Xilinx Constraints Guide

Defining From Timing Groups
Chapter 4: Xilinx Constraints
To create an area group based on a timing group, use the following User Constraints File
(UCF) and Netlist Constraints File (NCF) syntax:
TIMEGRP timing_group_name AREA_GROUP = area_group_name ;
• timing_group_name is the name of a previously defined timing group.
• area_group_name is the name of a new area group to be defined from the TIMEGRP
contents.
This is equivalent to manually assigning each member of the timing group to
area_group_name. The area group name defined by this statement can be used in RANGE
constraints, just like any other area group name.
TNM_NET Groups
In the AREA_GROUP definition, the timing_group_name is generally a TNM_NET
group. This allows area groups to be formed based on the loads of clock or other control
nets. Defining AREA_GROUP constraints from TIMEGRP constraints can improve
placement of designs with many different clock domains in devices that have more
clocks than clock regions.
TNM and TIMEGRP Groups
You can also specify
• A TNM group name, or
• The name of a user group defined by a TIMEGRP statement.
Edge qualifiers used in the TIMEGRP definition are ignored when determining area
group membership. In all cases, the AREA_GROUP members are determined after the
TIMEGRP has been propagated to its target elements.
TIMEGRP constraints can contain only synchronous elements and pads. Area groups
defined from timing groups also contain only these element types. If an AREA_GROUP
is defined by a TIMEGRP that contains only Flip-Flops or Latches, assigning a RANGE
to that group is useful only if ungrouped logic is also allowed within the area. For this
reason, don not define COMPRESSION for such groups.
PERIOD Specifications
If a TNM_NET is used by a PERIOD specification, and is traced into any DCM, PLL, or
MMCM, new TNM_NET groups and PERIOD specifications are created at the DCM,
PLL, or MMCM outputs. If the original TNM_NET is used to define an area group, and
if more than one clock tap is used on the DCM, PLL, or MMCM, the area group is
split into separate groups at each clock tap.
For example, assume you have the following UCF constraints:
NET "clk" TNM_NET="clock";
TIMESPEC "TS_clk" = PERIOD "clock" 10 MHz;
TIMEGRP "clock" AREA_GROUP="clock_area";
If the net clk is traced into a DCM, PLL, or MMCM, a new group and PERIOD
specification is created at each clock tap. Similarly, a new area group is created at each
clock tap. A suffix indicates the clock tap name. If the CLK0 and CLK2X taps are used,
the AREA_GROUP clock_area_CLK0 and clock_area_CLK2X are defined automatically.
When AREA_GROUP definitions are split in this manner, NGDBuild issues an
informational message, showing the names of the new groups. Use these new group
names, rather than the ones originally specified, in RANGE constraints.
Constraints Guide
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