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AR# 62118

Why are there LUT2, LUT3, LUT4, LUT5, LUT6 primitives as buffers for some of my clocks in the clock networks report? Why is the D-input of the register the load and not the C (clock) input?

Description

When analyzing the clock network, in some designs Vivado shows LUT primitive buffers for my clocks:

report_clock_networks-LUT-buffer.png
In some cases, the input of the register is shown in the clock network report:

screenshot_001.jpg

Is this expected behavior of the clock networks report?

Solution

"report_clock_networks" traces down every clock in design. 

A clock is a signal that drives either the G-input of a latch or the clock inputs of sequential elements like registers, block RAMs and DSPs.

As soon as a signal is identified as a clock, it will show the entire tree of that signal from the source to all of the loads, including the non-sequential elements like LUTs.

An improvement is planned to add a filter in the clock networks report and only show the sequential elements.


The clock networks report shows the real situation of the design. 

If the clock is buffered by a LUT, this will be shown in the clock networks report.

AR# 62118
Date Created 09/21/2014
Last Updated 11/19/2014
Status Active
Type General Article
Tools
  • Vivado Design Suite