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WouterPlatteau's avatar
WouterPlatteau
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5 years ago

3.3V supply for 3.0V LVCMOS

Hi,

I'm using a Cyclone 5 with a SDRAM.
I have a 3.3V supply on all my VVCIO's but for signal integrity we would use the
on chip series termination of 50Ohm.
This is not possible for 3.3V LVCMOS but it is for 3.0V LVCMOS.
What are the consequences if doing so? Turning my supply to 3.0V is not an option.

With kind regards,

Wouter Platteau

5 Replies

  • EngWei_O_Intel's avatar
    EngWei_O_Intel
    Icon for Frequent Contributor rankFrequent Contributor
    5 years ago

    Hi Wouter Platteau

    For those IO standard that doesn't support On Chip Termination (OCT), and yes for 3.3V LVCMOS, we should implement external termination.

    Thanks

    Eng Wei

    • WouterPlatteau's avatar
      WouterPlatteau
      Icon for New Contributor rankNew Contributor
      5 years ago

      I do understand that external termination is the proper solution in this case.
      But could you tell me the consequences of selecting 3.0 LVCMOS as the IO standard
      but supplying 3V3 on the VCCIO pins?

      • Elad's avatar
        Elad
        Icon for New Contributor rankNew Contributor
        5 years ago

        Assigning a bank a voltage in Quartus defines the bank as that voltage. In this case, the bank would become a 3.0v bank. Checking the Recommended Operating Conditions in the Datasheet, VCCIO for a 3.0v bank is allowed to be a max of 3.15v, so even at the nominal 3.3v value of a 3.3v VCCIO, it would be out of spec. That means that some percentage of the devices would not function correctly.

        Interestingly, VCCIO for a 3.3v bank is allowed to be a minimum of 3.135v, which overlaps with the 3.0v range. So it would be possible to tune the 3.3v supply down to 3.14v, and use it with both 3.0v and 3.3v banks.

        While this is technically feasible, it would require power supplies with very tight tolerances, which are much more expensive than adding external termination.

        The only clean solution that I see is to add another VRM to the board, and that is also probably the cheapest solution.

        Happy Friday!!!