+Bug:
+- with Nexus5 keyboard and mouse are very laggy.
+
+- 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+ - is_modified() is obsolete now.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.
+
+
+Power saving:
+- confirm suspend mode lufa.c: matrix_power_*, suspend_wakeup_condition
+- 8MHz clock
+- When not connected in a few minutes get into deep sleep to save battery life
+- CTS is needed for waking up from deep sleep? How deep sleep is activated?
+- firmware controlled 3.3V DC converter to switch on/off BT module
+- sleep MCU and BT module(keyboard is not used)
+- deep sleep MCU and BT module(keyboard is not used for long time)
+- deep sleep MCU and turn off BT module(keyboard is not used and not connected)
+- Battery ADC; switching, high resistance
+ - switching gnd end of divider with PF4
+ - high resistor 100K/1M?
+ capacitor 10nF
+ http://www.eevblog.com/forum/beginners/measuring-battery-voltage-without-consuming-current/
+- During USB suspend change clock source to internal RC from external Xtal(6.8)
+- FRZCLK: you can freeze clock for power saving. still WAKEUPI and VBUSTI interrupts are available while freezing.(21.7.3)
+- Suspend: Clear Suspend Bit, Freeze clock, disable PLL, MCU sleep(21.13)
+
+Improving:
+- BT LED; connecting, linked, sleeping, deep sleeping
+- Battry LED; blink(using timer?)
+- move rn42 to protocol directory when it becomes reusable stack
+- LUFA sendchar should be buffered and serial_uart.c buffur size is too large(256).
+- ADC resolution
+ AVR120
+ AVR32138