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USB Power Management

April 28, 2012. 3. Agenda. Platform Power for USBMobile platforms and USBPower states for USB devicesSummary. April 28, 2012. 4. Instantly Available PC. PC that acts like an applianceAppears off, but can respond to external eventsVery low power consumption (less than 5 Watts)Huge cost savings

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USB Power Management

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    2. April 29, 2012 2 USB Power Management Brad Hosler USB Engineering Manager Intel Corporation

    3. April 29, 2012 3 Agenda Platform Power for USB Mobile platforms and USB Power states for USB devices Summary

    4. April 29, 2012 4 Instantly Available PC PC that acts like an appliance Appears off, but can respond to external events Very low power consumption (less than 5 Watts) Huge cost savings in corporate environments Greater impact in consumer applications

    5. April 29, 2012 5 System Power States

    6. April 29, 2012 6 S3 Characteristics Much of system HW is not powered Enough alive to detect wake up events Limited power available Aux power supply typically provides 720ma at 5V shared between platform, PCI, and USB

    7. April 29, 2012 7 S3 Power Consumers

    8. April 29, 2012 8 USB Dynamic Power Suspend to Remote Wakeup transition Device can go to full power immediately Could be as high as 500ma Insertion of new device into empty port Device can draw up to 100ma Remote wakeup does NOT wake all USB devices Selective suspend is done at all ports

    9. April 29, 2012 9 Platform Dynamics Switch from Vaux to Main supply takes ~250msec HW controlled starting with any power event USB remote wakeup, PCI PME#, USB device insertion Power supply Vaux has dynamic characteristics Testing shows >3A for 500msec

    10. April 29, 2012 10 USB S3 Power Use 10ma/port rule for static power calculation Assume only one power event from USB Multiples within 1/4 second are unlikely Max for that one event is 500ma Dynamics of Vaux supply easily handle 500ma

    11. April 29, 2012 11 Mobile Platforms and USB

    12. April 29, 2012 12 C3 Processor State Low power state for processors Extends battery life by ~10% Entered when system is idle (including IO traffic) Can be entered in milliseconds (10 to 50) Any bus master activities keep processor alive USB host controllers poll memory every millisecond

    13. April 29, 2012 13 Use Selective Suspend New feature in Windows XP will stop HC polling if all ports are suspended This will allow process to go to C3 state Device drivers should suspend their device whenever it is idle Camera not open, disk drive not being accessed Use defined PM IOCTLs to manipulate device state and let the device do wakeup

    14. April 29, 2012 14 Power States for USB Devices

    15. April 29, 2012 15 USB Feature Specification Interface Power Management Specification defines some additional PM states for devices D0 - Fully on D1, D2, D3 - Less than fully on Relationship between Dx states and USB PM states (active/suspended) is orthogonal Device required to suspend/resume no matter what Dx state Suspend/resume doesnt effect device Dx state

    16. April 29, 2012 16 New Power Values If device is in D1, D2, or D3 and enabled for remote wakeup, then 100ma is current limit for suspend If device is in D1, D2, or D3 and signaling a remote wakeup, then device is limited to 100ma

    17. April 29, 2012 17 Budgeting USB Power New 100ma suspend current can exceed Vaux capacity If OS does power budgeting, OS will manage USB Dx states to stay within Vaux capacity OS has to know Vaux capacity If OS doesnt power budget, OS will make sure devices are in D0 before suspending Devices cant count on more power while suspended

    18. April 29, 2012 18 USB Driver Behavior OS will ask USB driver if system sleep is OK IRP_MN_QUERY_POWER Driver can fail the request in certain cases When doing critical IO (like modem connection) If able to suspend, driver should finish pending IOs and pass IRP down the stack Passing IRP without completing IOs can result in system hang

    19. April 29, 2012 19 Summary USB is an important feature of IAPCs USB power characteristics are easily handled by IAPCs Mobile platforms have power savings when USB device drivers suspend their device New USB power states dont guarantee higher suspend power Drivers need to be PM aware

    20. April 29, 2012 20 Call to Action OEMs: Use power budgeting to fully enable USB devices while in S3 state IHVs: Suspend your device when not in use Make your device mobile friendly IHVs: See if your peripheral can benefit from the USB PM feature specification Limit device power on remote wakeup More power to detect events

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