Hi,
Currently i am experiencing an unseen problem ! :
I designed a simple board that you can see it here :
(As asked before because of the big sizes i don't insert them in my message)
http://www.8052.com/users/farshidjh/project.GIF
http://www.8052.com/users/far...ct_pcb.GIF

without the microcontroller inserted push buttons voltage levels are readed prefectly (tested with a multimeter).
but whenever the AT89S51 is inserted P1.2,p1.3,p1.4 doesn't funtion properly as inputs,considering that the Push Buttons are pulled down ,so every time i press a button i should get high (+5) and when release them i should read GND (0v)
but the problem is when i release i get 2.3v-2.6v and if i disconnect the multimeter and then reconnect it to this pin it goes back down to zero (Noise ?)!.
I know this problem is not from my code, anyway here is the sample code i am using :

org 00h
mov p1,#0ffh
loop:
mov c,p1.2
mov p1.5,c
mov c,p1.3
mov p1.6,c
sjmp loop

(p1.5,p1.6 are LEDs)
My though is ,maybe i shouldn't connected pullups and pulldowns on a same port ?
Does anyone has any idea what is the problem that i am facing ?
(sorry there is no Reset superviser chip but hopfully i will include them in my future designs,considering that this is just a simple project for my friend and he want to use it once in non coomercial nor industrial environment)!

Any help would be greatly appreciated

Best Regards :
Farshid J.H.