Ways to Construct fi Objects - MATLAB & Simulink (original) (raw)

You can create a fi object by using a fi constructor function or you can build fi object constructors using theInsert fi Constructor dialog box. You can also use afi constructor function to copy an existing fi object.

numerictype and fimath properties can be specified directly in the fi constructor function or you can use an existingnumerictype or fimath object to construct afi object. The value of a property is taken from the last time it is set.

You can write a reusable MATLAB® algorithm by keeping the data types of the algorithmic variables in a separate types table.

Use fi Constructor to Create fi Objects

These examples show you several different ways to construct fi objects.

Construct fi Object with Default Data Type and Property Values

Create a fi object with the default data type and a value of 0.

a =

 0


      DataTypeMode: Fixed-point: binary point scaling
        Signedness: Signed
        WordLength: 16
    FractionLength: 15

The default fi constructor syntax creates a signedfi object with a value of 0, word length of 16 bits, and fraction length of 15 bits.

Note

The fi constructor creates the fi object using a RoundingMethod ofNearest and an OverflowAction of Saturate. If you construct a fi from floating-point values, the default RoundingMethod and OverflowAction property settings are not used.

For information on the display format of fi objects, refer to View Fixed-Point Data.

Copy a fi Object

To copy a fi object, use assignment.

a =

3.1416

      DataTypeMode: Fixed-point: binary point scaling
        Signedness: Signed
        WordLength: 16
    FractionLength: 13

b =

3.1416

      DataTypeMode: Fixed-point: binary point scaling
        Signedness: Signed
        WordLength: 16
    FractionLength: 13

Construct fi Object with Property Name-Value Pair Arguments

You can use property name-value pair arguments to set fi object properties in the fi constructor. Thefi object has three types of properties:

For example, specify the fimath object properties for the rounding method and overflow action to use when performing fixed-point arithmetic.

a = fi(pi,'RoundingMethod','Floor',... 'OverflowAction','Wrap')

a = 

3.1415

      DataTypeMode: Fixed-point: binary point scaling
        Signedness: Signed
        WordLength: 16
    FractionLength: 13

    RoundingMethod: Floor
    OverflowAction: Wrap
       ProductMode: FullPrecision
           SumMode: FullPrecision

If you specify at least one fimath object property in thefi constructor, the fi object has a local fimath object. The fi object uses default values for the remaining unspecified fimath object properties.

If you do not specify any fimath object properties in thefi object constructor, the fi object uses default fimath values and has no localfimath.

You can use the isfimathlocal function to determine whether a fi object has a localfimath associated with it.

Construct fi Object Using numerictype Object

You can create a fi object using anumerictype object. The numerictype Properties define the data type and scaling attributes of a fi object.

Create a numerictype object with default property values.

T =

      DataTypeMode: Fixed-point: binary point scaling
        Signedness: Signed
        WordLength: 16
    FractionLength: 15

Create a fi object from the numerictype object T.

a =

1.0000


      DataTypeMode: Fixed-point: binary point scaling
        Signedness: Signed
        WordLength: 16
    FractionLength: 15

You can use a fimath object and anumerictype object in the fi constructor.

F = fimath('RoundingMethod','Nearest',... 'OverflowAction','Saturate',... 'ProductMode','FullPrecision',... 'SumMode','FullPrecision')

F =

    RoundingMethod: Nearest
    OverflowAction: Saturate
       ProductMode: FullPrecision
           SumMode: FullPrecision

a =

1.0000


      DataTypeMode: Fixed-point: binary point scaling
        Signedness: Signed
        WordLength: 16
    FractionLength: 15

    RoundingMethod: Nearest
    OverflowAction: Saturate
       ProductMode: FullPrecision
           SumMode: FullPrecision

Note

The syntax a = fi(pi,T,F) is equivalent to a = fi(pi,F,T). You can use both statements to define afi object using a fimath object and a numerictype object.

Construct fi Object Using fimath Object

You can create a fi object using a specificfimath object. When you do so, a localfimath object is assigned to the fi object you create. If you do not specify any numerictype object properties, the word length of the fi object defaults to 16 bits. The fraction length is determined by best precision scaling.

For example, create a fimath object that specifies the rounding method, overflow action, product mode, and sum mode to use.

F = fimath('RoundingMethod','Nearest',... 'OverflowAction','Saturate',... 'ProductMode','FullPrecision',... 'SumMode','FullPrecision')

F =

         RoundingMethod: Nearest
      OverflowAction: Saturate
       ProductMode: FullPrecision
           SumMode: FullPrecision

Use dot notation to change the overflow action of thefimath object F.

F.OverflowAction = 'Wrap'

F =

         RoundingMethod: Nearest
      OverflowAction: Wrap
       ProductMode: FullPrecision
           SumMode: FullPrecision

Create a fi object using the fimath object F.

a =

3.1416

      DataTypeMode: Fixed-point: binary point scaling
        Signedness: Signed
        WordLength: 16
    FractionLength: 13

    RoundingMethod: Nearest
    OverflowAction: Wrap
       ProductMode: FullPrecision
           SumMode: FullPrecision

You can also create fi objects using afimath object while specifying variousnumerictype properties at creation time. For example, create an unsigned fi object with a value ofpi, word length of 8 bits, fraction length of 6 bits, andfimath F.

b = 

3.1406

      DataTypeMode: Fixed-point: binary point scaling
        Signedness: Unsigned
        WordLength: 8
    FractionLength: 6

    RoundingMethod: Nearest
    OverflowAction: Wrap
       ProductMode: FullPrecision
           SumMode: FullPrecision

Use the Insert fi Constructor Dialog Box to Build fi Object Constructors

You can build fi object constructors in MATLAB by using the Insert fi Constructor dialog box. After specifying the value and properties of the fi object in the dialog box, you can insert the prepopulated fi object constructor at a specific location in your file.

For example, create a signed fi object with a value ofpi, a word length of 16 bits and a fraction length of 13 bits.

  1. On the MATLABHome tab, in the File section, click New Script.
  2. On the Editor tab, in the Code section, click the Specify fixed-point data button arrow . Click Insert fi to open the Insert fi Constructor dialog box.
  3. Use the edit boxes and drop-down menus to specify the following properties of the fi object:
    • Value = pi
    • Data type mode =Fixed-point: binary point scaling
    • Signedness =Signed
    • Word length =16
    • Fraction length =13
  4. To insert the fi object constructor in your file, place your cursor at the desired location in the file, then clickOK on the Insert fi Constructor dialog box. Clicking OK closes the Insert fi Constructor dialog box and automatically populates the fi object constructor in your file.

Determine Property Precedence

The value of a property of a fi object is taken from the last time it is set. For example, create a numerictype object with theSigned set to true and a fraction length of 14.

T = numerictype('Signed',true,... 'FractionLength',14)

T =

      DataTypeMode: Fixed-point: binary point scaling
        Signedness: Signed
        WordLength: 16
    FractionLength: 14

Create a fi object that specifies thenumerictype property T after the Signed property. The resultingfi object is signed.

a = fi(pi,'Signed',false,... 'numerictype',T)

a =

1.9999

      DataTypeMode: Fixed-point: binary point scaling
        Signedness: Signed
        WordLength: 16
    FractionLength: 14

Create a second fi object that specifies thenumerictype T before the Signed property. The resultingfi object is unsigned.

b = fi(pi,'numerictype',T,... 'Signed',false)

b =

3.1416

      DataTypeMode: Fixed-point: binary point scaling
        Signedness: Unsigned
        WordLength: 16
    FractionLength: 14

Create fi Objects for Use in a Types Table

You can write a reusable MATLAB algorithm by keeping the data types of the algorithmic variables in a separate types table. For example,

function T = mytypes(dt) switch dt case 'double' T.b = double([]); T.x = double([]); T.y = double([]);

 case 'fixed16'
  T.b = fi([],1,16,15);
  T.x = fi([],1,16,15);
  T.y = fi([],1,16,14);

end end

Cast the variables in the algorithm to the data types in the types table as described in Manual Fixed-Point Conversion Best Practices.

function [y,z]=myfilter(b,x,z,T) y = zeros(size(x),'like',T.y); for n=1:length(x) z(:) = [x(n); z(1:end-1)]; y(n) = b * z; end end

In a separate test file, set up input data to feed into your algorithm, and specify the data types of the inputs.

% Test inputs b = fir1(11,0.25); t = linspace(0,10*pi,256)'; x = sin((pi/16)*t.^2); % Linear chirp

% Cast inputs T=mytypes('fixed16'); b=cast(b,'like',T.b); x=cast(x,'like',T.x); z=zeros(size(b'),'like',T.x);

% Run [y,z] = myfilter(b,x,z,T);

See Also

fi | fimath | fipref | numerictype

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