# FIR Filter Implementation in Verilog HDL

Started by December 3, 2008
1) I need a simple to code in Verilog HDL for implementing a FIR Filter.
Filter equation is as under :
y[n] = 1/3(x[n] + x [n-1] + x[n-2]

2) Is there any book availaible for implementing DSPs in Verilog HDL..

Thanks
The easiest (although maybe not the most efficient) way to implement this filter would be something like:

/** Verilog code */
module fir (
input signed [SAMPLE_WIDTH] new_sample, //incoming sample
input clk, //sample clock
output reg signed [SAMPLE_WIDTH] y); //filtered output

//use signed registers to perform signed arithmetic

always @ (posedge clk) begin

y <= (x_n + x_nm1 + x_nm2) / 3; //y[n]
x_nm2 <= x_nm1; //x[n-2]
x_nm1 <= x_n; //x[n-1]
x_n <= new_sample; //x[n]

end

endmodule
/** End of Verilog code */

The biggest problem with this code is that it uses a divide. As you probably know, division is very slow and expensive in hardware, and so you're much better off trying to massage your filter coefficients into powers of two in order to be able to perform division using a bit shift (shifting left by n corresponds to multiplication by 2^n, shifting right by n corresponds to division by 2^n). If you're working with a large filter with many taps, and can't make them all into powers of 2, you might be better off using a system clock that runs (much) faster than the sample clock, and sharing one multiplier between all filter taps.

For information on Verilog DSP work, I suggest you start by looking at some of the design examples given at http://www.altera.com/support/examples/dsp/exm-dsp.html.

Hope this helps.
--Tom
1) I need a simple to code in Verilog HDL for implementing a FIR Filter.
>Filter equation is as under :
>y[n] = 1/3(x[n] + x [n-1] + x[n-2]
>
>2) Is there any book availaible for implementing DSPs in Verilog HDL..
>
>Thanks
>
> 1) I need a simple to code in Verilog HDL for implementing a FIR Filter.
> >Filter equation is as under :
> >y[n] = 1/3(x[n] + x [n-1] + x[n-2]
> >
> >2) Is there any book availaible for implementing DSPs in Verilog HDL..

There's some Verilog code in Uwe Meyer-Baese's book on DSP with FPGAs. (I
have this book but haven't actually tried to use it yet, so can't give it a
thumbs-up or thumbs down.)

-- john, KE5FX
1) I need a simple to code in Verilog HDL for implementing a FIR Filter.
>Filter equation is as under :
>y[n] = 1/3(x[n] + x [n-1] + x[n-2]

The difference equation that was posted is simply a moving average. Couple things, look at the filter requirements (cutoff freq, position nulls, etc). If you change the order of the moving average to 4 you will not need the divide. There are resource efficient algorithms for moving average. A recursive structure (similar to a CIC filter) only requires adders.

I would also suggest using something like MyHDL (www.myhdl.org) to design, implement, and simulate any DSP HDL. It is much easier to integrate signal processing simulation with the HDL simulation. Makes analyzing and verifying DSP blocks much easier.

>
>2) Is there any book availaible for implementing DSPs in Verilog HDL..
>
The Uwe Meyer-Baese book is decent. As previously mentioned it has Verilog and VHDL examples. A general DSP book like Lyons' "Understanding Digital Signal Processing" would also be a good book. Couple other VSLI DSP books out there as well, Parhi and Wanhammar. The later books are a more in depth but good. I don't think the Parhi has HDL (Verilog/VHDL) examples but the others do.