Datasheet

In the case where the CTRLB.UPRSM bit is set while a host initiated downstream resume is already
started, the CTRLB.UPRSM is cleared and the upstream resume request is ignored.
32.6.2.15 Link Power Management L1 (LPM-L1) Suspend State Entry and Exit as Device
The LPM Handshake bit in CTRLB.LPMHDSK should be configured to accept the LPM transaction.
When a LPM transaction is received on any enabled endpoint n and a handshake has been sent in
response by the controller according to CTRLB.LPMHDSK, the Device Link Power Manager (EXTREG)
register is updated in the bank 0 of the addressed endpoint's descriptor. It contains information such as
the Best Effort Service Latency (BESL), the Remote Wake bit (bRemoteWake), and the Link State
parameter (bLinkState). Usually, the LPM transaction uses only the endpoint number 0.
If the LPM transaction was positively acknowledged (ACK handshake), USB sets the Link Power
Management Interrupt bit (INTFLAG.LPMSUSP) bit which indicates that the USB transceiver is
suspended, reducing power consumption. This suspend occurs 9 microseconds after the LPM transaction
according to the specification.
To further reduce consumption, it is recommended to stop the USB clock while the device is suspended.
The MCU can also enter in one of the available sleep modes if the wakeup time latency of the selected
sleep mode complies with the host latency constraint (see the BESL parameter in 32.8.4.4 EXTREG
register).
Recovering from this LPM-L1 suspend state is exactly the same as the Suspend state (see Section
32.6.2.13 Suspend State and Pad Behavior) except that the remote wakeup duration initiated by USB is
shorter to comply with the Link Power Management specification.
If the LPM transaction is responded with a NYET, the Link Power Management Not Yet Interrupt Flag
(INTFLAG.LPMNYET) is set. This generates an interrupt if the Link Power Management Not Yet Interrupt
Enable bit (INTENCLR/SET.LPMNYET) is set.
If the LPM transaction is responded with a STALL or no handshake, no flag is set, and the transaction is
ignored.
SAM D21 Family
USB – Universal Serial Bus
© 2018 Microchip Technology Inc.
Datasheet Complete
DS40001882D-page 805