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Power in USB 2.0 Devices

Posted: 16 Oct 2008 ?? ?Print Version ?Bookmark and Share

Keywords:USB 2.0? power? battery?

By Eric Huang
To USB or Not to USB

More devices are doing more things. The best example of devices that have more functions are phones.

Anyone shopping for a mobile phone, will see that the newest phones have more and more functions. Since the introduction of the iPhone, every company is introducing larger screens and touchscreens. The larger screens require more power to operate. More software to run more applications. More software to manage all the applications. More software to manage the power for the applications. More software to manage when the WiFi and Bluetooth are on-and-off. A touch screen that is polling all the time for data from the touch screen. And, in the case of the iPhone, an accelerometer that senses motion (like turning the phone to view pictures in the correct perspective. For a camera phone, an image capture device and maybe even a flash.

All these feature require power. Product makers must manage battery life or be doomed to 30 minutes of talk time. So the choices for managing are:

  1. Design the chip hardware to consume less power

  2. Design the software to manage power usage for each application

  3. Make the battery bigger to provide more juice

  4. Accept shorter battery life (hey, you don't get something for nothin')

Bigger batteries
The iPhone is larger both because of the screen, and probably to house a larger battery. Battery life is always relative to some other device. For example, my Blackberry would probably run for 2 weeks as a phone, but once I add data, it's probably 1 week, and with Bluetooth, maybe 6-7 hours of talk time, and it has to be recharged.

For change the hardware, we have done some studies with interesting results.

Some research by our own Mike Keating indicates that for some companies aggressively implementing hardware features to reduce power are nearing their limits. Phone companies are probably in the lead here. Keating and David Flynn have published our Low Power Methodology Manual (LPMM) which you can download for free here: http://www.synopsys.com/partners/arm/lpmm/lpmm.html.

If you really implement these features, you can squeeze out some extra battery life.

For me, the most interesting thing is that a lot of companies still do not employ these methods. These are consumer devices. There is still room on the hardware side by using our Low Power Methodology Manual. This is absolutely clear to me, however, you must implement multiple power domains, multiple power rails, and MTCMOS, among other things. Because of the time and effort required, many companies do not even attempt this.

To me, this means 2 things:

  1. A) Implement simpler hardware options for reducing power

  2. B) Implement our Low Power Methodology.

What does this have to do with USB?

For A) the fastest, easiest hardware solution is to use HSIC to implement add-on USB functions. HSIC uses a PHY that is 1/3 the power and area of a standard USB PHY. If you add the USB standard Link Power Management, LPM, you add the hardware capability to use LPM. (This is the USB standard LPM, not the Synopsys LPMM). Add the software for LPM and you can save a lot of power, possibly up to 20% of your battery life can be recovered depending on the kind of USB device you are using.

For B) if you adopt the Synopsys LPMM, you can get additional savings, but requires a more disiplined approach. More me, the most interesting thing about this option, is that it almost "free." It means, that if your engineers learn the methodology, there may be some additional silicon coast (egad Holmes) but would it be worth it to spend a few more pennies for the silicon if you could re-capture 10-40% more battery life? Most interesting, is that if the hardware hooks are in, then the software can really drive power usage down. The cost is minimal. You can use your existing IP.

Seems to me that B) offers a way to add value to raise the price of a product or sustain the product, even in potentially hard times.

Of course if you go the route of A), we have that too. (Shameless plug).

- Eric Huang is part of the Synopsys OpenCommunity.org. His blog, USB IP Blog: To USB or Not to USB, covers the latest trends and topics in USB IP.





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