Virtex-4 clocking

Dec 20, 2005 5 Replies

Hi fellow engineers,



I am about to design a PCB containing a Xilinx Virtex-4 FPGA. I have some experience in board design and have already used several Virtex and Virtex-II devices. I want an ICS8442 LVDS clock synthesizer to be the clock source of my Virtex-4. A look at the Virtex-4 User Guide (ug070.pdf) scared me a lot by stating "Clock inputs can be configured for any I/O standard, including differential I/O standards. Each clock input can be either single-ended or differential. All 16 or 32 clock inputs can be differential if desired. Global clock inputs can be configured for any I/O standard except LVDS and HT output differential standards. Only CSE output differential standards are supported by the global clock input pins." Does that mean that I am not allowed to connect an LVDS clock to the Global-clock inputs? Or did I get something wrong here? I have to use the ICS8442 because of the large frequency scale from 25-700 MHz.



Regards, Melanie


"Melanie Nasic" schrieb im Newsbeitrag news:do8vhr$sv9$ snipped-for-privacy@mamenchi.zrz.TU-Berlin.DE...

no nothing wrong, read it once more time

the clock input pins can not be used LVDS OUTPUT they of course can be used as LVDS input.

the restriction of some V4 lvds iopins of being input only for LVDS is new it is related to the 'low capacitance' of those pins I think

Antti

Antti is correct, but I, too, was confused by the wording in the user guide. I contacted our FAE and he confirmed that the restriction is only when using these pins as outputs.

Bob

Antti,

Yes, some pins do not have the LVDS output structure. The "low capacitance" that affords is a mere 0.5 pF, so I think more appropriate would be to say these pins just don't have these two IO output standards available to them.

Aust> "Melanie Nasic" schrieb im Newsbeitrag

"Austin Lesea" schrieb im Newsbeitrag news:do988l$ snipped-for-privacy@xco-news.xilinx.com...

Hi Austin

yes I know, I learned it the hard way, I was jump starting a FX12 board with LVDS _output_ clock connected in the banks that do not LVDS out (eg banks 0..3 for FX12-SF363) - after reading the datasheet, well its all in the datasheet but I guess there are more people around to find this issue the hard way.

Antti

I totally agree on that, Antti! The information given in the user guide is very misleading.

"Antti Lukats" schrieb im Newsbeitrag news:do998g$nc3$00$ snipped-for-privacy@news.t-online.com...

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