Visualize Differences Between Floating-Point and Fixed-Point Results

To complete this example, you must install the following products:

MATLAB^®
Fixed-Point Designer™
C compiler
See Supported Compilers.
You can use mex -setup to change the default compiler. See Change Default Compiler.

Inspect Example Files

Type	Name	Description
Function code	`myFilter.m`	Entry-point MATLAB function
Test file	`myFilterTest.m`	MATLAB script that tests `myFilter.m`
Plotting function	`plotDiff.m`	Custom plot function
MAT-file	`filterData.mat`	Data to filter.

The myFilter Function

The myFilterTest File

The plotDiff Function

Set Up Configuration Object

Create a coder.FixptConfig object.
```
fxptcfg = coder.config('fixpt');
```

Specify the test file name and custom plot function name. Enable logging and numerics testing.

fxptcfg.TestBenchName = 'myFilterTest';
fxptcfg.PlotFunction = 'plotDiff';
fxptcfg.TestNumerics = true; 
fxptcfg.LogIOForComparisonPlotting = true;
fxptcfg.DefaultWordLength = 16;

Convert to Fixed Point

Convert the floating-point MATLAB function, myFilter, to fixed-point MATLAB code. You do not need to specify input types for the codegen command because it infers the types from the test file.

codegen -args {complex(0, 0)} -float2fixed fxptcfg myFilter

The conversion process generates fixed-point code using a default word length of 16 and then runs a fixed-point simulation by running the myFilterTest.m function and calling the fixed-point version of myFilter.m.

Because you selected to log inputs and outputs for comparison plots and to use the custom plotting function, plotDiff.m, for these plots, the conversion process uses this function to generate the comparison plot.

The plot shows that the fixed-point results do not closely match the floating-point results.

Increase the word length to 24 and then convert to fixed point again.

fxptcfg.DefaultWordLength = 24;
codegen -args {complex(0, 0)} -float2fixed fxptcfg myFilter

The increased word length improved the results. This time, the plot shows that the fixed-point results match the floating-point results.