# Specify Fixed-Point Math Properties in MATLAB Function Block

This example shows how to specify fixed-point math properties in a MATLAB® Function block. You can specify `fimath`

properties to add fixed-point numbers without bit growth. The same methods also apply to subtraction and multiplication.

### Define Fixed-Point Variables with Attached `fimath`

Define fixed-point variables `A`

and `B`

with attached `fimath`

.

clearvars F = fimath(... 'RoundMode','Fix',... 'OverflowMode','Wrap',... 'ProductMode','SpecifyPrecision',... 'ProductWordLength',32,... 'ProductFractionLength',16,... 'SumMode','SpecifyPrecision',... 'SumWordLength',32,... 'SumFractionLength',16,... 'CastBeforeSum',true); A = fi(1,true,32,16,F); B = fi(1,true,32,16,F);

Define a structure `T`

containing prototypes of the variables `A`

and `B`

.

T.A = cast(0,'like',A); T.B = cast(0,'like',B);

The structure `T`

functions as a types table. The *values* of the fields of `T`

are not important. You will use the *data types* of the fields of `T`

later in this example to specify fixed-point types that carry the `fimath`

along with them.

Add the fixed-point variables `A`

and `B`

. In MATLAB, the `fimath`

attached to variables `A`

and `B`

specify that the sum is to be computed as 32-bit word length and 16-bit fraction length.

Y = A + B

Y = 2 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 32 FractionLength: 16 RoundingMethod: Zero OverflowAction: Wrap ProductMode: SpecifyPrecision ProductWordLength: 32 ProductFractionLength: 16 SumMode: SpecifyPrecision SumWordLength: 32 SumFractionLength: 16 CastBeforeSum: true

The `fimath`

propagates to the variable `Y`

.

### Default MATLAB Function Block Behavior

In general, the Simulink software does not propagate `fimath`

on fixed-point `fi`

objects. This rule applies even if the `fi`

objects have attached `fimath`

in a Constant block or are passed in as a MATLAB Function block parameter. However, any attached `fimath`

defined *inside* a MATLAB Function block are respected. One exception to the rule about parameters is described in the next section.

This function is defined in the block named `MATLAB Function 1`

in the `mParameterFIMath`

model. If you execute this function in MATLAB, it returns the same 32-bit data type as the `Y = A + B`

example.

function Y1 = default_behavior(A,B) Y1 = A + B; end

Simulate the `mParameterFIMath`

model to see how the MATLAB Function block executes this code.

model = 'mParameterFIMath'; open_system(model); sim(model); Y1 = logsout.get('Y1').Values.Data

Y1 = 2 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 33 FractionLength: 16

The `MATLAB Function 1`

block returns `Y1`

with a 33-bit word length instead of the 32-bit word length returned in MATLAB. To see why, open the **Property Inspector** pane for the `MATLAB Funtion 1`

block.

The `MATLAB Function 1`

block returns a 33-bit word length output because its **MATLAB Function **** fimath** setting has been set to

**Same as MATLAB**. The input parameters

`A`

and `B`

are stripped of their attached `fimath`

and instead use the default `fimath`

settings from MATLAB. The default `fimath`

in MATLAB does full-precision sums. Therefore, the sum of two 32-bit variables return a 33-bit result.To see the default `fimath`

settings in MATLAB, first reset any global `fimath`

settings, then enter `fimath`

in the MATLAB Command Window.

resetglobalfimath fimath

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

You can also to change the value of `globalfimath`

, but doing so changes the fixed-point math behavior globally. This method is not recommended.

### Pass `fimath`

into MATLAB Function Block from Parameter

In general, the MATLAB Function block strips `fimath`

from fixed-point inputs. The exception to this rule is if an input parameter is a non-tunable structure, such as this one.

T.A = cast(0,'like',A); T.B = cast(0,'like',B);

In the `mParameterFIMath`

model, the `MATLAB Function 2`

block has the structure `T`

defined as a non-tunable input parameter.

The fields `T.A`

and `T.B`

carry the data type and `fimath`

of `A`

and `B`

. If you cast the inputs `A`

and `B`

like `T.A`

and `T.B`

, respectively, you recover the `fimath`

that was defined in MATLAB.

function Y2 = cast_like_non_tunable_types(A, B, T) A = cast(A,'like',T.A); B = cast(B,'like',T.B); Y2 = A + B; end

The output of the `MATLAB Function 2`

block has the desired 32-bit word length.

`Y2 = logsout.get('Y2').Values.Data`

Y2 = 2 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 32 FractionLength: 16

This workflow provides a robust way of defining fixed-point data types in MATLAB Function blocks because it allows for the definition of different `fimath`

and data type for each different variable. This method has these advantages:

The algorithm and type specification can be separate, with the types controlled in a dictionary separate from the block. Changing the type does not change the algorithm or block.

MATLAB and MATLAB Function blocks in Simulink have identical behavior.

Each parameter can have its own

`fimath`

and data type.Each parameter can change to be types other than fixed-point. For instance,

`T.A = single(0); T.B = single(0);`

would change all types in this example to single without having to use global data-type override settings.

### Set `fimath`

for All Input Parameters

An alternative way to define `fimath`

in a MATLAB Function block is to declare the `fimath`

in the **Fixed-point properties** of the MATLAB Function block.

In the `mParameterFIMath`

model, the `MATLAB Function 3`

block contains this code.

function Y3 = mlfb_specify_fimath(A, B) Y3 = A + B; end

In the **Property Inspector** pane, under **Fixed-point properties > MATLAB Function **** fimath**, the option

**Specify Other**is selected. The

`fimath`

is defined as the variable `F`

from above.Alternatively, you can write the `fimath`

definition directly in the **MATLAB Function **** fimath** box.

Confirm that the output of this block has the desired 32-bit word length.

`Y3 = logsout.get('Y3').Values.Data `

Y3 = 2 DataTypeMode: Fixed-point: binary point scaling Signedness: Signed WordLength: 32 FractionLength: 16

This method has these limitations:

All MATLAB Function block input parameters get the same

`fimath`

.Each MATLAB Function block must be modified to specify the

`fimath`

.