its lid blocking 180 degrees of the view, and keyboard accessible from only one direction, seems mighty restrictive. And it’s now clear why those meetings at work seem even more boring these last few years – staring at all of those grey laptop lids is pretty claustrophobic, even if the people behind them are actually paying attention.
3. Size matters. Yeah, the iPad might be just a giant iPod Touch, but what’s wrong with that? In fact, applications like reading books and magazines, watching videos and movies, and browsing the Internet were compromises in the smart-phone form factor. Plus, everyone I’ve talked to over the age of 40 has said, “Whoa, I can actually see the screen!”
4. Saved by the virtual keyboard. I don’t know what percentage of people these days are trained touch-typists, but I’m not. I can keep up with most people in typing English, and consider myself “above average to superior” when typing in a typical computer program. (Remember how fast you used to be able to type “begin” and “end”?) My biggest problem has been that I need to look at the keyboard while I’m typing. I never realized how much of a disadvantage this is. I’ve gotten used to typing things like this blog very quickly, and then looking up to see if it all came out right. Now, with a large virtual keyboard, suddenly I’m looking at the keyboard and the output at the same time!! Ha! Take that, Mavis Beacon!!! It’ll take some practice, but I’m thinking this is a great combination for people like me.
5. More to come. One of the ways you know you’re onto something important is when you keep getting new ideas for extension, expansion, and innovation. After playing around with the iPad for a few days, I’ve put together a prototype sheet music library/displayer using pdf files, thought about kids no longer carrying around 30-pound backpacks, mused about the ultimate Internet-connected-digital-photo-frame app, and can even imagine an iPad (maybe connected to a TV or projector) along with four iPhones – as the ultimate gaming system! Watch out, Wii. Now’s a good time to start that iPod Touch wrist strap business…

On the downside: fingerprints. Ouch – this thing is a CSI field day! Along with those wrist straps, the “iScreenWipe” or “iGlove” or “iBottleOfHandSanitizer” would be a definite winner.

So how is all of this connected with our world of low-power engineering? What I haven’t complained about, or heard anyone else blog about since Saturday, is battery life. In fact, my iPad ran for more than 11 hours of movie watching, Internet surfing, multiplayer gaming, and app downloading on its first charge, and has been going strong ever since. Amazing. We’ll look at the numbers in a future blog.

The iPad has been a hit for everyone from ages 6 to 85 (I haven’t sampled outside that age range yet) – and I’ve already purchased a second one for the high end of that range. My parents saw our iPhone videos of our kids played on the iPad, and just had to have one. Grandkids’ videos and pictures – priceless!

This blog originally posted on the Low Power Engineering Community 3/11/10.  http://chipdesignmag.com/lpd/absolute-power

Driving in to work a few weeks ago in uncharacteristic fog, I noticed a streetlight was out. A little strange that there were no signs around, but I did the obligatory 4-way stop, and continued on my way. When I arrived in my office, my wife called, and said that the power was off in our neighborhood. Not a big deal, as it was only a little after eight in the morning, but I started poking around online at the office and through the magic of our modern communications infrastructure the picture began to emerge. First through Twitter, then blogs, then news stories: fog, plane crash, fire, transmission tower, you know the rest. Interestingly, my wife could have gotten all of this at home, as well, on her iPhone, but the networks were jammed with Palo Alto residents trying to get information.

I remember years ago when the power went out, the biggest concern was, “I hope the stuff in the freezer doesn’t thaw out.” Today, everyone’s first reaction is, “is the Internet still up?” We are addicted to getting our information in real-time, as it happens, from multiple sources – including live tweets and cell-phone-cam footage, as well as traditional media (who seem slow in comparison). And while that brings up some real questions about the future role of the media, our ability to communicate with each other and the world in real time is truly revolutionary. Mobile Internet devices are removing the last remaining tether preventing the information superhighway from taking flight: the power cord. Upcoming generations of devices will continue to improve functionality and power efficiency, and the sky’s the limit.

But all is not well in our low-power engineering community. The pilot of that small plane that crashed in the fog in Palo Alto three weeks ago was Doug Bourn, senior electrical engineer at Tesla Motors, who worked on the main power module of the Tesla Roadster. Prior to Tesla, Doug worked for many years at a small product design and development company called IDEO Product Development. And one of his close colleagues at IDEO was my wife. We are bummed.

This blog originally posted on the Low Power Engineering Community 2/11/10.  http://chipdesignmag.com/lpd/absolute-power

This blog originally posted on the Low Power Engineering Community 1/14/10.  http://chipdesignmag.com/lpd/absolute-power

Happy 2010 to everyone out there! To start the year out right, I thought it would be interesting to pull out a few “fun facts” about power and power consumption that we can chew on as we brave the new year.

For example, the average (resting) human body requires around 100W of power to keeping all systems running (heartbeat, cell division, a few brain waves, etc.). That seems a little high for “standby mode,” doesn’t it? But considering that we’re made up of between 10 trillion to 100 trillion cells, each consuming about 1 pW of power, it’s about the right ball park. Compare that with a 100 million-gate SoC today that might consume 10 to 100W of power, and we can see that we’re still perhaps 1,000,000x away from matching nature’s power efficiency, even ignoring the obvious mismatch in potential between a living cell and the proverbial “ASIC gate.”

On the higher-power front, a reasonably healthy human can generate about 1KW of peak output power during exercise. That 10:1 ratio between peak power and standby power isn’t very impressive compared to my smart phone, which has probably a couple of orders of magnitude better ability to really turn down the juice in standby, thanks to the magic of power shutdown. Imagine if we could do that with our bodies! Maybe an interesting insight into relaxation, meditation, and that old “Kung Fu” episode where Kwai Chang Caine gets stuck in the mine shaft…

And how about the REALLY big and powerful? An average car? 100 KW. A “supercar” (400+ hp)? 300 KW. An aircraft carrier? 200 MW. Nuclear reactor? 1 GW, which is only slightly less than the 1.21 GW required to power the famed DeLorean time machine in “Back to the Future.” And the estimated average power consumption of the human world this year? About 18 TW (1 terawatt is 10¹² watts).

So we run the gamut from picowatts (10^-12) to terawatts (10¹²). 24 orders of magnitude in power. What a year so far!

This blog originally posted on the Low Power Engineering Community 12/10/09.  http://chipdesignmag.com/lpd/absolute-power

Over the years, one of the most common questions I’ve gotten regarding low power is, “what’s more important, peak power, or average power?”  Technically speaking, they are important for different reasons – reliability, thermal concerns, and battery life, amongst others.  Power is simply the rate at which energy is transferred.  Average power seems intuitive to most people (it determines energy consumption and battery life), but the idea of peak power (especially “instantaneous” peak power) is somewhat more confusing, since the idea of “rate” doesn’t easily apply to an instantaneous situation.

One simple example might be to relate power (energy per unit time) to speed (distance per unit time).  If it takes me a total of six hours to drive between San Francisco and Los Angeles (say 360 miles), then my “average” speed comes out to be 60 mph.  However, that doesn’t mean that I drove any significant part of the trip at 60 mph.  I might have been driving 90 mph on I-5, 35 mph through the city, and 5 mph sitting in traffic near LA.  The CHP cares about “peak” speed, and if I were to approach the maximum 155 mph speed of my car, it would start to complain as well, but none of these necessarily has any appreciable impact on my 6-hour, 60 mph journey.

So a cell phone that draws 1 watt of power on average will run for 5 hours on a 5 watt-hour battery.  But actual power consumption could easily be 3 watts while viewing streaming video over the internet, 0.5 watts while talking on the phone, or just a few tens of milliwatts in standby mode.  To truly optimize for energy efficiency, we need to analyze the total energy consumed in each of these modes, including the time spent in each mode.

Oh, yeah – and my best real example to date of how “peak” and “average” are really independent: driving home from LA on the Sunday after Thanksgiving in a giant traffic jam on I-5 – “peak” speed 97 mph, total trip time 10.5 hours – you do the math!

This blog originally posted on the Low Power Engineering Community 11/12/09.  http://chipdesignmag.com/lpd/absolute-power

As we’ve all pretty much made the jump into “low power design” in the electronics industry over the last few years, I’d like to think a bit about where we’ve been and where we’re headed.  When we started this journey with clock gating and then multi-threshold design, the predominant goal for low power design was to achieve reasonable battery life for then-emerging battery powered devices like laptops, cellular phones, and early digital cameras. 

Today, the much ballyhooed “convergence” has happened – my smart phone (the one with the “GS” at the end) is a reasonable substitute for 90% of my telephone needs, 80% of my GPS needs, 80% of my electronic communications needs, 50% of my photography needs, 50% of my computing needs (including gaming), and maybe 20% of my video needs.   A single device that does all this, that I can carry in my pocket, and lasts though the day (well, sort-of) without a recharge?  Incredible!  And even more incredibly, my “vacation travel pack” of electronics, which just a couple of years ago looked like: phone, iPod, DS, Palm organizer, camera, video camera, laptop, games, DVD’s, tapes, batteries, and many, many chargers, has in many cases turned into: just my phone!  Nice.

Today’s mantra for low power is evolving into our larger call for efficiency – from saving the planet to saving the country to saving space in my pocket.  Power considerations in electronic design today are just as likely to be for efficiency in server farms and network infrastructure in the “cloud” as for the continuous need for extending battery life on our precious mobile devices.  The global green movement is starting to change the old American adage from “more is better” to “enough is enough.”   Our push for convergence is a big part of the solution – just the landfill savings in old batteries and chargers must be enormous!

My only complaint is that my wallet still takes the same amount of space in my pocket that it always has – and it’s the last thing I carry in addition to my phone.  To all you app developers out there – it’s time we finally made wallets obsolete!  Is there an app for that??