Electronic – First DDR2 Layout – How much of a data lane must have the same reference

That's the Cypress FX2LP USB microcontroller (I recognize it because I use it myself). If you're using the Hi-Speed USB transceiver, then you should really go with a 4-layer board. Without that ground plane right below the top layer, it will be near impossible to get the 90 ohm differential impedance that you want for the USB D+/D- lines.

http://www.cypress.com/?id=4&rID=34128 flat out states that 4 layers is required. It also states that controlled impedance is required, but in my experience you can usually get away without it, so long as you carefully research your fab's typical stack-up and work out the right width, space, and height.

http://www.cypress.com/?docID=25406 also provides more info on calculating the width, space, and height for the D+ and D- lines.

4-layers isn't that much more expensive; Advanced Circuits has a 66 each deal for 4 layer boards that I use quite often for projects that use that very chip, as opposed to the 33 each deal for 2 layers.

In regards to your actual question...use plenty of bypass caps, as close to the pins as possible. If you split the bottom layer to have VCC and GND, don't have a trace cross the split on the top layer. Keep all high-speed signals on the top layer because the via inductance can kill what fragile signal integrity a 2-layer board has.

Assuming you're only using USB-low-speed or full-speed, you should be fine.

Generally, layout considerations only really have to be taken if you're going long distances (many inches), or using USB-2.0. Even then, USB is surprisingly tolerant.

• USB 1.1 or USB2.0 low/full speed

  • You really don't need to worry. There are (possibly apocraphyal) stories of people running USB2.0 low-speed of 50' of CAT-5 wire. As long as you keep your wire-runs a few inches or less, I wouldn't worry.
  • The fastest edge you will need to worry about in low/full speed applications is 12 Mhz. As such, you're not really approaching the point where making sure your traces/wiring is properly transmission-lined/impedance-controlled is that important, at least as long as your overall uncontrolled-impedance sections are less then, say ~6".
  • As I said, most USB controllers are impressively tolerant of USB devices that are widely out-of-spec. If this is something for production, I would spend the effort to do it properly (there is one guy out there who has a motherboard that throws a hissy-fit if anything connected to it deviates from the spec by a tiny amount), but if it's just a test-board, I'd say just lay it out neatly, and don't worry about it.

• USB2.0 High-speed.

  • Here layout becomes more important. USB2.0 High-Speed has a maximum edge rate of 480 Mhz. As such, even short traces start to approach the wavelength of the data, and as such proper impedance-control becomes important.
  • Assuming you EDA package has proper impedance-controlled routing options, just set your differential-pair impedance to be ~90Ω, and you should be fine. Be careful to make sure you have a contiguous ground-plane, though

• USB3.0

  • So you hate yourself?