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AR# 10022

12.1 Constraints - How do I apply a FROM:TO constraint with generic time groups?

Description

What are generic time groups? How do I apply a FROM:TO constraint with generic time groups?

Solution

Overview of FROM:TO

Figure 1 - FROM TO example of design
Figure 1 - FROM TO example of design

There are four generic time groups: PADS, FFS, RAMS, and LATCHS. The generic time groups are keywords and should not be used when creating user-defined time groups.

- The PADS are all of the pads in the design, and are not considered synchronous elements.
- The FFS, RAMS, and LATCHS are all of the flip-flops, RAMs, and latches in the design, respectively. They are synchronous elements.

The FROM:TO constraint specifies the delays between the starting and ending point. The generic time groups can be used to cover the design shown in Figure 1. The following example of the UCF specification illustrates the use of generic time groups:

NET CLK PERIOD = 20;
TIMESPEC TS_p2p = FROM PADS TO PADS 35ns;
TIMESPEC TS_p2f = FROM PADS TO FFS 12ns;
TIMESPEC TS_f2p = FROM PADS TO PADS 15ns;

An example of the timing report is as follows:

=================================================================

Timing constraint: TS_COMB = MAXDELAY FROM TIMEGRP "PADS"

TO TIMEGRP "PADS" 35nS;

1 item analyzed, 0 timing errors detected.

Maximum delay is 9.076ns.

---------------------------------------------------------------------------------------------------

Slack:

25.924ns path $Net00003_ to $Net00004_ relative to
35.000ns delay constraint

For more details on timing constraints, please see the Timing Constraints User Guide: http://www.xilinx.com/support/documentation/sw_manuals/xilinx12_1/ug612.pdf

AR# 10022
Date Created 08/29/2007
Last Updated 12/15/2012
Status Active
Type General Article