Dear Prahlad J. Purohit,
with thanks for your informative reply,
first of all ,this is not my first board nor i am a beginer ,this is maybe my 50th board using MCS51 MCUs,
as shivani said RC reset is not the problem (as you se there is a manual reset available) and i have mentioned in my initial post that i know the benefits from it , but for this particular project i wasn't able to implement such a thing , sometimes if you use fewer ICs on your board you have less effort to teach your master what this chip will do ! (Hope you get it ?)
responding to your numbered sussgestiosn :
1- i don't think this setup is wrong , i know this is not regular ,but it should work , as far as you write 1's on you port it doesn't matter what you are doing outside and how you feed the pin input ,am i right on this ? what's others opinion about this ?
2- thanks for your tip, as i saied its my friends tipo , i am using a 6 Mhz crystal currentlty so feed a 1 Mhz Clock to the ADC so it is in range.
3- this dac wouldn't take more than 1 sample per second , so it wouldn't make any trouble , again consider it that input supply to this board is a regulated ,noise free one.
4- and this shcematic has been drawed using Egale Cadsoft, but not completly by me ,about the net box you are right !,I have asked from my friend to fix his schematic and PCB and i added a ground plane and some other corrections ,and as he don't wanted to change the basis of the project , i couldn't make everything better than this ,and these pull ups/downs is his NICE work ! ,
personally ,like Russell and you saied i never use a button connected to VCC along a pulldown ,but there is no reasoon from my viewpoint ,that such a design shouldn't even work !