AddSub - 2022.1 English

Vitis Model Composer User Guide (UG1483)

Document ID
Release Date
2022.1 English

The Xilinx AddSub block implements an adder/subtractor. The operation can be fixed (Addition or Subtraction) or changed dynamically under control of the sub mode signal.

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:
Specifies the block operation to be Addition, Subtraction, or Addition/ Subtraction. When Addition/Subtraction is selected, the block operation is determined by the sub input port, which must be driven by a Boolean signal. When the sub input is 1, the block performs subtraction. Otherwise, it performs addition.
Provide carry-in port
When selected, allows access to the carry-in port, cin.
Provide carry-out port
When selected, allows access to the carry-out port, cout. The carry-out port is available only when User defined precision is selected, the inputs and output are unsigned, and the number of output integer bits equals x, where x = max (integer bits a, integer bits b).
The Latency value defines the number of sample periods by which the block's output is delayed. One sample period might correspond to multiple clock cycles in the corresponding FPGA implementation (for example, when the hardware is over-clocked with respect to the Simulink model). Model Composer will not perform extensive pipelining unless you select the Pipeline for maximum performance option (on the Implementation tab, described below); additional latency is usually implemented as a shift register on the output of the block.
Output tab

This parameter allows you to specify the output precision for fixed-point arithmetic. Floating point arithmetic output will always be Full precision.

The block uses sufficient precision to represent the result without error.
User Defined
If you do not need full precision, this option allows you to specify a reduced number of total bits and/or fractional bits.
Fixed-point Output Type
Arithmetic Type
Signed (2’s comp)
The output is a Signed (2’s complement) number.
The output is an Unsigned number.
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.
Refer to the section Overflow and Quantization.
Refer to the section Overflow and Quantization.
Implementation tab

Parameters specific to the Implementation tab are as follows:

Use behavioral HDL (otherwise use core)
The block is implemented using behavioral HDL. This gives the downstream logic synthesis tool maximum freedom to optimize for performance or area.
Note: For Floating-point operations, the block always uses the Floating-point Operator core.
Core Parameters
Pipeline for maximum performance

The XILINX LogiCORE can be internally pipelined to optimize for speed instead of area. Selecting this option puts all user defined latency into the core until the maximum allowable latency is reached. If the Pipeline for maximum performance option is not selected and latency is greater than zero, a single output register is put in the core and additional latency is added on the output of the core.

The Pipeline for maximum performance option adds the pipeline registers throughout the block, so that the latency is distributed, instead of adding it only at the end. This helps to meet tight timing constraints in the design.

Implement using
Core logic can be implemented in Fabric, or in a DSP48, if a DSP48 is available in the target device. The default is Fabric.

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

LogiCORE™ Documentation

Adder/Subtractor LogiCORE IP Product Guide (PG120)

Floating-Point Operator LogiCORE IP Product Guide (PG060)