I will use separate crystals for both.

Good decision. You seem to be having more serious problems than trying to save a simple crystal.


89C51 generate some critical timing and receive data from a mechanical device and then filter as well as decode the data and send to 89C2051 by serially

Why is that? Is it that your 89c51 is kind off physically handicapped that it cannot do what a 89c2051 can do ?

device send 16 bytes in 450 milli seconds and another mechanical device complete the operation on these 16 bytes in 1 second

Maybe I am not clear as to what is to be done. But just going by the timings and byte numbers this whole job is child's play for a single 89c51. In fact most of the time it will be doing nothing ...

Sorry if I ended up confusing you. This normally happens when one happens to see only a portion of the full picture.

Raghu