0
\$\begingroup\$

I am learning FPGA programming. Going through example code/project. What is IOSTANDARD in constraints file. When to use LVCMOS33, LVCMOS25, LVDS_25.

Which one is best for high speed clock signals. Should SKEW for clock signal should be set as FAST or SLOW? How to decide property of an IOSignal? Could someone explain in simple terms.

\$\endgroup\$
2

1 Answer 1

0
\$\begingroup\$

These are IO signalling standards. In fact, their names are pretty self-describing:

  • LVCMOS33: Low-Voltage CMOS (with a 3.3V amplitude) single-ended
  • LVCMOS25: Low-Voltage CMOS (with a 2.5V amplitude) single-ended
  • LVDS_25: Low-Voltage Differential Signalling (with 2.5V differential swing)

Which one is best for high speed clock signals.

This question makes no sense, because we don't know what you're going to do with the signals. If the device attached to that output expects single-ended 3.3V amplitude, then you need to use LVCMOS33. If it accepts LVDS with a 2.5V voltage swing, LVDS, and so on.

If the choice is free because you're still designing what you'd attach, it depends, on how long things are, what your needs in cleanliness and spurious emissions are, what your power constraints are and so on.

Should SKEW for clock signal should be set as FAST or SLOW?

Depends, see above.

How to decide property of an IOSignal?

Mostly, this is the whole analog part of digital design! Your clock signal is still an analog signal and has to travel somewhere and drive some kind of receiver load. Can't really take the burden of learning the basics of high-speed signalling and transmission line theory of your shoulders, I'm afraid.

\$\endgroup\$
2
  • \$\begingroup\$ Thanks for comments and suggestion. \$\endgroup\$ Dec 15, 2021 at 17:17
  • \$\begingroup\$ You're welcome; but: no need to thank me in a comment. If the answer you've gotten was actually helpful, clicking on the upvote button next to it is the right way to say "thank you!". If it actually answered your question, using the "accept" button (the checkmark) is the right thing to do. \$\endgroup\$ Dec 15, 2021 at 18:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.