AN5097 DDR4 Layout Checklist Clarification for LS1028A Application

[jimmymm]

I am in the process of laying out the DDR4 interface for our LS1028A application and had a couple of questions related to the AN5097 checklist.

The last bullet on No. 27 states: "For 32-bit or 16-bit DDR4 data bus, the bit 0 (DQ[0]) and bit 1 (DQ[1]) of ECC
byte lane, bit-swap is not allowed." My initial interpretation of this is that bit 0 and bit 1 of the ECC's data bus cannot be bit-swapped. However, the reference design board has the following bit swapped interface:

DQ0->D1_MECC3

DQ1->D1_MECC1

DQ2->D1_MECC2

DQ3->D1_MECC0

I would like to implement the following bit-swapping order for Rank 1:

DQ0->D1_MECC2

DQ1->D1_MECC0

DQ2->D1_MECC3

DQ3->D1_MECC1

...and this order for Rank 2:

DQ0->D1_MECC0

DQ1->D1_MECC2

DQ2->D1_MECC1

DQ3->D1_MECC3

Is this bit swap permitted? I do not understand this rule.

My second question relates to No. 25 in the checklist, specifically this statement: "Spacing >= 2-3x distance from signal to adjacent ground plane in PCB stack-up". Is this the trace to trace spacing? If so, is it center-to-center or edge-to-edge?

[Bulat · Accepted solution]

1. Yes, your bit mapping is ok.

2. Yes, it is about trace to trace spacing. Accuracy of the recommendation is not too high (2-3x), treat it as edge-to-edge. The rule is really simple, greater spacing - lower interference.

Regards,

Bulat