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I am designing a board which include DDR3 Ram, AM335x Series MPU. I have a question at this topic.

How should it DDR Ram layouts at PCB? Can I draw at top layer? Or which layer the best? I want to work with this board without problem.

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  • \$\begingroup\$ Posting this question indicates that you have a lot to learn before you design something with DDR. Start by reading manufacturer design guidance, looking at the measures taken in typical designs, etc. Consider if you really should be doing this, or (if it needs to work the first time) either using an SoC+RAM module or bringing on someone with experience of these things. Or, if you can afford to go through a few experimental board spins, give it your best from whatever research you are willing to do, and have fun. \$\endgroup\$ – Chris Stratton Nov 18 '20 at 4:29
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Mert, Chris Stratton is correct, this is an advanced design topic/challenge and his friendly suggestions on how to go from beginner to being able to successfully do a DDR design are good. Look at "High-Speed PCB Design Guide" from Sierra Circuits as another starting point, is what I will contribute here. This is not "EGO", this is the real world, his suggestions and mine are not useless. You will need length tuning, impedance matching and possibly simulation, multilayer stackup knowledge, on and on.

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If you want to design a board "high speed signals" which are 50 MHz+ then you need to worry about transmission line effects and parasitic effects of capacitance, resistance and inductance of traces and vias. Unfortunately, this is the subject of books and too long for an answer here. The design of the traces should not create problems for the rise times if the signals. Typically this means keeping signals as short as possible and keeping the inductance low. Vias add inductance, so keeping signals in the top layer.

A very high speed interface 100s of MHz will also need to consider the length of the traces as the length will affect the rise times and arrival times (which will be in ns). The impedance of the PCB material and electric permeability will also need to be controlled.

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