hi,
you can refer to topic
jcding1980 has mentioned clearly, on 12 Feb 2005 22:53 ;
subject: 16F877
the difference between 16f877 and 16f877A as follows (as far as i know) :
i) 16f877A is an improved version of 16f877 with additional built-in comparator function.
ii) programmer that supports 16f877 might not support 16f877A.
iii) different in __CONFIG register bits for 16f877A and 16f877.
the rest are exactly the same. eg. pins compatibility, voltage supply, crystal oscillator frequency and etc.
however, there are slight different in electrical characteristic of these two ICs. but this does not really matter.
hope this help u..