PHB FPGA question

Am 27.12.20 um 21:50 schrieb snipped-for-privacy@highlandsniptechnology.com:

Maybe LVDS is not the way to go. The LV means they want it somewhat fast, but power still does matter. The game is different with LVPECL or CML.

Because it really depends.The clock could be routed in a thousand different ways on the chip. It is usually best to use one of the typically 4 global clock nets, but there may be local ones that are placed nicely to your outputs. And avoid tri-state buffers. They are sloooow, alone for the logic they contain.

I have not used Zyncs, but mostly Virtexes.

The canonic way to get short clock to out delays would be to use a global clock net without any logic in front of it and then write a constraints file with the specs you need. Leave the work to the router. Don't be too greedy in the first round, you can put on the thumb screws later. First make sure that what you spec is that what you want. Sometimes is is not intuitive to specify that.

You can see in the static timing verifier where the ps are lost and where work for improvements is futile.

Never tried that. But I have built pipelines 24 stages deep. Do less combinatorial stuff in one stage and start early/parallel enough.

I think, if that would be possible in a reliable way, Xilinx would spec it that way and would ask for more money.

Gerhard

The clock into the FPGA is from a differential PECL comparator, so using an LVDS input makes sense. The output will be 3.3v cmos.

Rumor has it that, in general, lvds i/o is about a ns faster than cmos.

That's the standard answer: it depends.

There are a zillion appnotes and class notes, none of which include the word "nanosecond."

The clock could be routed in a thousand

I want my output to transition immediately after the first clock edge. Or maybe before.

I'm not exactly sure what your goal is, but if you want to subtract out the clock routing delay, use an MMCM so that the clock to the flipflop will ha ve nearly the same phase as the clock at the input. You can also make sure that the flipflop is an output flop packed in the IOB so that the flop out put -> pin delay will be short and more deterministic. You can use a direc tive in the HDL to ensure the flop is in an IOB. You can also set a constr aint for the max delay out; otherwise the tools assume they have an entire clock period to get the signal from the flop to the pin. If the input is r eally asynchronous, you really ought to use a 2-flop synchronizer.

Some of the Xilinx parts have a separate column in the datasheet for a lowe r core voltage, which saves power but degrades timing. I definitely wouldn 't try increasing the voltage to beyond the spec. It might work, for a whi le... The best way to get better speed would probably be to ensure that th e junction temperature stays low.