Low Pass IIR filter using cyclone II FPGA

It is a classical problem of fixed point mathematics. The ideal Altera tool for your job is the altmult_accum megafunction which multiplies and accumulates in a single module.

The fractional number should be transformed to integer numbers (you may use Matlab/Octave for a simulation before) and the result should be scaled back considering the multiplication factor that you have applied to one of the operands before the multiplication.

If you need help about the Altera megafunction you can have a look at:

http://www.altera.com/literature/ug/ug_altmult_accum.pdf