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Altera_Forum's avatar
Altera_Forum
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13 years ago

Cyclone IV GX, DDR decoupling

Hi,

I'm actually doing schematics using the cyclone Iv gx evaluation board. I placed the same DDR2( Mircon MT47H16M16) but I wonder why there are only 40 caps (10x4 CN caps) when micron says to put caps on every signals ?

Are theses caps only needed for DQ DQS and DM ?

6 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    Post the schematics, but I suppose You're talking about caps after termination resistors to Vref?

  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    Yes, I'm talking about caps after termination resistors to Vref.

    Look page 11 on the attached schematic, it deals with CN3,CN4,CN9,CN11,CN2,CN1,CN15,CN17,CN16 and CN8.

    Thank you
  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    These are required, but not necessary. The more, the better, but usually there is no requirement to overload the PCB with caps.

  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    Ok, fine.

    However, if these caps are required but not necessary, for which signals are they required for, to the extent there are only 40 caps but 61 ddr2a signals ?
  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    These caps are placed to keep Vref plane as much clean as possible. You should add a cap near every termination resistor, but since You're probably using resistor networks (at least I did), then place at least one cap per resistor network. The common approach is to place 3 caps near one resistor network with values: 100nF, 10nF, 1nF.