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AR# 10023

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

Description

How can I apply a FROM:TO constraint with specific user-created time groups?

Solution

FROM:TO: Multi-Cycle Path

Figure 1 - FROM TO - Multi-Cycle Path
Figure 1 - FROM TO - Multi-Cycle Path

A FROM:TO constraint begins at a synchronous element and ends at a synchronous element. This is also referred to as a "multi-cycle path," because these paths are running faster or slower than the PERIOD constraint. The multi-cycle path can also mean that there is more than one cycle between each enabled clock edge.

The following example of the UCF specification illustrates the use of specific time groups:

NET CLK PERIOD = 20;
TIMESPEC TS_MULTI = FROM SOURCE TO DESTINATION 40ns;

An example of the timing report with this constraint:

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

Timing constraint: TS_MULTI = MAXDELAY FROM TIMEGRP "SOURCE" TO TIMEGRP "DESTINATION" 40 nS;

1 item analyzed, 0 timing errors detected.

Maximum delay is 3.108ns.

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

Slack:

36.892ns path DST to DO relative to
3.106ns total path delay
0.002ns clock skew
40.000ns 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

FROM:TO : Multiple Clocks

Figure 2 - FROM TO - Multiple Clocks
Figure 2 - FROM TO - Multiple Clocks

A FROM:TO constraint can cover the multi-cycle paths as described in Resolution 1, but it can also cover the paths between clock domains. (One clock covers a portion of the design and another clock covers the rest, but there are paths that go between these two clock domains.)

The designer must have a clear idea of the design specifics, and the multiple clock domains need to be taken into account.

An example of the UCF specification:

NET CLKA PERIOD = 20;
NET CLKB PERIOD = 15;
TIMESPEC TS_2CLKS = FROM CLKA TO CLKB 20ns;

An example of the timing report with this constraint:

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

Timing constraint: TS_2CLKS = MAXDELAY FROM TIMEGRP "CLKA" TO TIMEGRP "CLKB" 20 nS;
1 item analyzed, 0 timing errors detected.
Maximum delay is 3.289ns.

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

Slack:

16.711ns path FFB to DO relative to
20.000ns delay constraint

AR# 10023
Date Created 08/29/2007
Last Updated 12/15/2012
Status Active
Type General Article