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

[Abhi P Choksi]

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?

[Paul van Avesaath]

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.

[robertferanec]

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:

[saeid]

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 ?

[Abhi P Choksi]

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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[robertferanec]

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.

[saeid]

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 )?

[robertferanec]

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

[saeid]

Thank you so mach dear Robert.