RISETIME......I referred to this in my first post here:

http://www.8052.com/forum/read.phtml?id=69468

...but mentioned instead the "fall time". I did this becasue the active edge of your interrupt edge is most likely the low going edge which relates to the FALL TIME.

Michael Karas

PS....in these opamp type circuits it is VERY common that the FALLTIME and the RISETIME are not the same!!

MJK