Hallo Alkhadher,

I would highly recommend you to use a '51 derivative already containg this ADC. To proceed with 8255 and 555 is no good idea, because your board becomes so contaminated with ground noise, that result of ADC will be highly unreliable.
One trick to achieve good results is to invoke idle mode of microcontroller during conversion. Then digital switchings introducing noise to the ADC are highly minimized. But if you have an 555 all the time running, with counters etc. ground noise can be enormous.

Keep also in mind, that a careful separation of analog ground and digital ground must be performed. Both grounds may only be connected at one single point (ground star point).

And if sensor signals have to be handled, don't forget to obey the theorem of Nyqist-Shannon. So, some input signal low pass filtering might be essential.

Make a search here using keywords like "ADC", "ground plane", etc. to read more about this issue.

Kai