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Questions relating to designing PCBs
By winston
I'm building a PCB that uses the W5100 single-chip ethernet controller - this is the first time I've done anything with ethernet, so I'm a complete novice in this regard. I'm using a jack with integrated magnetics to reduce the PCB's complexity (since I'm limited to 2 layers). For starters, I'm making a breakout board which is just an implementation of the manufacturer's reference circuit for use with a mag jack.

Reading the manufacturer's notes on PCB layout raises a couple of questions.

Firstly, the usual recommendation about the crystal (25 MHz) and analogue signals - don't put them close to each other. However, the W5100's two XTAL pins are right next to Vcc 1.8v analogue - so it's unavoidable that for at least some distance, the 1.8v analogue supply is going to be only around 8 mil from one of the crystal input traces. I've tried to keep this as short as possible and as soon as there's room, route the crystal and analogue Vcc traces away from each other.

How close is "close"? The layout guide uses the word "close" a few times without really saying what's considered close (and when explaining that the chip should have short traces between it and the magnetics or magjack, says the maximum trace length is 19cm, which seems like a hell of a long way to me!)

Also on the issue of ground planes. The layout guide says there should be an isolation area between the jack and the PCB's main ground plane. It doesn't say what this is for - is this for static protection? It seems to suggest so because there's talk in the layout guide of making it wide enough that a static spark can't leap from the jack's chassis ground to the PCB's main ground plane.

I've had a look at a 3com ethernet card on my desk at work, and sure enough - the main ground plane is separated by what looks like about an 80 mil gap from the ground plane under the jack and magnetics BUT the jack's chassis is connected to the main ground plane.

However, the reference schematic shows the jack's chassis and the main ground plane connected together via a choke - this seems to contradict the layout guide if the isolation area is for static protection.

The documentation that I refer to is all at this URL: