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AR# 20964

12.1 Known Issue - Timing - Clock arrival time on falling to falling path incorrect

Description

When I do timing analysis, there are timing failures, which I believe do not exist because the clock arrival time for the destination clocks is not correct.

The Source clock arrival time is computed correctly:

TIMESPEC PHASE + PERIOD/2 = 1.67 (see UCF) + 2.5 = 4.170 ns.

The Destination clock arrival time is off by PERIOD/2. The arrival time should be:

TIMESPEC PHASE + DCM PHASE_SHIFT + PERIOD/2 = 4.625 + 1.288 + 2.5 = 8.413 ns.

Instead, the PERIOD/2 part is left off and it is calculated as:

TIMESPEC PHASE + DCM PHASE SHIFT = 4.625 + 1.288 = 5.913.

Why do the timing analysis tools produce the wrong clock edge and when is this going to be fixed?

Solution

You have is a set of constraints that are related to one another in the following way:

TS_A = 5ns;

TS_B = TS_A / 2 + PHASE 4.625;

TS_C = TS_B * 2 + PHASE 1.289;

To work around this issue, relate TS_C back to TS_A instead of TS_B.

This is scheduled to be fixed in the next major design tools release.

AR# 20964
Date Created 09/04/2007
Last Updated 05/12/2012
Status Archive
Type Known Issues