Actually the interrupt in 89c51 is falling-edge triggered. Hence after the interrupt occurs ( hi-to-lo) the level should be low.

SAW filters seems to me to be a mechanical way to perform acoustic signal processing operations. They convert acoustic signals into surface vibrations on a piezoelectric substrate and back. But they modify the physical properties of the substrate itself to perform operations such as bandwidth filtering, or delay-line etc. The acoustic signal out is hence a processed form of the signal in.

Try this link for some easy-2-read info:

http://www.lginnotek.com/saw/compa...any_2.html

I can't say what Infinite/finite is tho'
The only filters i know with those words desribing an aspect are FIR/IIR filters in DSP

Kundi