CEPD News 
Spring, 2011 - Vol 4, Issue 2


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In This Issue
Society News
IIR Filters continued
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Society News
Jason Fritz, PhD, and past chair of the IEEE SPS Denver section will be the presenter for the Signal Processing Society meeting on March 30.  Jason's talk is titled: The Polarimetric Characterization of Precipitation from High Frequency Synthetic Aperture Radar
When: 3/30/2011 06:00PM-07:30PM
Location: Integrated Teaching and Technology Lab (ITLL), Room 1B50, Engineering Drive, Boulder, Colorado United States 80302
Cost: Free
Please see the SPS Events page to register for this meeting and preview other upcoming events:
Calculus Bowl on April 12, 06:00PM
Photo Forensics Fort Collins April 18, 06:30PM
Photo Forensics Westminster April 19, 6:30PM
Why average when you can filter? (part 2)

In our previous newsletter, we presented a digital filter implemented with  floating point math in C. This time we present a filter using fixed point math in VHDL. A code snippet for a first-order IIR filter form is expressed as follows: 


PROCESS (resetn, lpfclk) BEGIN
   IF (resetn = '0') THEN
      w1 <= (others => '0');  y     <= (others => '0');
      x  <= (others => '0');  lpfout <= (others => '0');
   ELSIF (lpfclk'EVENT and lpfclk = '1') THEN
      w1 <= w0;       y      <= rightsum(31 DOWNTO 16);
      x  <= lpfin(15 downto 0); lpfout <= slpfout;
   END IF;

-- low pass filter --
A1W1 <= a1 * w1; -- use the negative of computed a1 for pos sum
-- make x value Q16 for Q16 addition, sign extend
xQ16 <= X"0000" & x & X"0000" when x(15)='0'
   else X"FFFF" & x & X"0000";  
leftsum <= xQ16 + A1W1; -- leftsum Q16 result
w0 <= leftsum(43 downto 16); 
rightsum <= w0 * b0;  --rightsum Q16 result
slpfout <= y;   -- output

-- low pass filter --


Check the code example link later this week for the full VHDL example code. Follow the filter design calculations and frequency response link to determine coefficient values. 


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