USBD automatically detects lack of activity on the USB bus for more than a defined amount of time.
When no activity has been detected for longer than tUSB,SUSPEND, 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 USBD in Low-power mode by 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. USBD is not able to respond to USB traffic unless HFXO is enabled and stable.