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Open REX : How value and Numbers of Bypass Capacitor is selected

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  • Open REX : How value and Numbers of Bypass Capacitor is selected

    For Open Rex Board i have below questions regarding Power Supply Decoupling network.

    (1)How value is selected of Bypass capacitor?
    (2)How to decide numbers of Bypass capacitor?

  • #2
    I am not sure About the open rex, buit for Xilinx and Altera devices they recommend a certain amount per IO/Bank / Power rail, in the datasheet or in their online calculators
    it used to be that each pin would need its own cap, but the newer Chips have a lot more on board local capacitors.. although they are small it iliminates a lot of bypass caps in newer designs.
    so my recommendation here is always check the documentation since it should provide additional support on this.

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    • #3
      As Paul van Avesaath mentioned, the same is for iMX6 CPU (which is used on OpenRex) - capacitors are recommended in design guides. Here is one of the examples (I think I saw somewhere also a table which exactly said number and values of caps for the core power rails, but can not find it right now) - screenshot from design guide:
      Click image for larger version

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      • #4
        I am going to design a board based on IMX6Q CPU. I have been checking open Rex and IMx6 Rex schematic as a reference. In the Open Rex board, the value of DDR3 decoupling capacitor are 220nf, whereas in the IMX6 Rex they are 100nf. I could not find 220nf in the DDR3 datasheet.

        What is the reason of that ?

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        • #5
          Hi Saeid,

          Please attach some snip which shows comparison.

          Apart from that, I can say that you may use any decoupling capacitor ranging from 100nF to 220nF for high-speed interfaces. But, better to go with recommendations.
          If DDR3 datasheet recommends 100nF decoupling capacitor then please use that. Don't go with 220nF.

          Thanks,
          Abhi

          ​

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          • #6
            I am going to design a board based on IMX6Q CPU. I have been checking open Rex and IMx6 Rex schematic as a reference. In the Open Rex board, the value of DDR3 decoupling capacitor are 220nf, whereas in the IMX6 Rex they are 100nf. I could not find 220nf in the DDR3 datasheet.
            I checked the schematics, they both have 220nF. Please, what 100nF do you mean?
            - iMX6Rex module schematic: https://www.imx6rex.com/wp-content/u...-Schematic.pdf
            - OpenRex schematic: https://www.imx6rex.com/wp-content/u...Production.pdf

            Maybe you were looking at decoupling of termination resistors? These are only on OpenRex as it is using fly-by. iMX6Rex module uses T-branch layout and doesn't need these caps.

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            • #7
              Thanks Dear Robert and Choksi for your replies. I made a mistake. As Robert mentioned both of them use 220nf for DDR decoupling capacitor. What I am trying to ask is how did you choose these capacitors? (220nf and 22uf )
              Why you did not use 100nf ? Because I could not find these value of capacitors in the DDR datasheet(Micron )?

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              • #8
                I used the same decoupling capacitors as they recommended and used in the CPU reference design.

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                • #9
                  Thank you so mach dear Robert.

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