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I really like your RP2040 chip and am using this to design a new small footprint PCB that offers USB and Ethernet access. The problem I am encountering is understanding your USB specs.

Here on page 11, the first two paragraphs: https://datasheets.raspberrypi.com/rp2040/hardware-design-with-rp2040.pdf

You mention the need for impedance matching the USB lines. I am not new to this concept and indeed if I use the numbers you provide for the traces, i.e. 1mm thickness FR4, 0.8mm track and separation of 0.15mm, we get ~90 impedance. That's all good, so why do I then need the 27ohm series resistors as this will make the total impedance ~143ohm differentially. This really does not make sense to me! I am happy to just implement it but it would be good to understand why when the traces meet the USB spec, you are adding additional resistors?

Nima Razavi
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Perhaps you have confused Impedance matching and Resistance matching? You simply cannot add up resistances and arrive at an impedance.

Wrt your question, all I can offer is this: Unless you have prior experience with design and layouts for USB interfaces, I would suggest you go with the recommendations.

Seamus
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