Using a FDDRCPE primitive. VIRTEX-II

[Anil Khanna]

Hi,

When using the DDR functionality, is it the user's responsibility to provide
the phase-shifted clocks or does the above mentioned primitive do this for
me? The documentation is not very clear.

Although using a DLL to phase-shift is the best practice, can I get by just
using an inverter between the two clocks? If so, does the Virtex-II IOB
provide a "free" inverter (gets absorbed in the IOB)?

Comments?

Anil

 

[Gabor Szakacs]

"Anil Khanna" <anil_khanna@mentor.com> wrote in message news:<40a3b3f0$1@solnews.wv.mentorg.com>...

Hi,

When using the DDR functionality, is it the user's responsibility to provide
the phase-shifted clocks or does the above mentioned primitive do this for
me? The documentation is not very clear.

The primitive has two clock inputs, they should both be connected.
Although using a DLL to phase-shift is the best practice, can I get by just
using an inverter between the two clocks? If so, does the Virtex-II IOB
provide a "free" inverter (gets absorbed in the IOB)?
Yes. You can connect clk to one input and !clk to the other and the

inverter is sucked into the IOB. Note that many people still use the
0 and 180 clock outputs from a DCM to do this which requires two global
clock buffers. Also remember that to work reliably with a single clock
and its inversion, your clock needs to have a 50% duty cycle.

Comments?

Anil