related and unrelated logic

[Szymon Janc]

"Note that once logic distribution reaches the 99% level through related
logic packing, this does not mean the device is completely utilized.
Unrelated logic packing will then begin, continuing until all usable
LUTs and FFs are occupied. Depending on your timing budget, increased
levels of unrelated logic packing may adversely affect the overall
timing performance of your design."

I'm getting this info durring mapping process. Why does it afect timing
performance? I've tried to google, but couldn't find an answer..

 

[Mike Treseler]

"Note that once logic distribution reaches the 99% level through related
logic packing, this does not mean the device is completely utilized. ....
I'm getting this info during mapping process. Why does it affect timing
performance?

A signal takes longer to move through two LUTs than one LUT.
Check the static timing Fmax to see if it is fast enough.
If this was a warning rather than an error you may be OK.
Sounds like the device is nearly full.

-- Mike Treseler

 

[David Spencer]

"Note that once logic distribution reaches the 99% level through related
logic packing, this does not mean the device is completely utilized.
Unrelated logic packing will then begin, continuing until all usable
LUTs and FFs are occupied. Depending on your timing budget, increased
levels of unrelated logic packing may adversely affect the overall
timing performance of your design."

Basically, the message says it all. The place and route will try and only
use each LUT for mutiple logic functions if those functions are
interconnected in the design. However, when all the LUTs are used up it can
try and squeeze the remaining logic into unused parts of LUTs that hold
totally unrelated functions. Your timing is likely to be compromised because
the placement will no longer be optimal and routing delays will be higher.

At the end of the day, as long as your timing requirements are met, it
doesn't matter how the design was finally placed or routed.