fanout violation when synthesis.

henry_wangjj

Hi,

suppose the given case:

the external system reset port rst_n is a high-fanout-net, however, when doing synthesis, using the constraints below

(1) I don't set_ideal_network attributes on rst_n, letting DC build the buffer tree.
(2) set_max_fanout 30 [get_ports rst_n]

when report_constraint -all_violators -max_fanout, violations occur on the port rst_n, showing a negtive slack.

Why doesn't this happen, and how do I fix the violation if I fix this during synthesis( not when implementating the layout).

Thanks in advance.

陈涛

did you set driven cell on rst_n?

henry_wangjj

回复 2# 陈涛


    No driven cell is set.

Now I think it's violated by the tight max_fanout constraint. I checked the library and found that the max_fanout of the output_pin of the
rst_n PAD is 600. In this case the slack is 600 - 150 = 450, which is a positive slack.And I could ignore this violation by remove the tight max_fanout constraint, or loosen the constraint.

Is this right?

Thanks in advance.

henry_wangjj

回复 2# 陈涛

No driving cell is set.

I checked the library and found that the output pins of the rst_n PAD has a max_fanout attribute of 600, In this case, the slack would be
600 - 160 > 0, which is a positive slack.
and I could remove the tight max_fanout constraint or loosen the constraint to get a no-violations result, right?

yeah?

thanks in advance.