I have a program working on AT89C52. Now I have to use AT89S52 instead of AT89C52. As I know these chips should be backward compatible. But when I write the same hex-file it works on C52, but does not work on S52. There is ALE-signal, but pins, I try to change in my program, do not change. Can anybody tell me, if I do something wrong, and is there significant difference between these controllers?