Virtex4 running at 360Mhz DDR

fastgreen2000 · 2005-05-10T18:48:57+00:00

I'm about to use Virtex 4, and wonder if this is achievable. Allliterature seems to indicate that it is, but I'd like hear what othersthink and perhaps point out where I need to be careful in the design.I'd be receiving an LVDS clock pair @ 360Mhz, running part of theinternal logic at 360. This internal logic includes DSP48 slices (butneed to be pipelined in the fabric since I need more than 48-bit 'C'input for adder). Preliminary testing indicates that it can go above360 with light user intervention. One thing I'm cautious about is, therest of logic runs much slower, at 90Mhz. Initially was thinking ofusing /4 version, but Peter Alfke's post regarding added skews due toloading differences in DCM outputs is making me think about itcarefully.For otuput, I'd be using ODDR to multiplex 360 Mhz logic, to send thedata out at 360Mhz DDR (so the data can look like 360Mhz 'clock').Data is LVDS, so is the forwarded LVDS clock pair @ 360Mhz. Thereceiving device will use both edges of the forwarded 360 Mhz clock tosample the data. Clock to output delay is not good, 3+ ns, but sincethe clock will be forwarded and will incur effectively the same delayas data (other than IOB-IOB clk skew), as long as I send out 180 degversion of internal 360 clock using ODDR, it should be ok. Not surewhat kind of SI issue there will be, however.I have an option of running it at 180Mhz if 360 is risky. Externaldevice will be different. Am I playing too safe by going to 180? Will360 be a challenge?I'd appreciate feedback.