Fixed-Point Designer

Accelerate Fixed-Point Simulation

This example shows how to accelerate fixed-point algorithms using fiaccel function. You generate a MEX function from MATLAB® code, run the generated MEX function, and compare the execution speed with MATLAB code simulation.

Description of the Example

This example uses a first-order feedback loop. It also uses a quantizer to avoid infinite bit growth. The output signal is delayed by one sample and fed back to dampen the input signal.

Copy Required File

You need this MATLAB-file to run this example. Copy it to a temporary directory. This step requires write-permission to the system's temporary directory.

tempdirObj = fidemo.fiTempdir('fiaccelbasicsdemo');
fiacceldir = tempdirObj.tempDir;
fiaccelsrc = ...
    fullfile(matlabroot,'toolbox','fixedpoint','fidemos','+fidemo','fiaccelFeedback.m');
copyfile(fiaccelsrc,fiacceldir,'f');

Inspect the MATLAB Feedback Function Code

The MATLAB function that performs the feedback loop is in the file fiaccelFeedback.m. This code quantizes the input, and performs the feedback loop action :

type(fullfile(fiacceldir,'fiaccelFeedback.m'))
function [y,w] = fiaccelFeedback(x,a,y,w)
%FIACCELFEEDBACK Quantizer and feedback loop used in FIACCELBASICSDEMO.

% Copyright 1984-2013 The MathWorks, Inc.
%#codegen

for n = 1:length(x)
    y(n) =  quantize(x(n) - a*w, true, 16, 12, 'floor', 'wrap');
    w    = y(n);    
end

The following variables are used in this function:

  • x is the input signal vector.

  • y is the output signal vector.

  • a is the feedback gain.

  • w is the unit-delayed output signal.

Create the Input Signal and Initialize Variables

rng('default');                      % Random number generator
x = fi(2*rand(1000,1)-1,true,16,15); % Input signal
a = fi(.9,true,16,15);               % Feedback gain
y = fi(zeros(size(x)),true,16,12);   % Initialize output. Fraction length
                                     % is chosen to prevent overflow
w = fi(0,true,16,12);                % Initialize delayed output
A = coder.Constant(a);               % Declare "a" constant for code
                                     % generation

Run Normal Mode

tic,
y = fiaccelFeedback(x,a,y,w);
t1 = toc;

Build the MEX Version of the Feedback Code

fiaccel fiaccelFeedback -args {x,A,y,w} -o fiaccelFeedback_mex

Run the MEX Version

tic
y2 = fiaccelFeedback_mex(x,y,w);
t2 = toc;

Acceleration Ratio

Code acceleration provides optimizations for accelerating fixed-point algorithms through MEX file generation. Fixed-Point Designer™ provides a convenience function fiaccel to convert your MATLAB code to a MEX function, which can greatly accelerate the execution speed of your fixed-point algorithms.

r = t1/t2
r =

  103.2215

Clean up Temporary Files

clear fiaccelFeedback_mex;
tempdirObj.cleanUp;