The following example shows how to create a fixed-point, lowpass,
direct form FIR filter in Simulink^{®}. To create the FIR filter,
you use Fixed-Point Designer™ software and the MATLAB Function block. In this example,
you perform the following tasks in the sequence shown:

Place a MATLAB Function block in a new model. You can find the block in the Simulink User-Defined Functions library.

Save your model as

`cgen_fi`

.Double-click the MATLAB Function block in your model to open the MATLAB Function Block Editor. Type or copy and paste the following MATLAB

^{®}code, including comments, into the Editor:function [yout,zf] = dffirdemo(b, x, zi) %#codegen %codegen_fi doc model example %Initialize the output signal yout and the final conditions zf Ty = numerictype(1,12,8); yout = fi(zeros(size(x)),'numerictype',Ty); zf = zi; % FIR filter code for k=1:length(x); % Update the states: z = [x(k);z(1:end-1)] zf(:) = [x(k);zf(1:end-1)]; % Form the output: y(k) = b*z yout(k) = b*zf; end % Plot the outputs only in simulation. % This does not generate C code. figure; subplot(211);plot(x); title('Noisy Signal');grid; subplot(212);plot(yout); title('Filtered Signal');grid;

Define the filter coefficients *b*, noise *x*,
and initial conditions *zi* by typing the following
code at the MATLAB command line:

b=fidemo.fi_fir_coefficients; load mtlb x = mtlb; n = length(x); noise = sin(2*pi*2140*(0:n-1)'./Fs); x = x + noise; zi = zeros(length(b),1);

Add blocks to your model to create the following system.

Set the block parameters in the model to these Fixed-Point FIR Code Example Parameter Values.

From the model menu, select

**Simulation**>**Model Configuration Parameters**and set the following parameters.Parameter Value **Stop time**`0`

**Type**`Fixed-step`

**Solver**`discrete (no continuous states)`

Click

**Apply**to save your changes.

Open the Model Explorer for the model.

Click the

**cgen_fi**>**MATLAB Function**node in the**Model Hierarchy**pane. The dialog box for the MATLAB Function block appears in the**Dialog**pane of the Model Explorer.Select

**Specify other**for the**MATLAB Function block fimath**parameter on the MATLAB Function block dialog box. You can then create the following`fimath`

object in the edit box:fimath('RoundingMethod','Floor','OverflowAction','Wrap',... 'ProductMode','KeepLSB','ProductWordLength',32,... 'SumMode','KeepLSB','SumWordLength',32)

The

`fimath`

object you define here is associated with fixed-point inputs to the MATLAB Function block as well as the`fi`

object you construct within the block.By selecting

**Specify other**for the**MATLAB Function block fimath**, you ensure that your model always uses the`fimath`

properties you specified.

Run the simulation by selecting your model and typing

**Ctrl+T**. While the simulation is running, information outputs to the MATLAB command line. You can look at the plots of the noisy signal and the filtered signal.Next, build embeddable C code for your model by selecting the model and typing

**Ctrl+B**. While the code is building, information outputs to the MATLAB command line. A folder called`coder_fi_grt_rtw`

is created in your current working folder.Navigate to

`coder_fi_grt_rtw`

>`cgen_fi.c`

. In this file, you can see the code generated from your model. Search for the following comment in your code:This search brings you to the beginning of the section of the code that your MATLAB Function block generated./* codegen_fi doc model example */

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