Answers Database

XC5200: BUFG - different skew between clock pin and non clock pin.


Record #1305

Product Family:  Hardware

Product Line:  5200

Problem Title:
XC5200:	BUFG - different skew between clock pin and non clock pin.


Problem Description:
The BUFG of the XC5200 family of devices drives clock signal
to the flip-flops.  A low skew is guaranteed, typically less
than 1ns.

The BUFG of the XC5200 family is also capable of driving non
clock signals such as CE or LUT (Look-up Table) inputs . The
skew between CLBs that are in the same column is still low.
However the skew between the CLBs placed in different columns
can increase if the fanout and/or the type of loading on the
vertical longline related to this global signal is differs
from one column to another.

For instance :

BUFG drives a signal 'global'.	The signal 'global' is connect-
ed to CE of CLB_R1C1, CLB_R2C1, CLB_R3C1, and CLB_R1C2, and
also to LC0.F2 of CLB_R4C1, CLB_R5C1, and CLB_R6C1.

The skew on 'global' between CLBs in column 1 and CLBs in
column 2 could be greater than 1ns.


Solution 1:

Having the same fanout/load on each vertical longline, can
reduce clock skew.



End of Record #1305