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AM3517 DDR Connection clarification

John Bukowski
Prodigy 300 points

Other Parts Discussed in Thread: AM3517

The portion of the AM3517 datasheet that explains how to route the DDR portion of the PCB is on pages 135-147.  My question is:  If we had to choose between “T-routing” the control lines OR the address lines, which set should we “T”? 

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    TI__Mastermind 41665 points

    John Bukowski said:
    If we had to choose between “T-routing” the control lines OR the address lines, which set should we “T”? 

    I am not sure I properly understand your query, both the address lines and associated control signals (ADDR_CTRL net class) need to be routed in a T fashion, or as close to a T fashion as possible while meeting the layout specs provided in table 6-20 or 6-31 depending on if you are using LPDDR or DDR2. All the signals in the net class reference each other from a timing standpoint, thus they need to meet the various skew requirements, having them in a T layout helps to ensure the skew is minimal, thus the stem (A of figure 6-21) of the T should be as long as possible with the branches (B and C of figure 6-21) equal and as short as possible. I don't believe a 90deg T is a hard requirement, just minimizing the lengths of the branches and maximizing the length of the stem to give the best skew performance.