Raspberry Pi - Hardware Flaws and Fixups?

Hi Pete,

I love the work you have done on this, I am having great fun and learning loads with my Pi.

I do have one suggestion/request.

In my current setup, I have an external USB hub powered with a 3A PSU (which I know is overkill, but it was handy).  For the wiring I have a micro USB lead into the hub just for power, and then I have a standard USB A to USB B lead from the hub into one of the USB ports on the Pi.

This seems to be one cable more than I should need.  To make the cabling cleaner and simpler looking, I have tried removing the micro USB lead and the Pi starts to boot up, but then resets when it gets to the USB Hotplug boot code.

Why couldn't the power flow from the powered external USB hub into the Pi via one of its two USB ports via a USB A to USB B lead?  Then I would only need the one lead from the Pi into the hub for both power and data.

It is a very small point, but I think it would be a nice touch.

Thanks

Gary.

GizmoB73 wrote:

In my current setup, I have an external USB hub powered with a 3A PSU (which I know is overkill, but it was handy).  For the wiring I have a micro USB lead into the hub just for power, and then I have a standard USB A to USB B lead from the hub into one of the USB ports on the Pi.

Hi Gary,

I'm not Pete, but I think I understand what is going on. I had planned to do exactly what you wanted to do before i found out why it doesn't work. As apparently you've verified... :-)

There are several auto-resetting fuses on the board. There is a 700mA one between the micro-usb power-port and the rest of the system. And there are two 140mA ones between the 5V of the system and the outgoing USB ports.

So, apparently your Raspberry Pi manages to partially boot through one 140mA fuse before apparently the fuse decides to do what it's supposed to do: enough is enough! and it shuts down the power.

Pete has said that those fuses, although rated for 140mA will hold out until 280mA, and then suddenly start to limit the current to 140mA. Apparently the RPI manages to work on less than that 280mA.

What I THINK you observed is that at that stage the USB gets enabled, and that also means the ethernet chip. That's what makes the difference between the Model A and Model B, and that causes a reported difference in current-requirements.

It sounds very much like you could be right, that would explain why it kept rebooting.

Presumably that means that one of those two fuses could be swapped for a larger value then.  I realise why that has been done, but there could be an understanding that the bottom port could double up as the supply perhaps.

Jumpers could be another solution, but i doubt that there is sufficient spare space on the board for that?

Obviously i won't be hacking this one to test it out!

Yes Roger is spot on.

I can see the value in your suggestion, but I was worried simply that if someone parks something on the second USB that takes a fair bit of current, adds something to the GPIO and then tries to feed the whole board by back feeding from a USB we were going to run into 'use case' related issues and related support.  (OK - like now)

There is an argument for at least up rating to 500mA but the whole concept of back feeding is a topic in itself.  I always thought this would be a world of unintended consequences as IMO it should not really be supported anyway?  It also presents problems in that then the input fusing and tracking needs to be reviewed to just make sure it will cope with it - which I do so if you want to hack your own Pi you can.

The SMSC does have reporting for USB power issues , but I don't think the software guys have implemented in the drivers - moot point if the fuse rips the rug from under you anyway.

This whole topic looks like a thread in itself with additional (valid) discussion on PoE for new versions.  Would anyone be willing to put up a thread collecting all the salient stuff so far in the head?

Cheers

Pete

Hi Pete,

thanks for providing this excellent "technical window" to the Raspberry Pi and for the interesting dialogue.

Have you ever considered using a power management IC instead of the several LDO's ?

While I think that PoE sounds interesting I've the impression that probably will be cost prohibitive to implement, I'm still on the search for a cost effective PoE solution (meaning a IEEE802.3af fully complaint.)

Not sure if I missed the answer, but it has been asked a couple of times if and when full schematics of the board will be released ?

So far the only section that seems to be available is the power supply.

One suggestion I'd add is replacing the microUSB connector by something more sturdy like a DC Power Jack.

Thanks again & Regards

Jorge

Hi Pete,

thanks for providing this excellent "technical window" to the Raspberry Pi and for the interesting dialogue.

Have you ever considered using a power management IC instead of the several LDO's ?

While I think that PoE sounds interesting I've the impression that probably will be cost prohibitive to implement, I'm still on the search for a cost effective PoE solution (meaning a IEEE802.3af fully complaint.)

Not sure if I missed the answer, but it has been asked a couple of times if and when full schematics of the board will be released ?

So far the only section that seems to be available is the power supply.

One suggestion I'd add is replacing the microUSB connector by something more sturdy like a DC Power Jack.

Thanks again & Regards

Jorge

Someone always has something they don't like when choices are being made.

I think the decisison to go for microUSB is a good one. Adapters providing "5V, 1A charging current" for devices like HTC smartphones are common. The charging connector has been standardized to "microUSB" which now nicely fits the Raspberry Pi. IMHO this all fits together nicely.

Do you have any (anecdotal?) "proof" that the microUSB connector is (too) weak?

Schematics are going to be published, I've heard. Sometime after the design has been "finalized" which means more than a few boards going to customers.

I think the micro USB is an OK choice, but I think a 2.5mm dc power jack (positive tip) would have probably been better. They are two a penny, very common psu's and tend to be more robust sockets when physically anchored.

I work for a hardware manufacturer who use these micro USB sockets in a couple of our products. I have seen plenty where the socket is rattling around inside the case after someone has pushed the plug in  too hard and the socket has ripped clean off of the pcb.  The socket can also get loose internally when forces are applied with the plug inserted.

This is not intended to be criticism, merely my observations, from existing products, and I doubt that school children will be gentle.

It is not an issue of liking it or not, I remember reading numerous reports (you can google for it) of customers complaining about issues with the microUSB connector in several products, such as the Nokia N900, Nokia E7, HTC EVO, BlackBerry PB, etc. Some people even admit that is their fault when the connector is not lined up properly (ie. upside down) and by insisting and putting extra pressure the surface mount female jack gets removed from the pcb or if the traces are too thin they get cut due the mechanical stress.

About availablity of power supplies, nowadays you can find zillions like the one distributed with the Roku box which uses a standard DC jack, they sell them for $9.99, but you can find many other sources with a $5-$10 price range.

I agree with GizmoB73, not intended to be criticism, being the main target user school kids, anything that make the boards more reliable and child proof IMHO is a plus.

Didn't check, but in large volumes I'd also look at what is the cost difference between a decent DC jack and the microUSB.

The problem with micro USB is when you apply some vertical force on the plug,  a small force to the connector may produce a significant force on the tracks of the pcb, welding holds, but the tracks come off.

Generally, the design of housing will prevent it.

Hey GizmoB73,still connected, did you give up with the media player ?

Is XBMC ok ?

Did you get some help or info from the foundation about the low frame rate and the lack of sound ?

Maybe Pete has the solution.

It's certainly something I worried about.  The micro USB's are a bit of issue because they latch the connector in, and the amount of force you require to extract it is significant. The only problems I have had reported is when trying to extract they have used the electrolytic cap on the PSU input as a counter point to hold the board and broken that off. Interestingly they all still work perfectly - just look a little sad.

I deliberately chose one that had through hole anchor pins on it as I could see that the fully surface mount ones are (well to be blunt) - rubbish in terms of abuse withstand.  To mitigate against users trying to force the connector in upside down the part also has a solder land under the front of the connector which solders the formed body together and also to the board. We also chose an HDMI socket that had good through hole retaining pegs for similar reasons.

In terms of why we chose micro USB - simply the wide range of cheap phone charger that now exist and the standardisation of voltage supply spec that saves us another regulator.

Right call? - time will tell (still need to move that cap though)!

Cheers

Pete

Thanks for the follow up Pete. As you said time will tell, and hope you guys can sail smoothly through the compliance tests and put the boards on people's hands, that will really be the ultimate test.

Good call on the HDMI one, yes those cables can put a lot of tension on the connector.

Cheers

Jorge

I'd agree the 2.5mm jack is more robust, but it's a bit *too* common. My company's first product that used a 2.5mm jack, running on 5V and it ended up being bundled by some retailer with a completely different product that used the same jack but came with a 24 V supply(!) and you can guess the results. That quickly got fixed with a redesign but the protection circuit cost some money and board space.

Jacques, I am mobile at present with no access to the Pi.

I will try what I can when I return next week.  I have three Mede8ers around the house running from a NAS so the XBMC is an amusement, not something I really expect to be using.

Yes, I guess that one of the side effects of using USB connectors is that you have some guarantee that the input will be ~5V to meet the USB standard.

For some of the projects I've been working on lately I started using the TI SWIFT step down DC-DC converters, such as the TPS54231, it provides a wide input range and is highly efficient, obviously you need a source higher than 5V but I added a full bridge rectifier so you can get AC/DC input not worrying about polarity, but as you said it represents additional cost and board space.

I also asked Pete if they ever considered using a power management IC, I believe that Eben at some time mentioned that the plan was to use a buck regulator.

I know cost is a big issue, but there are some IC's that can take multiple power sources, include a LiPo charger and multiple outputs with protection.

Good job Pete, it seems that you have choosen the best USB connector.

Ready for a Mars mission