vhdl problem for iir filter

[Altera_Forum]

hi: guys  

i have a problem makes me crazy.  how to use vhdl codes to implement iir filter  y(n)= 2cos(w)*y(n-1) - y(n-2)  and the initial conditions is y(-1)=sin(w),y(-2)=sin(2w).  which means i need output this filter every cycle calculation like following y(0),y(1)....  y(0)=2cos(w)*y(-1)-y(-2);  y(1)=2cos(w)*y(0)-y(-1);  ...  i use a mac and a rounding operation to do it.  but its not work like i describe above.  how should i do. Am i need plus some selfcycle computation on testbench or on original filter codes. And how to plus it into this codes.   thanks

[Altera_Forum]

Hello, 

 I can't indentify a filter, cause there is no signal input. If you are actually trying to design an oscillator, then you should probably choose a structure less sensitive to rounding errors or use an NCO instead.  But apart from this consideration, the shown IIR structure can surely be synthesized in VHDL. I understand, that cos(w) is a constant that either can be calculated at compile time or loaded as a parameter (which is big difference for FPGA implementation), the same with the said initial values. Next you have to define a numeric format (probably signed) and a resolution. Then the algorithm can be build from a multiplier and a substractor megafunction or infered from HDL. Two registers are needed to store previous y(n).   Basic operation can be evaluated also in a spreadsheet tool, e. g. Excel to study rounding error effects with different numerial resolution.  Regards,  Frank

[Altera_Forum]

to Frank: 

thank you pay attentation to my problem. yes, as you said i am doing a digital oscillator utilized a iir structure.  you said i need two registers to store previous y(n). i show u my codes as attached, may be when u see my codes you can understand my problem totally. Please check out my codes, and give me some advise agian. Thank you

[Altera_Forum]

the archive seems to be broken. Can't open it.

[Altera_Forum]

sorry to Frank 

the forum can not attached the .rar files, so please show me your email address, i send it to you directly.