Xilinx 8.2i manual Offset in Constraint Examples, Offset in Header, Offset in Path Details

Chapter 12: TRACE

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OFFSET IN Constraint Examples

This section describes in detail a specific example of an OFFSET IN constraint as shown in the Timing Constraints section of a timing analysis report. For clarification, the OFFSET IN constraint information is divided into the following parts:

•OFFSET IN Header

•OFFSET IN Path Details

•OFFSET IN Detailed Path Data

•OFFSET IN Detail Path Clock Path

•OFFSET IN with Phase Clock

OFFSET IN Header

The header includes the constraint, the number of items analyzed, and number of timing errors detected. Please see PERIOD Header for more information on items analyzed and timing errors.

Example:

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

Timing constraint: OFFSET = IN 4 nS BEFORE COMP "wclk_in" ;

113 items analyzed, 30 timing	errors detected.
Minimum allowable offset is	4.468ns.

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The minimum allowable offset is 4.468 ns. Because this is an OFFSET IN BEFORE, it means the data must be valid 4.468 ns before the initial edge of the clock. The PERIOD constraint was defined with the keyword HIGH, therefore the initial edge of the clock is the rising edge.

OFFSET IN Path Details

This path fails the constraint by 0.468 ns. The slack equation shows how the slack was calculated. In respect to the slack equation data delay increases the setup time while clock delay decreases the setup time. The clock arrival time is also taken into account. In this example, the clock arrival time is 0.000 ns; therefore, it does not affect the slack.

Example:

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

Slack:	-0.468ns (requirement - (data path - clock path
- clock arrival + uncertainty))
Source:	wr_enl (PAD)
Destination:	wr_addr[2] (FF)
Destination Clock:	wclk rising at 0.000ns
Requirement:	4.000ns

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