I especially liked the section on DDR memory PCB routing and how it is less complicated than people make it out to be. At least for this application. Makes one want to try something like this
On a past project we had two DDR RAMs and a NOR Flash on the same bus. There was one hardware engineer, and he laid out the memory subsystem in about a day. (The power supplies were dramatically more complicated.)
It almost worked first time. I noticed about one memory error per day per device, which I repro'd with repeated `md5sum /tmp/largefile`.
We just dropped the clock rate in U-Boot.
Failed devices? Heat it up with a hot air gun for a while. Fixed 90% of them. Still doesn't work? Throw it away.
There are no mysteries, just stuff you don't know yet.
Once you get into flyby or T-routing topologies - good luck!
If you are playing with a 2x2" EVK then 8 or 10 layers isn't a budgetary problem. When the board starts getting larger it's now it's a huge cost adder as opposed to low speed 2-layer PCB.
This is why designers switch to doing it with SOMs. The SOM can be a high-speed part and the carrier PCB is cheap and slow.
PCBs are built in a sandwich stack. The middle dielectric layer, or "prepreg", is way thicker than the dielectric layers in the outer layers. This doesn't seem like a big deal, but for the speeds that a DDR interface runs at, it creates a higher impedance return loop for whatever you end up routing on layer 3. (Typically DQs, in my experience.)
You can avoid this entirely by building with a six layer stackup, and routing your DRAM signals on layers 1/2/3, keeping layer 2 as an unbroken ground plane. The result: nice, low impedance return tracks.
Yes, Jay is right - most of the time, this doesn't create problems. But when it does, it's pretty challenging to fix without a board spin, and when you're doing this professionally, that loses you time, creates risk, and is generally hella stressful.
And you still need to pass radiated emissions testing. Even those right-angle DIMM connectors for the SOMs spray RF noise everywhere when you put video on them.
:)