User guide

System Generator for DSP User Guide www.xilinx.com 113
UG640 (v 12.2) July 23, 2010
Using FDATool in Digital Filter Applications
The filter coefficients can be displayed in the MATLAB workspace by typing:
>> xlfda_numerator('FDATool')
These useful functions help you find the maximum and minimum coefficient value in
order to adequately specify the coefficient width and binary point:
>> max(xlfda_numerator('FDATool'))
>> min(xlfda_numerator('FDATool'))
For this tutorial, the coefficient type has been set to be Fix_12_12, which is a 12-bit
number with the binary point to the left of the twelfth bit. The result of the max()
function above shows that the largest coefficient is 0.3022, which means that the binary
point may be positioned to the left of the most significant bit. How do you reason that?
A Fix_12_12 number has a range of -0.5 to 0.4998, meaning the dynamic range is
maximized by putting the binary point left of the most significant bit. If you moved the
binary point to the right (by using a Fix_12_11 number) you would lose one bit of
dynamic range because a Fix_12_11 number has a range of -1 to 0.9995, which is more
than you require to represent the coefficients.
4. Click on the Reference Filter block and the MAC Based FIR block and verify the
parameter values for coef, coef_width, coef_binpt, data_width, data_binpt and Fs as
shown below.
Click OK on each dialog box