Multiple IDELAYCTRLs in V-5: how, and why?

I'm on Virtex-5, and have a lot of IODELAYs, for fixed and variable
input delays. I've read chapter 7 of the User Guide, and AR #39966,
about IODELAY_GROUPs. I need to instantiate one or more IDELAYCTRLs,
and my options are (1) to instantiatiate one of them, and let the tool
fix everything up, or (2) instantiate lots of them, and do it all by
hand.

Here's the bit I don't understand. What's wrong with (1)?

Page 342 of ug190 says "The most efficient way to use the IDELAYCTRL
module is to define and lock down the placement of every IDELAYCTRL
instance used in a design....Xilinx strongly recommends using
IDELAYCTRL with a LOC constraint", but the UG doesn't seem to say
anywhere why this is the case. What's the point? And why don't the
tools just conenct up the IDELAYCTRLs anyway, without being told to?
The IODELAY is useless without it.

Thanks.

 

[Gabor]

nospam@nospam.com wrote:

I'm on Virtex-5, and have a lot of IODELAYs, for fixed and variable
input delays. I've read chapter 7 of the User Guide, and AR #39966,
about IODELAY_GROUPs. I need to instantiate one or more IDELAYCTRLs,
and my options are (1) to instantiatiate one of them, and let the tool
fix everything up, or (2) instantiate lots of them, and do it all by
hand.

Here's the bit I don't understand. What's wrong with (1)?

Page 342 of ug190 says "The most efficient way to use the IDELAYCTRL
module is to define and lock down the placement of every IDELAYCTRL
instance used in a design....Xilinx strongly recommends using
IDELAYCTRL with a LOC constraint", but the UG doesn't seem to say
anywhere why this is the case. What's the point? And why don't the
tools just conenct up the IDELAYCTRLs anyway, without being told to?
The IODELAY is useless without it.

Thanks.


The IDELAYCTRL is one of the most poorly handled structures in the V5.

That being said, I believe the user guide was written before the
introduction of the IODELAY_GROUP to the tools. Using groups is
the new "preferred" method. In a simple system where you have only
one 200MHz reference clock and don't use the LOCK output of the
IDELAYCTRL, you can either use the original option (1) - one
IDELAYCTRL and no LOC constraint - or you can assign all IODELAYs
to the same IODELAY_GROUP. If you're unfortunate enough to work on
an XPS / EDK design, then you might need to have multiple IODELAY_GROUP
constraints to avoid hand placement of all IDELAYCTRL's.

-- Gabor