Electronic – PCB trace length

It's a common design strategy to have a GND copper pour, which covers most of the bottom copper layer. In this strategy, the bottom layer is used for routing as little as practical in order to keep the GND copper pour uninterrupted. The goal is to mimic a ground plane like in a 4-layer board.

Here's an example:

Let's see how this would agree with the design rules in the O.P. I treat them as general rules. I'm ignoring more specific things, such as low noise circuits or power circuits, which may require a different interpretation of these rules.

1. A ground plane is under the signal trace. The DC component of the return current follows the path of least resistance. The AC component of the return current follows the path of least impedance, and it runs under the signal trace.
2. Star ground. Each sub-circuit can sit on its own local GND copper pour. Local GND pours are connected to one point, which will be the center of the star. Alternatively, slits can be made in the GND pour between the sub-circuits.
3. Minimize the return path length may conflict with rule 1. Which would it be: minimal path or same as the signal?
4. Ground connection should have low impedance. Usually, ground traces are wider than signal traces. I doubt that "same trace width as the signal" is a good rule for the general case.

Design rules similar to the ones in the O.P. are also discussed here and here. (Analog devices has quite a few application notes on grounding. Search their site.)

Length matching is about timing so if you want to know how tightly you have to length match you have to understand the timing budget for your interface. Signals will leave your source, and arrive at your destination with some timing relationship. Your receiver requires a certain timing relationship between clock and data in order to be guaranteed to function correctly. This is usually defined as setup and hold time, or how long before the clock edge your data needs to be valid, and how long after it must stay valid.

There are several things in a system which eat into this budget, one of which will be your routing. Sometimes a manufacturer will tell you this information, other times you must derive it from the input and output timing data of your transmitter and receiver. Of course it's easy to just say well I must match exactly, because then you don't have to think about it :)

But let's think about it for a minute. You have a 170Mhz signal? That's a period of 5.882ns. What would happen if you routed all your data to within one inch of clock. What would be the worst case timing difference. Propagation time for a top layer trace, a microstrip is about 150ps/in. So a 1 inch difference will skew a data signal from clock +/- 150ps. That's really not bad at all considering your 5.882ns clock period. In fact 170Mhz is really not that fast.

If you understood the output skew from your transmitter, and your setup and hold times for your receiver you could come up with a number for the acceptable routing delay. Of course there are other factors, clock jitter, ISI, etc, but this should give you a good idea of what you can do.