VoCore Ethernet with only capacitors

[brad]

If you are like me and are a bit cheap / don't have any magnetics in the electronics lab and want ethernet right now. Then you can connect the ethernet with some capacitors you have laying around. Any capacitor from about 33nF to 0.1uF will be fine for this. 33nF is recommended.



It works just like using magnetics and is a perfect solution if you have networked devices on one pcb, I actually think this should have been made on the original board design so we could just connect the ethernet cable direct to the vocore without needing external magnetics, It is a recognized way to do ethernet without magnetics and I got this idea from reading a TI spec on how to use capacitors instead of magnetics (link below). This is a pretty clean solution if you just want to use something you have in your component box.

Page 5 of this shows the same schematic of what my changes are. VoCore already has the resistors, except they only go to ground and we need to pull them up to 3.3v and then add the capacitors.Connecting 2 vocore you only need one set of capacitors.

http://www.ti.com/lit/an/snla088a/snla088a.pdf

EDIT: use 33nF caps, not 0.1uF.

EDIT: Added picture of how I soldered it. I did this with a standard tip cheap ass soldering iron. So no excuses folks. The white stuff is "white out" to stiffen the wire, since its a single strand from a stranded cable.

[Vonger]

I was thinking this is impossible, but you done it

[brad]

Yeah, its a nice solution to have Ethernet without the usual bulky parts. Can also connect two vocore's real easy to double the GPIOs and cpu cores. Very elegant indeed.
If you make a V2, please do Ethernet like this, or have a solder jumper to add in the 3v pullup. This is the best board for embedded development. Great job!

[SteeevO]

I've been trying to get ethernet to work by connecting cat-5- cable directly to the board, and the other end with an RJ45 plugged into a network switch. this configuration had failed to get any connectivity or even a link light on the switch.
Can you elaborate on what is needed to get this working properly and why?

[brad]

For one you can't just connect direct. There is impedance matching as Ethernet is practically an RF signal. The line needs to be a 3v pullup and the decoupling capacitors are definitely needed on each data line. Vonger did the design for "magnetics" which have a center tap of a coil at 1.8V. So there is no 1.8v pull up on the data lines. Which is correct for ethernet. But, like everything in this world, there is usually another "not approved" method.
The information on not using magnetics is defined in this document quite clearly and I will not explain it further. You can read it for yourself. http://www.ti.com/lit/an/snla088a/snla088a.pdf Check page 5, this the schematic we use and the wiring is shown in my first post.

Basically, you need to pull the ethernet data lines up to 3v. This is easy, since the 50ohm resistors are on the board and a 3v supply is close by. You could also use a gpio port if you were a control freak. I use a single strand of super thin wire to join the 3v to the non ground side of the capacitor shown in my diagram. As this location connects to the two 50ohm resistors for that data line (I will take a photo when I get back home to show how I wired it). Then all you need is a capacitor between each ethernet connector (this is external to the vocore). This capacitance works best at 33nF. It needs this capacitor to retain the same electrical characteristics, that is, blocking the DC. For short tracks it works great. I have tested with an ethernet cable 20cm long going to a switch and this also works.

[nbash]

How can I make similar for VoCore2 ?

I have made it according to the scheme from http://www.ti.com/lit/an/snla088a/snla088a.pdf but doesn't work.

vocore2-switch.jpg (28.34 KiB) Viewed 57195 times

Really only the transformer will help?

[Vonger]

default the lan ports are used as gpio/sd/uart, so change this will not effect.