The iPad 2,4 Review: 32nm Brings Better Battery Lifeby Anand Lal Shimpi on May 4, 2012 12:50 AM EST
When Apple launched the 3rd generation iPad (as the new iPad), it also dropped the price of the entry-level 16GB WiFi iPad 2 to $399. Apple's products tend to hold their values exceptionally well, so this two-tablet strategy made sense. Apple also proved the success of discount-the-previous-gen strategy with its iPhone line, where you can now buy current, n-1 and n-2 generations of iPhones at prices separated by $100.
What's different with the $399 iPad 2 is that Apple used it as a vehicle to introduce a new hardware platform, or more specifically, a new SoC.
The 32nm HK+MG Apple A5 SoC
Prior to the new iPad announcement there were three versions of the iPad 2:
|Apple iPad 2 Lineup|
|iPad 2,1||iPad 2,2||iPad 2,3||iPad 2,4|
|A5 SoC||45nm LP||45nm LP||45nm LP||32nm LP|
|Connectivity||WiFi||WiFi + GSM||WiFi + CDMA||WiFi|
The 2,1 was WiFi-only, the 2,2 was GSM and the 2,3 was CDMA. The new addition to the family is the iPad 2,4. The 2,4 replaces the original iPad 2,1. It's also only available in a single capacity.
There's no known way to tell whether you're getting an iPad 2,4 vs. the older iPad 2,1 without opening the box. The 2,4 unit I ended up with was made in China, ruling out manufacturing region as a way of telling. The external box looks identical, as does the device itself.
The newer iPad 2,4 units should come with iOS 5.1 preloaded, while any older iPad 2,1 stock may have 5.0.1 or older. But the most accurate way to tell is by looking at what a utility like Geekbench will tell you about the hardware (Update: there are other free utilities that can serve the same purpose, Linpack and Battery Life Pro are two examples):
This particular iPad 2,4 sample came from Best Buy, and several attempts to find one elsewhere came up short. All indications seem to point to the iPad 2,4 being relatively rare, which makes sense considering what's inside it.
Although the iPad 2,1 and its 3G brethren all used a 45nm Apple A5 SoC, the iPad 2,4 uses a die-shrunk 32nm version. The performance remains the same, but the die is much smaller. This isn't however just a normal die shrink, as Apple is using Samsung's 32nm high-k + metal gate LP transistors for this new A5 die. Intel was first to make the HK+MG transition back at 45nm in 2007 and correctly predicted that no one else would make the move until 32nm at the earliest.
Transistors are amazingly complex to fully understand, but at a high level they're quite simple. Imagine a transistor as a silicon based switch. When on, current flows, and when off, current stops flowing. The smaller you make a transistor, the more likely it is to misbehave. If current flows while the transistor is off, you waste power. This is known as leakage current and can come from a number of sources.
One such source is the gate oxide/gate dielectric, a particularly thin part of modern day transistors - on the order of a handful of atoms thick. Thinning the gate dielectric is desirable up to a certain point, after which the dielectric simply leaks too much power. Switching to a different material here, specifically one with a higher dielectric constant (a higher k-value), can significantly reduce leakage current and mitigate this issue. This is exactly what the first part of Samsung's 32nm high-k + metal gate process does.
The second half of the new process is the introduction of a metal gate electrode. Switching from a polysilicon to a metal gate electrode results in higher drive current by elimination of a region of depleted conducting carriers between the gate electrode and gate dielectric.
The combination of these two innovations results in less wasted current and more efficient current delivery, which in turn can give us a more power efficient chip. It's a net win. It makes manufacturing more complex, and there's definitely a learning curve to implementing it, but after you get over that hurdle it becomes just another part of the process.
The More Cost Effective Die
Traditionally the move to a smaller process node brings about an increase in transistor density. As transistors get smaller, you can fit more of them into the same space (or the same number into a smaller space). It's this basic principle that makes Moore's Law work. If you can keep shrinking transistor size by about 50% every two years, you'll theoretically be able to double transistor count at the same cost every two years (or cut cost in half every two years). In practice it doesn't work this well. Newer processes are always more expensive than their predecessors initially and logic scaling is never perfect.
It's rare these days that we actually see a pure die shrink anymore. With Intel's tick-tock model we almost always see increases in functionality to accompany each process node shift. In the case of Ivy Bridge, we actually saw a significant increase in transistor count thanks to an improved GPU. With Apple's 32nm A5 however, we truly end up with a die shrunk version of the 45nm A5 SoC. About the only part of the computing world where we see these pure shrinks is in the console space where performance doesn't have to go up within a generation, but cost must go down.
45nm A5 (left) vs. 32nm A5 (right) - Source: Chipworks
The original 45nm A5's die measured approximately 122mm^2. The new 32nm A5 has a surface area of only 69mm^2. That's actually amazingly good scaling at 57% of the old die size, as perfect scaling from 45nm to 32nm would be around 50.5%.
Assuming Apple could make full use of a 300mm wafer (which it can't, wafers are round, chips are rectangular at best so there are some unusable chips), Samsung could deliver 579 45nm A5 die to Apple. The move to 32nm would give Apple 75% more die per wafer at 1015 chips. Again both of these numbers are over estimates as they assume full usage of the surface area of a wafer as well as 100% yields, but you can see the benefit of a smaller die. As long as wafer costs increase by a factor less than the 75% increase in number of die per wafer, Apple can effectively reduce SoC cost by going this route.
These ARM based SoCs are already fairly cheap - all selling well below $30 (many around $15) - so there's not a whole lot of cost savings here. On a product like the $399 iPad 2, where Apple needs to do its best to maintain margins while holding onto (and growing) market share, every last dollar matters.
Gate density vs. process node at Samsung
There's another motivation for Apple however. Just as with any good microprocessor company, its best to introduce a new process technology on a known architecture. It's also a good idea to introduce a new process technology on lower volume products. The combination of both of these minimize risk. Should there be something wrong with the new process, introducing a new architecture on it just means you now have two very complex things to debug - the process technology and the chip's architecture. Should the new process not yield very well initially, you'd be similarly screwed if you were depending on it for your highest volume parts.
32nm A5 in iPad 2,4 (Source: Chipworks)
Apple decided to try out Samsung's 32nm HK+MG process on the A5 used in the 3rd generation Apple TV and some of the new iPad 2s. The former is a relatively low volume product for Apple, while the latter still moves in significant quantities. To deal with that fact, Apple is continuing to ship the original 45nm iPad 2,1 alongside the new 32nm iPad 2,4. Any hiccups in Samsung's production of the A5 and there are still more than enough iPad 2,1s to go around. The risk of moving to 32nm is effectively mitigated, while the learnings Apple gains from building the 32nm A5 will pay off later this year as Apple ramps up production of a 32nm SoC for use in the next iPhone. It's a very smart strategy, one you would expect from an experienced chip company - not a device vendor. When you consider that Apple employs chip architects who have worked on everything from the Athlon 64 to the Cortex A15, Apple's behavior is no longer that surprising.
Apple gets two benefits from the iPad 2,4: lower manufacturing costs, and experience with Samsung's 32nm HK+MG process which it will later use in much greater volumes. What about customers who end up with an iPad 2,4? Better battery life and cooler operation, of course.
Impact of HK+MG at Samsung
Remember the basics of Samsung's 32nm HK+MG process: a 40% performance improvement at the same leakage, or a 10x reduction in leakage at the same switching speed. As the iPad 2,4 retains the same clocks as the initial iPad 2, the benefit realized is a significant reduction in leakage current. This translates to tangibly better battery life.
Post Your CommentPlease log in or sign up to comment.
View All Comments
Tuvok86 - Friday, May 4, 2012 - linkdo you think Apple will introduce the 32nm soc on iPad 3 silently or they will wait for the next iteration?
guidryp - Friday, May 4, 2012 - link"Note that idle power is actually higher than video playback simply because there's more that's being driven on the display, at a higher intensity on the iOS home screen than on a dark 16:9 video."
That really doesn't make sense. This is LCD, not OLED. Displaying black/white should take about the same amount of power, because the backlight is uniform and covers the whole display. It doesn't turn off on dark areas of the screen.
Perhaps Apple cheats and turns down the brightness while a movie plays. Create a movie that has a pure white screen and measure nits...
André - Friday, May 4, 2012 - linkNo need to call shenanigans. You simply failed to read what was written.
It isn't the screen that uses less power but the SoC because it has dedicated hardware for video playback.
guidryp - Friday, May 4, 2012 - linkHe is clearly talking about screen content, when he mentions "dark 16:9 video" which implies that power changes drastically dependent on what is displayed.
This is true for OLED screens, but untrue for LCD screens.
Also while dedicated Video decode HW offloads work from CPU/GPU, they essentially have no work to offload while idle. Really great dedicated Video decode HW could get you to idle consumption, but I don't see how it could get you below idle.
Something very strange here.
PeteH - Friday, May 4, 2012 - linkWhat about bandwidth? 8-bit 4:2:0 video is a bit less than 40% of the size of 8-bit ARGB (for equivalent source dimensions).
Anand Lal Shimpi - Friday, May 4, 2012 - linkThere could be a handful of things happening here:
1) It is possible to have a small reduction in LCD power consumption by displaying black vs. white. The gap isn't anywhere near as dramatic as it can be on an OLED display, but it can be measurable. This alone can't explain the difference, I agree.
2) It's possible that Apple is putting the SoC into an even lower power state when a video playback scenario is triggered. Apple does quite a bit of aggressive power management so this wouldn't be too far fetched.
3) It's not clear to me if Apple is doing any localized dimming, which would have a significant impact on video decode power consumption.
guidryp - Friday, May 4, 2012 - linkI think the back-light as the largest power draw (especially in idle), is the easiest source of savings. Displaymate measured 7watts at max for the back-light alone on iPad 3 !
Easy reductions either from a simple general brightness reduction when a movie is playing, or dynamic backlight control while a movie is playing. Done right it gets some savings while being hard to notice.
I really don't think you can get a much lower power state while playing video, than while just displaying a start screen. In either case the CPU is still running, still instantly ready for touch, or sensor reactions.
However they did it, it is a slick piece of engineering.
Pshooter - Friday, May 4, 2012 - linkhttp://www.displaymate.com/iPad_ShootOut_1.htm
BSMonitor - Friday, May 4, 2012 - link"This particular iPad 2,4 sample came from Best Buy, and several attempts to find one elsewhere came up short. All indications seem to point to the iPad 2,4 being relatively rare, which makes sense considering what's inside it."
Was one of the other attempts Apple's own website??
Anand Lal Shimpi - Friday, May 4, 2012 - linkThe other tries were at an Apple store (3x) and a Walmart (1x).