Xilinx Virtex II Output Register

[jc]

I have a input signal thats get registered and then outputed on
another signal (pad) One register delay. I would like the Output
register to used instead of the Input register. I am using Xilinx ISE,
VHDL and a Virtex II device. Everytime I implement the Input register
is used, how do I use the Output register. I am trying to improve the
clock to pad time.

Thanks
John C

 

[Brannon King]

Set IOB=FALSE on the input instance name and IOB=TRUE on the output instance
name, or use the primitives IBUF for the input buffer and OFDXI for the
output.


"jc" <jcocozza@juno.com> wrote in message
news:8e698b60.0401060636.6fc1d7a8@posting.google.com...

I have a input signal thats get registered and then outputed on
another signal (pad) One register delay. I would like the Output
register to used instead of the Input register. I am using Xilinx ISE,
VHDL and a Virtex II device. Everytime I implement the Input register
is used, how do I use the Output register. I am trying to improve the
clock to pad time.

Thanks
John C

 

[Bret Wade]

jcocozza@juno.com (jc) wrote in message news:<8e698b60.0401060636.6fc1d7a8@posting.google.com>...

I have a input signal thats get registered and then outputed on
another signal (pad) One register delay. I would like the Output
register to used instead of the Input register. I am using Xilinx ISE,
VHDL and a Virtex II device. Everytime I implement the Input register
is used, how do I use the Output register. I am trying to improve the
clock to pad time.

Thanks
John C

John,

Since this is a Virtex-II device and you already have IOB FFs enabled,
all you need to do is add a BEL constraint to OFF1 or OFF2:

INST "FF_Name" BEL = OFF1 ;

IOB BEL constraints aren't supported for architectures older than
Virtex-II, so for a Virtex-E design, one of the following map
constraints could be used:

1. Give FF and output pad the same BLKNM constraint.
2. Give FF and output pad the same LOC constraint.
3. Apply a KEEP constraint to the FF D input net, assuming fanout of
1.

Regards,
Bret