DSP Blockset

DSP Constant

Generate a discrete-time or continuous-time constant signal

Library

DSP Sources

Description

The DSP Constant block generates a signal whose value remains constant throughout the simulation. The Constant value parameter specifies the constant to output, and can be any valid MATLAB expression that evaluates to a scalar, vector, or matrix.

When Sample mode is set to Continuous, the output is a continuous-time signal. When Sample mode is set to Discrete, the Sample time parameter is visible, and the signal has the discrete output period specified by the Sample time parameter.

You can set the output signal to Frame-based, Sample-based, or Sample-based (interpret vectors as 1-D) with the Output parameter.

Dialog Box

Opening this dialog box causes a running simulation to pause. See Changing Source Block Parameters in the online Simulink documentation for details.

Constant value

Specify the constant to generate. This parameter is tunable; values entered here can be tuned, but their dimensions must remain fixed.

If you specify any data type information in this field, it is overridden by the value of the Output data type parameter.

Sample mode

Specify the sample mode of the output, Discrete for a discrete-time signal or Continuous for a continuous-time signal.

Output

Specify whether the output is Sample-based (interpret vectors as 1-D), Sample-based, or Frame-based. When Sample-based is selected and the output is a vector, its dimension is constrained to match the Constant value dimension (row or column). If Sample-based (interpret vectors as 1-D) is selected, however, the output has no specified dimensionality.

Sample time

Specify the discrete sample period for sample-based outputs. When Frame-based is selected for the Output parameter, this parameter is named Frame period, and is the discrete frame period for the frame-based output. This parameter is only visible when Discrete is selected for the Sample mode parameter.

Show additional parameters

If selected, additional parameters specific to implementation of the block become visible as shown.

Allow overrides from DSP Fixed-Point Attributes blocks

If you select this parameter, and if the Output data type parameter is set to Fixed-point, fixed-point data types for this block may be set by DSP Fixed-Point Attributes blocks in your model. If this parameter is unselected, the data types are always set by the parameters in the block mask.

Output data type

Specify the output data type in one of the following ways:

• Choose one of the built-in data types from the drop-down list.
• Choose Fixed-point to specify the output data type and scaling in the Word length, Set fraction length in output to, and Fraction length parameters.
• Choose User-defined to specify the output data type and scaling in the User-defined data type, Set fraction length in output to, and Fraction length parameters.
• Choose Inherit from `Constant value' to set the output data type and scaling to match the values of the Constant value parameter.
• Choose Inherit via back propagation to set the output data type and scaling to match the following block.
  The value of this parameter overrides any data type information specified in the Constant value parameter.

Word length

Specify the word length, in bits, of the fixed-point output data type. This parameter is only visible if Fixed-point is selected for the Output data type parameter.

User-defined data type

Specify any built-in or fixed-point data type. You can specify fixed-point data types using the sfix, ufix, sint, uint, sfrac, and ufrac functions from the Fixed-Point Blockset. This parameter is only visible if User-defined is selected for the Output data type parameter.

Set fraction length in output to

Specify the scaling of the fixed-point output by either of the following two methods:

• Choose Best precision to have the output scaling automatically set such that the output signal has the best possible precision.
• Choose User-defined to specify the output scaling in the Fraction length parameter.
  This parameter is only visible if Fixed-point or User-defined is selected for the Output data type parameter, and if the specified output data type is a fixed-point data type.

Fraction length

For fixed-point output data types, specify the number of fractional bits, or bits to the right of the binary point. This parameter is only visible if Fixed-point or User-defined is selected for the Output data type parameter, and if User-defined is selected for the Set fraction length in output to parameter.

Supported Data Types

• Double-precision floating point
• Single-precision floating point
• Fixed point
• 8-, 16-, and 32-bit signed integers
• 8-, 16-, and 32-bit unsigned integers

To learn how to convert your data types to the above data types in MATLAB and Simulink, see Supported Data Types and How to Convert to Them.

See Also

Constant	Simulink
Signal From Workspace	DSP Blockset

Also see Creating Signals Using Constant Blocks for how to use this and other blocks to generate constant signals.

Downsample

DSP Fixed-Point Attributes