Electronic – Why is length matching important for high-speed signals

I highly recommend the first thing you do is purchase High Speed Digital Design: A Handbook of Black Magic. Read it twice, then read it again :)

One important point. The crystal frequency doesn't matter here, you need to know the speed of the signals on the lines in question (which can be many times the crystal frequency). More over its actually rise / fall times that drive almost all signal integrity issues, not the digital frequency of the signal.

Designing for DDR isn't really that simple. High speed design can be a bit of a 'voodoo' art, even if you have $10,000+ simulation software. In other words, don't expect to nail the design the first time without putting in the work to understand the issues involved, a check list really won't cut it.

What I mean is, you really should start by reading the book I linked. It will give you enough background that the information in AN2582 will make sense (side note you linked the wrong pdf in the op). It will also allow you to understand the design trade offs you'll likely have to make when laying out the PCB.

That being said, here are my thoughts:

Routing Guidelines:

High level things to consider / avoid:

1) Route on a single layer, with a solid ground plane under it. Avoid vias like the plague. If this isn't possible, the DQ and ADDR groups are most critical, route those first, try to only move signals as groups to different layers.

2) Make sure you impedance match the traces: 50-60ohms, whatever comes out to the 'nicest' trace width for the design. Note the difference between differential and single ended lines and match the impedance appropriately.

3) Maintain proper signal spacing (i think 3*signal line width is preferred). This will help limit crosstalk between signals.

4) Match trace length of all related signals / groups (differential pairs, data bus, address bus, etc). Try to keep all traces to roughly the same length, that is you don't want the address group to be 1cm longer than the data group if you can avoid it.

5) Use source termination. You probably don't need parallel termination nor a Vtt given your board size and use of a single ram ic.

6) Pay special attention to Vref, it needs to be stable: well decoupled, fat traces. For a single ram module you can generate it with a simple resistor divider.

7) Don't use resistor banks for the termination, use individual resistors.

8) Expect that you'll need to 'play' with the source termination resistor values on the first prototype. Basically put a scope on the signal and try various values in the region of (trace_impedance - driver output impedance) = R. Look for the value that results in the cleanest signal (read up on eye patterns).

Signal Groups:

They are (NOTE: Taken from AN2910 and this is for a 64bit + 8bit ECC module, you don't have all these pins):

Data Group: \$MDQS(8:0), \overline{MDQS}(8:0), MDM(8:0), MDQ(63:0), MECC(7:0)\$

Address/CMD Group: \$MBA(2:0), MA(15:0), \overline{MRAS}, \overline{MCAS}, \overline{MWE}\$

Control Group: \$\overline{MCS}(3:0), MCKE(3:0), MODT(3:0)\$

Clock Group: \$MCK(5:0)\$ and \$\overline{MCK}(5:0)\$

Stack Up:

There are lots of ways to do this. Micron gives their recommendation for 6 layer stack ups with 3 or 4 signal layers in app note TN-46-14.

Really stack up is an entire topic of its own, but if your device has the 'standard' assortment of devices on it, these recommendations should work fine.

Other Stuff:

I think the rest of your questions are answered in the linked pdfs or AN2582. There is another checklist available in AN2910.

Your intuition is correct, depending on edge speed and how close those serpentine paths are you can cause your self problems. They absolutely will couple to each other like you're wondering. In fact if it's tight enough the high frequency component may just couple straight through the S curves like they aren't even there.

The question then becomes will that coupling be a problem in your application. They look far enough apart in that picture for DDR3 but it's hard to tell. Of course simulation of the path would always be best, but I know we don't all always have access to expensive tools when we need them :)

You seem to be on the right path though. Here's Johnson talking a little more about it.