NRF52820-QDAA-R datasheet(375/437 Pages) NORDIC

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Peripherals

6.26.7 USB suspend and resume

Normally, the host will maintain activity on the USB at least every millisecond according to USB

specification. A USB device will enter suspend when there is no activity on the bus (idle) for a given time.

The device will resume operation when it receives any non idle signalling.

To signal that the device shall go into low power mode (suspend), the host stops activity on the USB bus,

which becomes idle. Only the device pull-up and host pull-downs act on D+ and D-, and the bus is thus

kept at a constant J state. It is up to the device to detect this lack of activity, and enter the low power

mode (suspend) within a specified time.

The USB host can decide to suspend or resume USB activity at any time. If remote wake-up is enabled, the

device may signal to the host to resume from suspend.

6.26.7.1 Entering suspend

The USBD peripheral automatically detects lack of activity for more than a defined amount of time, and

performs steps needed to enter suspend.

When no activity has been detected for longer than tUSB,SUSPEND, the USBD generates the USBEVENT event

with SUSPEND bit set in register EVENTCAUSE. The software shall ensure that the current drawn from

the USB supply line VBUS is within the specified limits before T2SUSP, as defined in chapter 7 of the USB

specification. In order to reduce idle current of USBD, the software must explicitly place the USBD in low

power mode through writing

LowPower to register LOWPOWER.

In order to save power, and provided that no other peripheral needs it, the crystal oscillator (HFXO) in

CLOCK may be disabled by software during the USB suspend, while the USB pull-up is disconnected, or

when VBUS is not present. Software must explicitly enable it at any other time. The USBD will not be able

to respond to USB traffic unless HFXO is enabled and stable.

6.26.7.2 Host-initiated resume

Once the host resumes the bus activity, it has to be responsive to incoming requests on the USB bus

within the time TRSMRCY (as defined in chapter 7 of the USB specification) and revert to normal power

consumption mode.

If the host resumes bus activity with or without a RESUME condition (in other words: bus activity is

defined as any non-J state), the USBD peripheral will generate a USBEVENT event, with RESUME bit set

in register EVENTCAUSE. If the host resumes bus activity simply by restarting sending frames, the USBD

peripheral will generate SOF events.

6.26.7.3 Device-initiated remote wake-up

Assuming the remote wake-up is supported by the device and enabled by the host, the device can request

the host to resume from suspend if wake-up condition is met.

To do so, the HFXO needs to be enabled first. After waking up the HFXO, the software must bring USBD out

of the low power mode and into the normal power consumption mode through writing

ForceNormal

in register LOWPOWER. It can then instruct the USBD peripheral to drive a RESUME condition (K state)

on the USB bus by triggering the DPDMDRIVE task, and hence attempt to wake up the host. By choosing

Resume in DPDMVALUE, the duration of the RESUME state is under hardware control (tUSB,DRIVEK). By

choosing

J or K, the duration of that state is under software control (the J or K state is maintained until a

DPDMNODRIVE task is triggered) and has to meet TDRSMUP as specified in USB specification chapter 7.

Upon writing the

ForceNormal in register LOWPOWER, a USBEVENT event is generated with the

USBWUALLOWED bit set in register EVENTCAUSE.

The value in register DPDMVALUE on page 404 will only be captured and used when the DPDMDRIVE

task is triggered. This value defines the state the bus will be forced into after the DPDMDRIVE task.

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