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The Power Management model on Windows Mobile
Posted on 2-Aug-2005 10:59 by M Freitas. | Tags Filed under: Blog.

The Windows Mobile Team blog brings more information on how Pocket PC and Smartphone devices work, this time explaining power control.

That is an interesting read, like the previous entries on why Windows Mobile is moving to persistent storage. Here are some parts of the post:

"It all comes down to this: PocketPCs sleep, and Smartphones don't. Smartphones have a model that's very easy to understand. The device can be on, or it can be off. When it's on, everything works. When it's off, nothing does. When it's off, you can't receive phone calls, you don't get meeting reminders, and nothing is burning power. You can pull out the batteries and have no detrimental effects..."

"Sleep is the main way that PocketPCs conserve power. So the PocketPC is always trying to fall asleep. He's like a bored security guard on the graveyard shift staring at a TV screen with his eyes drooping, only to jolt himself back awake every few minutes. In Settings you get to tell your PocketPC how long to stay awake. Three minutes is typical. And then you've got to actively work to keep him up. If three minutes go by without you pressing any buttons or touching the screen, he'll fall back to sleep. (You can put him right to sleep by pressing the power button.) Apps that want to keep him awake longer need to be proactive about it. There's a function apps call (SystemIdleTimerReset) every 30 seconds when they want the system to stay awake. This is how Media Player keeps the music playing for longer than three minutes. This is also how Active Sync can make sure it syncs everything. And this is how PIE can download huge files without the system falling asleep in the middle of them. Etc..."

"As I said before, things are much simpler in the Smartphone world. Smartphones follow a model we call "Always On." These guys never sleep. You can turn them off completely, but when you do that, you don't expect to receive phone calls (at least, I hope you don't. You'll be disappointed otherwise). Sure, if you don't touch any keys for a bit, the backlight will turn off. And if you don't touch any keys for a while longer, the screen will turn off. And with the screen off, he looks asleep. But he's not. He's just resting his eyes. Programs are still running, and everything is still going.

"So why did Smartphone change it? You're going to find this hard to believe, but in a connected world, the Always On model actually burns less power than the Sleep model. Yes, you read that correctly. Staying on all the time actually burns less power than going to sleep. Here's why.

The issue is that it takes a "long" time to go to sleep and a similarly long time to wake back up. When a PocketPC goes to sleep, we have to notify every device driver so that they can each save any important information (their "state") and shut off the hardware they're controlling. Then, on wake up, we need to notify every driver again and have them turn all their hardware on. This process can take up to three seconds in each direction.

Smartphone, on the other hand, can come out of his idle in a millisecond, do what he needs to do, and go back to idle a millisecond after he's done.

Imagine that your device receives a SMS message. The sleeping PocketPC will need to run the CPU for around six seconds to handle it. The Smartphone will do the same task in a few milliseconds. Waking up is much more efficient on a device that doesn't sleep. It turns on only the devices necessary, uses them for the minimum amount of time needed, and then immediately shuts them back off.

Now, imagine a device that gets an SMS every time it moves from one cell tower to another, and imagine being in an area where you're on the boundary between two towers. Or, consider being signed in to an Instant Messenger client and having it frequently updating your friends list. Or, imagine a process that downloads data you care about every few minutes. Etc. As these things become more pervasive, we'll see the Always On power model being much more energy efficient than the sleep model..."

Wow! Now this explains a lot on why Pocket PC Phone Edition devices based on the current power management model have such a poor battery life. You have to read the full article...

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