Xilinx Constraints Guide

Chapter 4: Xilinx Constraints
OFFSET IN (Offset In)
Architecture Support
Applicable Elements
The OFFSET IN (Offset In) constraint:
• Specifies the timing requirements of an input interface to the FPGA device.
• Specifies the clock and data timing relationship at the external pads of the FPGA
device.
An OFFSET IN constraint specification checks the setup and hold timing requirements
of all synchronous elements associated with the constraint.
The OFFSET IN constraint is specified using a clock net name. The clock net associated
with the OFFSET IN constraint is the external clock pad. Because the constraint specifies
the clock and data relationship at the external pads of the FPGA, the OFFSET IN
constraint cannot be specified using an internal clock net. However, the OFFSET IN
constraint automatically accounts for any phase or delay adjustments on the clock path
due to components such as the DCM, PLL, MMCM, or IDELAY when analyzing the
setup and hold timing requirements at the capturing synchronous element. In addition,
the constraint propagates through the clock network and automatically applies to all
clocks derived from the original external clock.
The OFFSET IN constraint is global in scope by default. In the global OFFSET IN
constraint, all synchronous elements that are clocked by the specified clock net, and
capture external data, are covered by the constraint. The scope of the synchronous
elements covered by the constraint can be restricted by specifying time groups on a
subset of input data pads, a subset of the capturing synchronous elements, or both.
Applies to all FPGA devices and all CPLD devices.
• Global
• Net-Specific
• Pad Time Group
Syntax Examples
The following examples show how to use this constraint with particular tools or
methods. If a tool or method is not listed, you cannot use this constraint with it.
Although User Constraints File (UCF) examples are given below, Xilinx® recommends
specifying the OFFSET IN constraint using Constraints Editor.
Global Method
The global method is the default OFFSET IN constraint. The global OFFSET IN
constraint applies to all synchronous elements that capture incoming data and are
triggered by the specified clock signal.
Global Method UCF Syntax Example
OFFSET = IN “offset_time” [units] [VALID [UNITS]] {BEFORE|AFTER}
“clk_name” [{RISING|FALLING}];
Global Method PCF Syntax Example
OFFSET = IN “offset_time” [units] [VALID [UNITS]] {BEFORE|AFTER}
COMP “clk_iob_name” [{RISING|FALLING}];
Constraints Guide
194 www.xilinx.com UG625 (v. 13.2) July 6, 2011