Video and Image Processing Suite User Guide

ID 683416
Date 4/04/2022
Public
Document Table of Contents

8.3. FIR Coefficient Specification

You can either specify the FIR filter IP filtering operation coefficients as fixed values that are not run-time editable, or you can turn on an Avalon memory-mapped agent interface to edit the values of the coefficients at run time.

The IP requires you to define a fixed-point type for the coefficients. The user-entered coefficients (shown as white boxes in the parameter editor) are rounded to fit in the chosen coefficient fixed-point type (shown as purple boxes in the parameter editor).

  • For run-time editable coefficients, you must enter the desired coefficient values through an Avalon memory-mapped agent control interface at run time, and you can update the coefficient values as often as once per frame.
    Note: The coefficient values all revert to 0 after every reset, so you must initialize coefficients at least once on start-up.
  • To keep the register map as small as possible and to reduce complexity in the hardware, the IP reduces the number of coefficients that it edits at run time when you turn on any of the symmetric modes.
  • For T unique coefficient values after symmetry, the register map contains T addresses into which you should write coefficients, starting at address 7 and finishing at T+ 6.

Fixed Coefficient

For fixed coefficients, you specify the values for the coefficients with a comma-separated .csv text file and there is no Avalon memory-mapped agent interface. The selected coefficient values take effect immediate effect at reset.

Regardless of the symmetry mode, the text file must contain a full listing of all the coefficients in the N×M array i.e. the file must always contain N×M comma-separated values. When the .csv file is parsed in Platform Designer to create the list of compile time coefficients, the IP checks the values entered against the selected symmetry mode and provides warnings if the coefficients are not symmetric across the selected axes. The values specified in the .csv file must be in their unquantized format. For example, if you want a value of 1.7 for a given coefficient, the value in the file should be 1.75. When the file is parsed in Platform Designer, the coefficients automatically quantize according to the precision you specify.

Note: The quantization process selects the closest value available in the given precision format. If you select the coefficients arbitrarily without reference to the available precision, the quantized value may differ from the desired value.

Run-time Editable Coefficients

To keep the register map as small as possible and to reduce complexity in the hardware, the number of coefficients that are edited at run time is reduced when you turn on any of the symmetric modes.

If the IP has T unique coefficient values after symmetry, the register map contains T addresses into which you should write coefficients, starting at address 7 and finishing at T+ 6.

Write coefficient index 0 (as described in the symmetry section) to address 7. Then write each successively indexed coefficient at each following address. The updated coefficient set does not take effect until you issue a write to address 6. You can write any value to address 6. The action of the write forces the commit.

The new coefficient set then takes effect on the next frame after the write to address 6.
Note: The coefficient values you write to the register map must be in prequantized format as the hardware cost to implement quantization on floating point values is prohibitive.