Mi sa che usa il dtr
da Arduino-usbserial.c
void EVENT_CDC_Device_ControLineStateChanged(USB_ClassInfo_CDC_Device_t* const CDCInterfaceInfo)
{
bool CurrentDTRState = (CDCInterfaceInfo->State.ControlLineStates.HostToDevice & CDC_CONTROL_LINE_OUT_DTR);
if (CurrentDTRState)
AVR_RESET_LINE_PORT &= ~AVR_RESET_LINE_MASK;
else
AVR_RESET_LINE_PORT |= AVR_RESET_LINE_MASK;
}