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electrical interface problem LVPECL - LVDS multi-inputs

Hi there,

I'm currently having a serious problem: I got an LVPECL clock synthesizer and I want to connect it to several clock inputs on my FPGA. The FPGA features 2 LVDS interfaces, whereas each LVDS pair is located at opposite sides of the device, meaning there will be some extensive routing to do. I designed a resistor network for the level conversion from LVPECL to LVDS. What I'd like to know now is a) Can I route the 2x2 lines (two times differential to the two opposite sides of the FPGA) one-to-one out of my clock device to the inputs or should I use a dedicated buffer / repeater IC for clock distribution? b) If clock buffer are needed, should I use LVPECL buffers and do the conversion to LVDS level afterwards or should I perfom the conversion before the buffer and then use an LVDS IC? c) Where should I place the level conversion network? Is is better to place it right at the LVPECL output or is it more advisable to do it right before the FPGA inputs after a transmission line length of about 7 cm? Any help, comments, advice is highly appreciated!

Thank you all very much. Kurt

Reply to
Kurt Kaiser
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Why do you need to connect the same clock to more than one clock input on your FPGA? What kind of FPGA are you using?

If I had to buffer and translate, I would rather translate, then buffer. Unless you are very careful with your resistors and interconnect, a resistor translator may not work too well.

Andy

Kurt Kaiser wrote:

Reply to
Andy

"Andy" schrieb im Newsbeitrag news: snipped-for-privacy@l12g2000cwl.googlegroups.com...

Thanks so far for your answer. I'm using Virtex-4 FX60. Depending on the implemented designs I will use either the clock input on one side or the other, not both the at the same time of course. So I'm hoping for more flexibility and freedom of clock routing if I have two clocks available and stick to the one that fits better from design to design. The FPGA's input is LVDS.

Reply to
Kurt Kaiser

Those clock inputs cannot be configured as LVPECL? I know general purpose differential inputs can be LVPECL. You have to terminate on board though.

Are there other things on board clocked off the same clock? If not (i.e. nothing external is synchronous to that clock somewhere, then what does it matter which side you get into the FPGA on; just pick one and build the board that way. I can't imagine the FPGA clock design would be nearly as complex (or risky) as what you want to do on the board.

Andy

Kurt Kaiser wrote:

Reply to
Andy

My understanding of LVDS inputs is that you have a differential receiver that switches at the signal crossing point within the specified timing as long as your common mode voltage is within the databook spec. I haven't used Virtex-4 yet, but at least in earlier Xilinx parts the LVDS input common mode range was broad enough to support LVPECL. This will most likely depend on the VCCio for that bank. The datasheet seems to imply that you have the full GND to VCCio range for signal swing. Common mode range would be less.

Reply to
Gabor

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