i.MX6 DDR3 PCB Layout Notes

i.MX6 DDR3 PCB layout can be a daunting task for someone who does not often design high speed PCBs. Also, because i.MX6 chips are quite similar across variants in many ways, these notes can apply to any chip of the series. I have put these notes down while designing an i.MX6 Solo X based custom PCB.

If I go on to make other Linux SBC PCBs based on 1GHz or faster SoCs that use DDR memory, I will update this article with as much information as I can find.

In short, this article is the set of notes that I use when routing i.MX6 boards. All the resources that are suggested reading material are linked in the references section below.

Optimal Placement for i.MX6 DDR3 PCB Layout

i.MX6 SoCs have all the DDR DRAM related balls on one side of the chip. The data lanes and data lane strobes are all along the outer edge of the chip and therefore it is possible to connect all the data lanes to the DDR chips using 2 layers of the PCB.

To figure out which data bus pin will connect to what DDR3 chip data lane, you can start with a simple data bus pin map. Here is a map for the i.MX6 Solo X chip and a couple of 2Gb DDR3L DRAM chips.

The colors are just used to identify the byte lanes (DQ0 to DQ7, etc). You can also see the byte lanes on the DDR3 BGA-96 chips. The symmetry makes it easy to lay out the i.MX6 to DDR3 interface for the data lanes.

What I do for laying out the data buses (assuming 6 layers) is:

Lay out differential data lane strobes on layer L1 and L3 for DDR3 on the left
Lay out length matched data lane bits and DQM for DDR3 on the left
Length match everything based on matching guidelines below
Mirror this routing pattern over to the DDR3 on the right side

DDR3 Trace Length Matching Strategy

A T-topology is possible in a symmetric arrangement like this [ref.1]. We do not have a lot of DDR chips and capacitive loading should not really be a concern here with short signal trace lengths. In a case like this, the best way to route traces is as follows [ref.2].

For the data lanes, length should be matched within a data lane. For i.MX6 Solo X, only two 2-byte groups are used for a 32-bit wide data bus.

DDR3 PCB Layout Length Matching Rules and Constraints

Routing DDR3 requires strict length matching. However, for SoCs that run at speeds lower than 1GHz like the i.MX6 Solo X and the i.MX6ULL chipsets, the length tolerance is not as strict as it would be otherwise. Also, in my experience, you could get away with breaking the rules by a little bit as well.

Because DDR3 chips are almost always meant to operate at higher than 1GHz (higher speed grade), and the i.MX6 chips often run the DDR3 at speeds less than 500 MHz, the chips allow some more relaxation in length matching because of their high slew rate IOs meant for 1+ GHz bus operation.

Here is a summary of the DDR3 PCB layout length matching rules and restrictions [ref.2].

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Change Log

Initial Release: 30 April 2021

References

Reference 1: Whitepaper on DDR3 Routing Topologies
Reference 2: i.MX6 Series Layout Recommendations
Reference 3: i.MX6 Sabre OrCAD PCB Design Files