Xilinx Constraints Guide

Chapter 4: Xilinx Constraints
VHDL Syntax
Declare the VHDL constraint as follows:
attribute drive: string;
Specify the VHDL constraint as follows:
attribute drive of {component_name |entity_name|label_name} : {component|entity
|label} is “value”;
For a list of the constraint values, see the UCF and NCF Syntax section below.
For more information about basic VHDL syntax, see VHDL Attributes.
Verilog Syntax
Place the Verilog constraint immediately before the module or instantiation.
Specify the Verilog constraint as follows:
(* DRIVE = “value” *)
For a list of the constraint values, see the UCF and NCF Syntax section below.
For more information about basic Verilog syntax, see Verilog Attributes.
UCF and NCF Syntax
This section gives UCF and NCF Syntax examples for the following:
• IOB Output Components (UCF)
• SelectIO Technology Output Components
IOB Output Components (UCF)
For Spartan®-3 and higher devices or Virtex®-4 and higher devices:
INST “instance_name” DRIVE={2|4|6| 8|12|16 |24};
12 mA is the default
SelectIO Technology Output Components
This section applies to the following components:
• IOBUF_SelectIO
• OBUF_SelectIO
• OBUFT_SelectIO
The following table shows syntax examples for the named standard with Spartan-3
devices and higher or Virtex-4 devices and higher. The default in each case is 12 mA.
Standard Syntax
LVTTL INST “instance_name” DRIVE={2|4|6|8|12|16|24};
LVCMOS12
LVCMOS15
LVCMOS18
LVCMOS25
LVCMOS33
INST “instance_name” DRIVE={2|4|6|8|12|16};
INST “instance_name” DRIVE={2|4|6|8|12|16|24};
Constraints Guide
106 www.xilinx.com UG625 (v. 13.2) July 6, 2011