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AR# 60769

2013.4 Vivado implementation - Input FFs are transformed to IDDR with no user control

Description

A case has been reported where for an input path driving multiple FFs, two of the FFs are transformed into an IDDR which has to be placed in the ILOGIC component.

This configuration was causing hold time issues that could be solved by preventing the IDDR transformation.

The transformation is done by a phase called Mandatory Logic Optimization which cannot be bypassed.

It was found that IOB=FALSE and DONT_TOUCH had no effect on preventing the optimization as expected.

Solution

A Change Request has been filed so that the IOB=FALSE and DONT_TOUCH properties will be respected by Mandatory Logic Optimization.

This is expected to be fixed in release 2014.3.

The recommended work-around is to instantiate a LUT1 buffer in the input path and apply a DONT_TOUCH property to prevent its removal.

This should isolate the FFs from the I/O connectivity and avoid the IDDR transformation.
AR# 60769
Date Created 05/21/2014
Last Updated 05/22/2014
Status Active
Type General Article
Devices
  • Kintex-7
Tools
  • Vivado Design Suite - 2014.1
  • Vivado Design Suite - 2013.4