" ... 1- was it a correct decision to chose AT89s8252?"

Why would you choose a chip that you don't know anything about? If this is a school project, the first step is find out what resources are available to you, i.e. dev boards, compilers, programmers, prototyping components, test equipment, lab availability, etc. Base your design on that. I microcontroller that you cannot program is useless.

"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."

Your Atmel chip has EEPROM onboard - why not use that? This is an academic exercise, so I wouldn't worry about the size. If size counts, use a different chip or use external mem.

"3- what about the TIME?

see what my doctor said to me:

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."

I don't know much about this area. I would assume that battery backing the micro would only be useful if it was accompanied by a reduced oscillator frequency. I think that there are battery-backed NVRAM modules with built in RTC's.. That might be something to look into. For all I know, that could be the description for bunches of RTC's..


"but doctor gave me all design options.. i can do what i want to reach the goal.

but my idea is to use the the softwared RCT using the 8252 crustal itself>>>>>is it good idea? what about the problem of approximation? i reed that with every mov instruction, this will take 2 cylcle ((as i remember)) which will make the big error after long time because the 8252 will be powered forever?"

Software RTC is timer interrupt driven. Instruction length is not an issue.

"which choice you say?"

The question I always ask myself when professors give me freedom is what do I want to learn? Because I am preparing for a career in hardware design, I try to use projects like this to further my own aims. Do you want to be a programmer? Do the software RTC. Do you want to be a hardware designer? Use serial ADC's, RTC's and NV mems.

Although your professor might have a solution that he would like to see.. It just depends on the prof. Some will only give full credit to a design that fits HIS criteria (whether stated criteria or not), others are lenient.


"4- last point: the digital scale:

read this:
(((microcontroller, which includes an A/D analog-to-digital converter for the weight signal.)))

does my chip provide A/D converter?
if yes how to use it?

not that i can use ready scale. but how to connect it to the 8252 and if there are companies provide a special scale for my project."

I will not answer whether or not your micro has an A/D - you need to open the datasheet sooner or later.

Does your prof want the whole system, or the hardware design? If it were me, I would use a pot to "simulate" a scale - the important part is that it provides a signal from the real world. Again, depends on what the prof wants.

- Lee