input to IIR filter in Verilog

I don't think, that you can't use FIR filters succesfully without some minimal understanding of number representation in HDL or digital logic generally. But this has been the problem so far. A 1st order low-pass, as addressed with the above code, is the most simple filter of the world and doesn't involve issues with coefficient sensitivity or stability.

I won't say: Oops, you don't manage to design a 1st order IIR, it's much easier with FIR... Also the FIR tool requires some basic knowledge of filters, e.g. relation of fs, fc and number of coefficients. Previous Altera forum discussions demonstrate some attempts to ignore these basics.