+- lose USB connection during power-down mode
+ - USB initialize code in main() causes this - WRONG
+ - Do not power-down during USB connection is active - DONE 11/11
+ (USB_DeviceState == USB_DEVICE_Configured) is used to check USB connection
+ matrix_power_down() matrix.c - 11/23
+
+Todo:
+
+Design:
+- remove MCU dependent power saving code from core/keyboard
+ - it should be located in project matrix.c - DONE 11/23
+- HHKB matrix.c needs matrix_prev?
+ - is_modified() is obsolete now. really needs?
+
+
+LUFA:
+USB connection check: state of USB deivce
+- USB_DeviceState:
+ USB_Deivce_State_t { Unattached, Powered, Default, Addressed, Configured*, Suspended* }
+- USB_IsInitialized: state of LUFA core setup
+ becomes true in USB_Init() USBController_AVR8.c
+ becomes false in USB_Disable() USBController_AVR8.c
+- USB_VBUS_GetStatus(): state of VBUS(power/connection)
+- USB_Disable() detaches, disables all interrupts, controller, PLL, regulater.
+
+