External non-USB DC supply

I was going to make a ‘cape’ for my Beaglebone to both provide an RS232 interface plus also an external DC supply - in the environment of interest there will be DC available, but it will probably be higher than +5V so I was planning on placing a local regulator etc on the cape that provided +5V output for a ‘range’ of inputs.

Reading the Beaglebone Reference manual it suggests that the ‘VDD_5V is the main power supply from the DC input jack’ and also this same signal is also to be found at both pins 5 & 6 of the expansion header P9 - the circuit diagram seems to also suggest/confirm this!

However! Measuring continuity between the DC jack’s centre pin and P9’s pin 5 or 6 however returns a high resistance,

The manual also quotes:

It is possible to provide 5V to the main board from an expansion board. By supplying a 5V signal into the

VDD_5V rail, the main board can be supplied. This voltage must not exceed 5V. You should not supply any voltage into any other pin of the expansion

connectors. Based on the board design, this rail is limited to 1A per pin to the BeagleBone.

Has anybody any experience/knowledge of this area please?

Sure. We do this all the time. The 7" LCD for example has a 5V jack on it. We connect a 5V supply to it and it powers the BeagleBone just fine.

If you were to read the schematic, you would understand the issue here. The DC jack does NOT connect direct to the expansion header, but instead goes to a NCP349 over voltage detection device which is turned off when there is no DC plugged in. The expansion pin connects to the output of that device. So, what you are reading is correct.


Sorry Gerald - BUT if I look @ the schematic (and I already had) the DC jack DOES go direct to the expansion header - YES the NCP3489 input is connected but paralleled with the header and its ‘protected’ output is referred to a DC_IN. On both the diagram and the manual VDD_5V is referred to at the Jack input and header.

As a further experiment - on my Beagle bone I have powered it and there is a +5V connection @ pins 5 &6) if (and only if the jack rather than USB is present power source is provided)

According to me either the circuit diagram I have is wrong or something else is.

You might well be right as to things working OK with a cape board, but what I have seen certainly does not make sense.

If you were to read the schematic, you would understand the issue here.
The DC jack does NOT connect direct to the expansion header, but instead
goes to a NCP349 over voltage detection device which is turned off when
there is no DC plugged in. The expansion pin connects to the output of
that device. So, what you are reading is correct.

The schematic at https://github.com/CircuitCo/BeagleBone-RevA6/blob/master/BEAGLEBONE_REV_A6A.pdf?raw=true does not show that.

It shows P5-1 and P9-5 both tied to VDD_5V.

However, it doesn't act that way... schematic error?

- Mike

You have old schematics. That was changed a while back. What revision is your board?



I'm looking at the Rev A6 PDF schematics at:

Are there newer schematics somewhere?

- Mike

Well my board is (I think) a REV A6.

As for the schematic - I only downloaded that the other day (PC say 07th August) from http://beagleboard.org/static/beaglebone/latest/README.htm
and they are labelled ‘Saturday, November 05, 2011’’ which I notice has very recently changed to ‘Thursday, February 09, 2012’ (+ is portrait mode rather than the original landscape). BUT it is still wrong! as is exactly the same as the original here - both input + expansion labelled VDD_5V.

Incidentally - thank you Mike - your schematics are dated ‘Thursday, June 28, 2012’ + suggest revision A6A but still show exactly the same - just like you/I said!

Richard Croucher

Let me look into this. It will take me a while to get to it.


OK. I just took an ohmmeter and I am getting 0 ohms between the jack center pin and P9 pins 5 and 6 on the BeagleBone.

Sorry for the confusion. The rev A1 board had the VDD_5V on the other side of the NCP349. I moved it on the A3 version before release to production.


Ok - I can easily believe this - but where is the most recent + up to date documentation as everything I have seen so far says otherwise.

This includes both the schematics and the reference manual!

I can only assume @ present that this 'change' means that there is no 'over-voltage' protection via the header connection and that the NCP349 does not mind having a DC supply shoved towards its output?

I does indeed mind. That is why it was moved to the input. And yes the NCP349 is still there. The center pin of the DC jack connects to the input of the NCP349 and to connector P9. The latest schematics and SRM can be found here and they do show this connection. And I verfied the connection with my mutlimeter as I have already indicated.



Gerald - I am now getting confused.

On my Beaglebone - believed to be Version 6 the centre pin of the DC input certainly does not connect directly to expansion pins which is why I initially raised the question.

As I only received this board a couple of weeks ago + it is marked to be version 6 - why is this?

As previously indicated - I carried out a similar multimeter test to yours and there is high resistance. This board certainly does not appear to reflect the 'up to date' or even diagrams over a year old!

This can only suggest that the connection has not been changed on this board - whatever version it actually is. Can you estimate the date that you 'updated; things please?

But it does. When you turn the board over and you measure from the center tab, which connector are you measuring to? Left of the DC jack or right of the DC jack?


My A6 does connect P5-1 to P9-5 - verified with multimeter.

Also, I was using P9-5 to power an external board, with no troubles.

Are you sure of your measurements?

- Mike

I was measuring to the pin/connection that comes out of the back - which certainly looked like the centre pin. Plus after failure to get an expected result I also tried both tabs that penetrate the PCB either side.

Or at least I though I did!

I will have to dismount the board and have a closer examination of this tomorrow when I again have access to it (I am in the UK -hence 21:30). Will come back with whatever I find!

At least this conversation has confirmed that my original idea of powering everything from the expansion headers is sound. My initial measurements were an attempt to confirm this - hence my raising the question when I got what looked like a negative result.

With the DC jack facing you and the backside of the board facing up, P9 is on the left.


I was already in the correct area thanks.

But you are right! - my continuity measurements today confirm the link. Why things appeared to fail yesterday I have no idea, and I even repeated the measurement with the same result + made some voltage measurements later so I must have been in the correct pin/positions.

I do not know how many sites contain circuit diagrams etc. but this exercise has demonstrated that they are not all up to date etc. Can I please suggest that someone keeps the http://beagleboard.org/static/____beaglebone/latest/README.htm <http://beagleboard.org/static/__beaglebone/latest/README.htm - which I would imagine to be one of the most ‘official’ ones up to date!

Apologies for any confusion caused on my part - plus the additional amout/repeat of things attached to my replies etc yesterday. Many of these were done directly via google mail, rather that visiting this site and the necessity to cut off earlier texts was not obvious.