USB suspend not implemented

[Dakkaron]

I'm using an ATmega32u4, but the issue seems to affect all devices.

According to the manual for that chip, the software is supposed to freeze the USB clock and optionally switch to the internal clock whenever it receives an USB suspend interrupt (SUSPI), and revert whenever it receives the USB wakeup interrupt (WAKEUPI). (Section 6.10 of the manual).

The USB core hints at that. It has the functions USB_ClockDisable() and USB_ClockEnable(), which are referenced in a TODO comment in the ISR (see

ArduinoCore-avr/cores/arduino/USBCore.cpp

Line 791 in 6309212

//USB_ClockEnable();

and

ArduinoCore-avr/cores/arduino/USBCore.cpp

Line 800 in 6309212

//USB_ClockDisable();

), but both aren't actually activated.

Since the USBCore doesn't properly implement suspending, the host device needs to always keep its USB driver active and wakelocking, no matter if the USB connection is actually used. This causes significant battery drain in battery-powered devices.

It would be highly appreciated if this function is implemented.