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AR# 1354

My CBxxx counter is not fast enough for my design


Keywords: CB, CC, counter, speed

Urgency: Standard

General Description:

In most cases, using a CBxxx counter for a XC4000/XC5200 architecture is
not the best choice, since these macros do not utilize dedicated fast carry
logic. Furthermore, without RLOCs on the flip-flops within the CBxxx
macros, there is no guarantee that the place-and-route software will place
the counter flip-flops optimally.


Using the XBLOX COUNTER module is the best choice in most situations, since
XBLOX will choose the best configuration for a given architecture. Another
choice is to use one of the CCxxx counter macros, which do use dedicated
fast carry logic and use RLOCs for effective placement of logic resources.
If the counter you desire is larger than any of largest counters in the
library (CC16xxx), you may need to create your own counter macro, which
would be based on a CC16xxx macro, for the proper number of counter bits.
CCxxx macros should NOT be cascaded= this creates a less-than-optimal
counter due to the added delay from coming off and getting back on the fast
carry path, which occurs at the point where the two CCxxx macros would be
AR# 1354
日期 05/24/1999
状态 Archive
Type 综合文章