Issues with BoB2

25 Nov 2009

BoB2 is a nice idea but there are a few things not quite right with it:

1N3595 or 1N4001 are poor choices for D2. The forward voltage drop is too high and does not leave enough voltage to power the mbed module. You need to use a Schottky diode.

You need to connect the MagJack transformer primary centre taps to +3.3V. These pins are not shown on your schematic symbol, but are pins 3 and 4 on the Tyco datasheet. Check out the DP83848 datasheet, Section 5.1, Figure 11. You also need to decouple these pins with 100nF capacitors to GND located close to the pins. If you don't make these connections the Ethernet signal amplitudes will be wrong.

The Ethernet tracks should be routed as differential pairs with a differential impedance of 100 ohms. Not doing this will introduce reflections which increase the jitter on the signals.

The USB tracks should also be routed as a differential pair. However, this is less critical than Ethernet.

USB interfaces can be susceptible to damage through ESD. It would be prudent to put some protective circuitry on BoB2. NXP do some low capacitance diode arrays that are suitable.

I recommend you thicken the power track from the USB connector to D2 to minimise the voltage drop. You might like to do the same on the VIN track to the PWM headers and the VIN_MBED track.

The present connection of the buttons and switches conflicts with the second SPI interface. I would like to keep that available for connection to LCDs such as the Nokia 6610.  I think it would be better to connect the buttons and switches to p27 - p30.

A push button reset switch connected to NR would be useful. I think there is just room to squeeze one in.

Incidentally, why did you specify an expensive POE MagJack when you are not using the POE function? A plain vanilla MagJack would do the job and costs around half the price.

I look forward to seeing the Mk2 BoB2. If you make these changes, I will definitely buy one!


25 Nov 2009

Hi Paul,

Thanks for the feedback. All the points you raise are completely valid :-)

mbed-BoB2 was done as a Friday evening hash up, mainly to learn how to use BatchPCB, and get used to Eagle. You might have spotted that I'm not a PCB layout guy :-)

Most of the components were chosen to suit what people are likely to have laying around. Sure the 1N4001 isnt the ideal choice, but most people will have them. I did use a BAT60A Schottky with a Vf of 0.1v on the original BoB, but it turned out to be tricky to get hold of, and pain to solder. I am sure there is a suitable part that would fit in place of the 1N4001. The PoE connector is on there as the original BoB had one (as well a PoE adpator), you are right, it should be a cheaper generic one.

The rest of the pinout stuff was fairly arbitrary, although I was hoping to lay components out according to where they connect, so it is clear.

I've got a couple on order, so i'll have a play with them when they get back - I'll probably also learn a bunch more things that can be improved by doing that.

I'm happy to roll in the changes you suggest, but it's not top ot the list of things to do at the moment, so it will be a few weeks. The Eagle files (.sch and .brd) are published on the page, so if you fancy having a bash at it yourself, feel free.






25 Nov 2009

Hi Chris,

Thanks for your reply.

I am willing to have a go at modifying BoB2. I have never used Eagle before so it should be interesting! I visited the BoB2 cookbook page ( but could not find the .sch and .brd files. Am I looking in the wrong place? If not, could you post the files there, or send them directly to my email address?


25 Nov 2009

Hi Paul,

Great stuff! Eagle seems pretty intuitive, and there are loads of good tutorials and resources as i found when i was getting to grips with it.

The .sch and .brd fles are at the bottom of the cookbook page as an zipfile attachment.

Shout if that link isnt doing it for you.


26 Nov 2009

Hi Chris,

The zip file does not contain the .brd and .sch files. It only seems to contains the manufacturing files.


08 Jan 2010

Paul / Chris,

Did you guys get anywhere with these changes?  I really like the breakout board, but I also understand that the changes that Paul is suggesting are useful, and would like to be able to order the PCB from the gerber files and schematic of  the 'new and improved' BoB3...



08 Jan 2010

Hi Jason,

Thanks for your interest.

If you haven't already done so, have a look at my notebook.

The BoB3 pcb's are currently being made at PCB Pool. They should dispatch them any time now, so hopefully I will get them early next week. I have already got the parts from Farnell, and will make one up straight away. As soon as I am happy there are no major problems I will generate some assembly documentation and post that, the Eagle files and some photo's in my notebook.

At present I don't have any Gerber files. This is because PCB Pool worked directly from the Eagle BRD file.


19 Nov 2010 . Edited: 19 Nov 2010


Check this Eagle library which is designed for mbed kit