Fixed-Point Blockset

Block Parameters

Many Fixed-Point Blockset blocks use the same parameters, which you configure through the block dialog box. Some common block parameters are associated with these blockset features:

• Parameter and output data type selection
• Parameter and output scaling selection
• Autoscaling
• Rounding
• Overflow handling

Block-specific parameters are described in the block reference pages.

Selecting the Data Type and Scaling

For many fixed-point blocks, you need to associate data type and scaling information with numerical parameters and output signals. Fixed-Point Blockset blocks often provide you with the option of inheriting information from an input signal, from the next block downstream, or by an internal rule. Alternatively, you can often specify the data type and scaling yourself in the dialog. You control this option with the Output data type mode and Parameter data type mode parameters. These drop-down lists often support one or more of the following four choices:

• Specify via dialog--You explicitly specify the output data type and scaling with the Output data type and Output scaling value parameters, or the parameter data type and scaling with the Parameter data type and Parameter scaling value parameters.
• Inherit via back propagation--Specified data type and scaling information is inherited by backpropagation from the next block downstream. In many cases, you will find that the Data Type Propagation block provides you with the most flexibility when back propagating the data type.
• Inherit via internal rule--The specified data type information is inherited from the input(s). The goal of the inheritance rule is to select the "natural" data type and scaling for the output. The specific rule that is used depends on the block operation.

• For example, if you are multiplying two signed 16-bit signals, the Product block produces the natural output of a signed 32-bit data type. An "unnatural" output is produced if the inputs have different signs and different sizes. In this case, some trial and error may be required to achieve satisfactory results.

  If you are adding signals, two natural choices for the output data type and scaling are possible: to preserve the precision or to prevent overflow. However, blocks only support one rule. For example, the Sum block preserves precision. If your goal is to prevent overflow, then you should manually configure the data type and scaling.

• Same as input--The output data type and scaling are the same as the input signal.

In addition, the Output data type mode and Parameter data type mode parameters often include built-in data types in their drop-down lists for easy selection. Built-in data types can also be entered into the Output data type or Parameter data type parameter if Specify via dialog is selected for the Output data type mode or Parameter data type mode parameter.

The supported fixed-point data types that may be entered into the Output data type or Parameter data type parameter and their default scalings are shown below.

Output Data Types and Default Scaling

Data Type	Description	Default Scaling
float	Floating-point number	None
sfix	Signed generalized fixed-point number	None
sfrac	Signed fractional number	Right of the sign bit
sint	Signed integer	Right of the least significant bit
ufix	Unsigned generalized fixed-point number	None
uint	Unsigned integer	Right of the least significant bit
ufrac	Unsigned fractional number	Left of the most significant bit

In the Fixed-Point Blockset, the word size in bits of fixed-point data types is given as an argument to the data type. For example, sfix(16) specifies a 16-bit signed generalized fixed-point number. Word sizes from 1 to 128 bits are supported in simulation.

Floating-point data types are IEEE-style and are specified as float('single') for single-precision numbers and float('double') for double-precision numbers. Nonstandard IEEE-style numbers are specified as float(TotalBits,ExpBits) where TotalBits is the total number of physical bits and ExpBits is the number of exponent bits.

For more information about supported fixed-point data types and their default scaling, refer to Data Types and Scaling.

If you select Specify via dialog for the Output data type mode or Parameter data type mode parameter, you must also explicitly specify the output or parameter scaling with the Output scaling value or Parameter scaling value parameter. The supported scaling modes for generalized fixed-point data types are given below. Default scaling is used for all other fixed-point data types.

Scaling Modes for Generalized Fixed-Point Data Types 

Scaling Mode	Description
Radix point-only	Specify radix point-only (powers-of-two) scaling. For example, a scaling of 2^-10 (or pow2(-10)) places the radix point at a location 10 places to the left of the least significant bit.
[Slope Bias]	Specify [Slope Bias] scaling. For example, a scaling of [5/9 10] specifies a slope of 5/9 and a bias of 10. When using this mode, you must specify a positive slope.

Note that some blocks provide a form of radix point-only scaling for constant vectors and constant matrices. Refer to Example: Constant Scaling for Best Precision for more information.

Locking the Output Scaling

If the Lock output scaling against changes by the autoscaling tool check box is selected, then the automatic scaling tool autofixexp will not change the Output scaling value parameter. Otherwise, the automatic scaling tool is free to adjust the scaling. You can run autofixexp directly from the command line, or through the Fixed-Point Settings interface, fxptdlg.

Rounding

You can choose the rounding mode for the block operation with the Round integer calculations toward parameter list. The available rounding modes are shown below.

Rounding Modes  

Rounding Mode	Description
Zero	Round the output towards zero.
Nearest	Round the output towards the nearest representable number, with the exact midpoint rounded towards positive infinity.
Ceiling	Round the output towards positive infinity.
Floor	Round the output towards negative infinity.

Handling Overflows

Overflow handling for fixed-point numbers is specified with the Saturate on integer overflow check box. If selected, fixed-point overflow results saturate. Otherwise, overflow results wrap. Whenever a result saturates, a warning is displayed.

Common Block Features

Block Icon Labels