1. (1) 9V Battery is considered to be useable upto terminal voltage of 8.18V (calculation: Ni-Cd Battery Cell Reference. Float Volt: 1.1V, Final Volt: 1.0V.)
(2) Safe operating voltage of ATmega328P MCU at 16 MHz is 4.5V. UNO's on-board AMS1117-5.0 type fixed voltage regulator is gurantted to maintain 15V/5V regulation at 800 mA current.
2. I never experinecd such event.
3. 28-Pin DIP package vs 32-pin Flat ackage:
IO line: 23 / 23
ADC: (6)/(6) + 2
Vcc: 1/2
AVcc: 1/1
AGND: 1/1
DGND: 1/2
AREF: 1/1
Total: 28/32
4. (Quoted fro Post-2) "Only what is needed; the full library will be compiled but the linker will throw away what is not needed."
5. A variable will not be saved in flash unless qualified by keywords const PROGMEM. For example:
byte const PROGMEM abf = 3;
Regardless of the length of an identifier/variable name, it is always evaluted to an 16-bit (12-bit?) unique RAM location address (if saved in RAM) or 14-bit flash location address (if saved in flash).
6. Yes!
7. (Quoted from Post-2)" 1. Libraries usually consist of one or more .h files and one or more .cpp files. The .h file that you include tells the compiler which functions and variables are there, the .cpp file contains the implementation of the functions. You only include the .h file."