Constant - 2020.2 English

Vivado Design Suite Reference Guide: Model-Based DSP Design Using System Generator (UG958)

Document ID
Release Date
2020.2 English

This block is listed in the following Xilinx® Blockset libraries: Basic Elements, Control Logic, Math, Floating-Point and Index.

The Xilinx Constant block generates a constant that can be a fixed-point value, or a Boolean value. This block is similar to the Simulink® constant block, but can be used to directly drive the inputs on Xilinx blocks.

DSP48 Instruction Mode

The constant block, when set to create a DSP48 instruction, is useful for generating DSP48 control sequences. The the figure below shows an example. The example implements a 35x35-bit multiplier using a sequence of four instructions in a DSP48 block. The constant blocks supply the desired instructions to a multiplexer that selects each instruction in the desired sequence.

Figure 1. Example of Constant Block Creating a DSP48 Instruction

Block Parameters

The block parameters dialog box can be invoked by double-clicking the icon in your Simulink® model.

Basic tab

Parameters specific to the Basic tab are as follows:

Constant Value

Specifies the value of the constant. When changed, the new value appears on the block icon. If the constant data type is specified as fixed-point and cannot be expressed exactly in the specified fixed-point type, its value is rounded and saturated as needed. A positive value is implemented as an unsigned number, a negative value as signed.

Output Type
Specifies the data type of the output. Can be Boolean, Fixed-point, or Floating-point.
Arithmetic Type
If the Output Type is specified as Fixed-point, you can select Signed (2’s comp), Unsigned, or DSP48 instruction as the Arithmetic Type.
Fixed-point Precision
Number of bits
Specifies the bit location of the binary point of the output number, where bit zero is the least significant bit.
Binary point
Position of the binary point in the fixed-point output.
Floating-point Precision
Specifies single precision (32 bits).
Specifies double precision (64 bits)
Activates the field below so you can specify the Exponent width and the Fraction width.
Exponent width
Specifies the exponent width.
Fraction width
Specifies the fraction width.
Sample Period
Sampled Constant
Allows a sample period to be associated with the constant output and inherited by blocks that the constant block drives. (This is useful mainly because the blocks eventually target hardware and the Simulink sample periods are used to establish hardware clock periods.)
DSP48 tab
DSP48 Instruction
The use of this block for DSP48 instructions is deprecated. Please use the Opmode block.

Other parameters used by this block are explained in the topic Common Options in Block Parameter Dialog Boxes.