May 17, USB Power Management Brad Hosler USB Engineering Manager Intel Corporation

May 17, Agenda w PC Power Management Overview w USB PM Characteristics w Power states for USB devices w Summary

May 17, Instantly Available PC w PC that acts like an appliance – Appears off, but can respond to external events – Very low power consumption (less than 5 Watts) w Huge cost savings in corporate environments w Greater impact in consumer applications USB Is Very Important in These PCs, Especially As PCs Become ‘Legacy Free’

May 17, System Power States S0 - Fully ON USB operational S1 - Sleeping USB suspended S3 - Suspend to Ram USB suspended S4 - Suspend to Disk USB suspended S5 - Off USB off

May 17, S3 Characteristics w Much of system HW is not powered – Enough alive to detect wake up events w Limited power available – Aux power supply typically provides 720ma at 5V – shared between platform, PCI, and USB S3 Is the ‘OFF’ State for IAPCs

May 17, S3 Power Consumers Base Board Memory Subsystem PCI Remote Wakeup Slots Other Slots USB~100ma~100ma 375ma 20ma 375ma 20ma10ma/port

May 17, USB Dynamic Power w Suspend to Remote Wakeup transition – Device can go to full power immediately – Could be as high as 500ma w Insertion of new device into empty port – Device can draw up to 100ma w Remote wakeup does NOT wake all USB devices – Selective suspend is done at all ports

May 17, Platform Dynamics w Switch from Vaux to Main supply takes ~250msec – HW controlled starting with any power event u USB remote wakeup, PCI PME#, USB device insertion w Power supply Vaux has dynamic characteristics – Testing shows >3A for 500msec

May 17, USB S3 Power w Use 10ma/port rule for static power calculation w Assume only one power event from USB – Multiples within 1/4 second are unlikely – Max for that one event is 500ma w Dynamics of Vaux supply easily handle 500ma Use USB Static Power for Vaux Sizing Vaux Supply’s Easily Handle USB Dynamic Power

May 17, Power States for USB Devices

May 17, USB Feature Specification w Interface Power Management Specification defines some additional PM states for devices – D0 - Fully on – D1, D2, D3 - Less than fully on w Relationship between Dx states and USB PM states (active/suspended) is orthogonal – Device required to suspend/resume no matter what Dx state – Suspend/resume doesn’t effect device Dx state

May 17, New Power Values w If device is in D1, D2, or D3 and enabled for remote wakeup, then 100ma is current limit for suspend w If device is in D1, D2, or D3 and signaling a remote wakeup, then device is limited to 100ma

May 17, Budgeting USB Power w New 100ma ‘suspend’ current can exceed Vaux capacity w If OS does power budgeting, OS will manage USB Dx states to stay within Vaux capacity – OS has to know Vaux capacity w If OS doesn’t power budget, OS will make sure devices are in D0 before suspending – Devices can’t count on more power while suspended

May 17, USB Driver Behavior w OS will ask USB driver if system ‘sleep’ is OK – IRP_MN_QUERY_POWER w Driver can ‘fail’ the request in certain cases – When doing ‘critical’ IO (like modem connection) w 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 Device Drivers Must Be PM Aware Device Drivers Must Be PM Aware

May 17, Summary w USB is an important feature of IAPCs w USB power characteristics are easily handled by IAPCs w New USB power states don’t guarantee higher suspend power w Make your device driver PM aware

May 17, Call to Action w OEMs: Use power budgeting to fully enable USB devices while in S3 state w IHVs: See if your peripheral can benefit from the USB PM feature specification – Limit device power on remote wakeup – More power to detect events Make Sure Your Device Driver Is PM Aware