Regarding PCB design layout of Raspberry Pi (RPI)


(viviantern) #1

I have query regarding PCB design layout of Raspberry Pi (RPI). I wanted to know how LAN9512 chip(http://www.kynix.com/Detail/226404/LAN9512-JZX.html) is connected to Ethernet RJ45.

I found one of PCB layout picture which suggests there is line on PCB which goes from LAN9512 chip to Ethernet RJ45 which I guess mark as R23.

What does R23 represent in PCB design, is it some kind of system bus or USB bus?

Same way how CPU/GPU is connected to component like D2 & D3 (RAM)?


(Original Image)


(Original Image)

PS: I’m not at all familiar with PCB design.


(sdfgeoff) #2

On PCB’s, things labelled with an “R” are almost always a resistor. If it’s between the ethernet jack and the LAN chip, it may be to do with impedance matching. It’s hard to explain impedance matching without getting technical, but if you imagine that you have a heavy object and a light object and they hit each other, the light object will go flying while the heavy object won’t move much. If you have a wire with high resistance and one with low resistance, then you can get some weird electrical effects (only a problem at high signal speeds such as high-speed-ethernet).

More useful that looking at the PCB layout is to find the schematic - unfortunately the Pi is not open source hardware, so you can’t actually get the schematic for recent models. Fortunatly, the LAN chip has a datasheet: http://ww1.microchip.com/downloads/en/DeviceDoc/00002304A.pdf
It doesn’t show the signal side, but it does show some of the requirements for the power supply schematics. If you are planning to implement a PCB with these components, be aware that high-speed PCB design is not trivial.