Hallo Adel,

first: Don't worry so much about everything. Next months and years are becoming very exiting for you. Don't worry, be happy. You will see very fascinating things and you must learn, that 95% of developement is to find best compromise. And you must (and will) learn that trying to make things as uncomplicated as possible from beginning on is really essential. Things are becoming complicated enough while proceeding developement, anyway.

There is another topic: Talk with your professor that you are in contact with this forum and ask him whether there are any problems. And it's better to 'clean' such things at the beginning.

'Register/clock is an IC consists of registers decicated to maintaing time. additionally, it requiers its own crystal oscillator. Optionally, it can be powered by a backup power supply to maintain time in the event that power is removed from the main circuit.'

If you want to use this timer feature, chip MUST be powered by a backup power supply. Do it just like datasheet says.

'2- if i do not want to use EEPROM to store the 10 user's data (passwords+height+last weight), what will be your opinion if i use the familiar RAM memory that built in my 8252 and the 8252 will be powered by a backup power supply becuse i do not want these data to be lost.'

There is indeed the possibility of having the whole microcontroller in data retention mode, with the help of global backup power supply. BUT: This is a job for a professional, because reset function is not so easy to implement. Lots of complicated details have to be taken into account. I would not start with such option. Please do use that internal EEPROM, it's just implemented for applications like your's. There is absolutely no reason for you not to use this EEPROM!

'1- was it a correct decision to chose AT89s8252?'

Take microcontroller that makes life easiest for you. If it is AT89C8252 then use it.

Finally, one more hint: Read datasheet as carefully as possible!!! If you don't understand something just ask professor or other people for help. Also, have a look at website of manufacturer of chip. Then you will find many additional details in application notes, erratas a.s.o. Also, have a look at google.

Good luck,
Kai