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 in INTFLAG(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 EXTREG register).
Recovering from this LPM-L1 suspend state is exactly the same as the Suspend state (see Section 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(INTFLAG.LPMNYET) is set. This generates an interrupt if the Link Power Management Not Yet Interrupt Enable bit in INTENCLR/SET (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.