hello,
I have a related question .I am new to using microcontrollers .
I have a AT89C52 20PC (20 mhz) . This indicates that the max
input clock to the chip is 20 mhz not that the chip can accept only
input of 20mhz . Am i right ?

As to using an external clock , the data sheet says that the
rise and fall times must be less than 20 ns which i think could be
difficult to ensure from an external clock .

bye
ramc