Incompatible Self-Powered Hub

I'm not sure it's the RPi design that's wrong, more that USB hubs are not always very careful about where their power goes. For instance... I have a Logik LP4HUB10. But, it sends power through the hub-Pi cable (just like Morgaine's problem) and I had to follow instructions http://www.raspberrypi.org/phpBB3/viewtopic.php?f=28&t=8926 here that meant taking apart the hub, snipping a wire and putting it back together again.

I guess the only thing the Pi could do is to prevent incoming power into the two USB ports... But then again why should it - power going through the wrong wire in a hub is a power drain, surely?

Michael, the Pi's power design is wrong in various areas, but in this thread I'm referring only to the Pi's USB host end by the type A sockets, ie. not to the micro-USB end.  Other problem areas are for other threads.

The Pi's power design is very badly wrong at the USB host end because even if a downstream hub were supplying more than 500mA per port (which the USB spec does not require it to do), and even if a self-powered hub's type B port were supplying that current upstream through the type-B to type-A lead (which it is not required to do), and even if that upstream current were sufficient to power the Pi (which it is certainly not required to do), then even under the combination of all these auspicious circumstances the Pi will still not boot properly when powered through this B-A upstream path because the F1/F2 polyfuses have far too high a resistance to keep the upstream voltage above 4.75V as per the USB spec.

This end of the power design is wrong whichever way you look at it.  The voltage drop across an F1/F2 polyfuse (eg. 4 ohms on my Pi) at the Pi's nominal 700mA is around V=IR = (0.7 * 4) = 2.8V, so even a perfect infinite-current supply at the upper end of the USB voltage spec (5.25V) would provide the SoC with (5.25 - 2.8) = 2.45V through the polyfuse, which cannot possibly work.

There is simply no in-spec way for the Pi to be powered from a downstream self-powered hub via that A-B path, and therefore the Pi design should prevent A-B power from being applied upstream.  And because the Pi power design doesn't block it, the Pi can go into rapid reboot cycles and damage HDMI equipment with perfectly in-spec hubs, as in this case.

Of course there are ways to hack hubs and cables to prevent the worst of this, but that's beside the point.  And applying power from the hub to the micro-USB charger socket is completely outside the topic of this thread.  That doesn't work either with this low-current unit, but it's not my topic here, and I can't criticize this cheap little hub for that --- it's not intended to be a Pi power supply.  It is however intended to be a working self-powered hub for USB devices downstream of the Pi, and in that it fails completely for reasons already described.

Morgaine.

Morgaine Dinova wrote:

 

There is simply no in-spec way for the Pi to be powered from a downstream self-powered hub via that A-B path, and therefore the Pi design should prevent A-B power from being applied upstream.  And because the Pi power design doesn't block it, the Pi can go into rapid reboot cycles and damage HDMI equipment with perfectly in-spec hubs, as in this case.

Morgaine.

I stated upstream that a hub should not be sending current upstream.  Just to make sure, I took a quick look at the USB 2.0 Standard and found this:

Section 7.2.1:

 

No device shall supply (source) current on VBUS at its upstream facing port at any time. From VBUS on its upstream facing port, a device may only draw (sink) current.

 

If a USB hub is sending current upstream, it's not in compliance with the USB 2.0 standard.  IMO, it's not RasPi's responsibility to work around non-compliant hubs, but it may be in RasPi's interest to do so anyway since there seem to be a lot of non-compliant hubs out there.

In general, I find the USB 2.0 Spec to be quite readable.  An exception to this is the software sections, which help me appreciate why it's so difficult to write host controller software well.  The spec is available for free at http://www.usb.org/developers/docs/.

Is it possible to put something (i.e. simply solder) on the Pi so that it stops the upstream voltage? (Not from a it's-the-Pi's-problem point of view, more of a hobbyists question).

As for the USB spec... I choose not to read that. Please don't make me ;-)

Michael Horne wrote:

 

Is it possible to put something (i.e. simply solder) on the Pi so that it stops the upstream voltage? (Not from a it's-the-Pi's-problem point of view, more of a hobbyists question).

As for the USB spec... I choose not to read that. Please don't make me ;-)

According to my read of the schematics, all you need to do is remove F1 or F2, depending on which RasPi USB port you want to connect to the hub.  The LAN9512 can still detect whether +5V is present or not using D11 or D10.  I don't see any other connections to +5V0_USBA/B.

Aw c'mon, the USB spec is mostly quite readable, and has lots of pretty pictures.  But whatever you do, start with Chapter 4.  Chapters 1 - 3 can cure insomnia instantly.

If the USB spec says "No device shall supply (source) current on VBUS at its upstream facing port at any time" then I agree with John that no hub that does this is certifiably USB compliant.

Farnell are then in an even worse situation than I feared.  Not only should this hub be removed from the Farnell Pi accessory page since it is not Pi-compatible, but all alleged "USB" hubs that do likewise should either be removed from stock or no longer be described as "USB" hubs.  It's very common, so this would have a massive impact.

Of course, we know that the latter is not going to happen.  Pragmatically, one has to accept the situation as it is and work with it.

Engineers are typically very pragmatic.  The standard engineering mantra of "Be generous in what you accept but strict in what you send" is a very good mantra to apply in this area, and it should have been applied when the Pi was designed.  It wasn't, and so the Pi design has given Farnell and RS a big headache, rendered many a hub product on the market unusable with Pi, and endangered HDMI gear through the pathological rapid boot cycling.  Not good.

Pi design issues aside, I don't think it's good for Farnell to be listing a Pi non-compatible device as a Pi accessory.  At its exceedingly low price though, it's sure to be worth a simple hack to bring it into compliance.  Of course, official Pi accessories shouldn't need hacking to work.

Morgaine.

The Pi's power design is very badly wrong at the USB host end

I was curious about how the USB host end worked for a more normal x86 PC, so picked a random Asus board that happened to be lying around. You know what, it seems to use something very similar to the Pi. The polyfuses appear to be  1.6A (0.14ohm so unlikely to be 160mA) and there's a ferrite bead between the fuse and the usb connector. Difficult to tell if there's a diode to allow detecting a fault condition or not.

Of course the big difference is that the other side of the polyfuse is directly onto the main 5v rail - in this case something that can do 5v @ 25A. So even if we assume a 4 port powered hub has a 5v 2A supply it's rather unlikely to have any chance of powering something that can potentially draw 10x that current. 

Still, these non-compliant powered hubs can't be doing themselves any favours trying to dump whatever power they have available back into a load like that. The Pi has the disadvantage(!) of being such a low power device that this reverse power can cause the symptoms you describe.

Aye, the high resistance of the F1/F2 polyfuses causes a variety of problems, and this is just one.

As we've been saying for many months now, F1/F2 should be specc'd to trip at a far higher current than at present because low trip points entail high values of resistance which are a design error when present in a USB power line.  Using them as a 100-140mA USB current limit is a misapplication of the devices and a total disregard for voltage regulation.

Once the Pi is available ex-stock, I bet it'll be common to short out F1 and F2 and power the Pi from such badly behaving hubs through their upstream host A-B lead, thus eliminating one mains adapter from the system.

Just for fun, I looked for the USB certification mark on this hub.  The hardware doesn't have one.  That's not uncommon of course, but the point is that the hardware isn't lying about compliance.

For even more fun, I checked the packaging which by some miracle hadn't been thrown out yet, and discovered two interesting things:

• The packaging not only carries a Certified USB Hi-Speed mark, but also proudly proclaims "USB2.0 and USB1.1 Compliant and USB-IF certified" ("USB-IF" is the USB Implementers Forum).
• The packaging has a "2A" legend next to the power supply symbol, despite the fact that the hub's mains adapter is marked "MODEL: YD-001 OUTPUT: 5V 1000mA".

So, my guess is that Dynamode bought in a bulk load of uncertified hubs of unknown compliance, didn't bother to check whether they were USB2.0 compliant or not, bundled them with a random mains adapter, and their marketing division made up fictional packaging without reference to what was actually being provided.  And Farnell buyers fell for the false advertising by Dynamode.

It's a funny old world.

Farnell Purchasing Dept --- please give Dynamode a heap of pain, plus added ridicule for incompetence.

Morgaine.

Morgaine Dinova wrote:

 

For even more fun, I checked the packaging which by some miracle hadn't been thrown out yet, and discovered two interesting things:

 

• The packaging has a "2A" legend next to the power supply symbol, despite the fact that the hub's mains adapter is marked "MODEL: YD-001 OUTPUT: 5V 1000mA".

 

I decided to have some fun too and see what the USB 2.0 spec has to say about how much current a self-powered hub should supply:

Section 7.2.1:

 

Self-powered hubs: ... Hubs that obtain operating power externally (from the USB) must supply five unit loads [500 mA] to each port. Battery-powered hubs may supply either one or five unit loads per port.

So  a 4-port hub needs 2A and a 7-port hub 3.5A to comply.  The adapter obviously didn't read the spec.

And people think engineers don't have fun!

John Beetem wrote:

 

So  a 4-port hub needs 2A and a 7-port hub 3.5A to comply.

Belkin get a lot of flak for poor quality hardware (mostly deserved, from my own experience), but interestingly their own 7-port hub model FSU307 has a 3.8A mains adapter, does not feed +5V upstream through the A-B lead, and works fine as a downstream self-powered hub for Pi.

What's more, despite supplying only 5.05V out open-circuit, it can power the Pi perfectly through an A to micro-USB lead, and a fairly high-resistance one at that (100mV drop when Pi is quiescent).  So, kudos to Belkin there.  However, that hub cost more than the Pi !!!

Unfortunately I have to put the Belkin back where it was after this test as its beefy power was needed there, but it provided useful comparison.  I don't need high power for the Pi's hub, just USB compliance and Pi compatibility, which the Dynamode lacks.

Morgaine.

John Beetem wrote:

 

So  a 4-port hub needs 2A and a 7-port hub 3.5A to comply.

Belkin get a lot of flak for poor quality hardware (mostly deserved, from my own experience), but interestingly their own 7-port hub model FSU307 has a 3.8A mains adapter, does not feed +5V upstream through the A-B lead, and works fine as a downstream self-powered hub for Pi.

What's more, despite supplying only 5.05V out open-circuit, it can power the Pi perfectly through an A to micro-USB lead, and a fairly high-resistance one at that (100mV drop when Pi is quiescent).  So, kudos to Belkin there.  However, that hub cost more than the Pi !!!

Unfortunately I have to put the Belkin back where it was after this test as its beefy power was needed there, but it provided useful comparison.  I don't need high power for the Pi's hub, just USB compliance and Pi compatibility, which the Dynamode lacks.

Morgaine.

Funny enough, i'm not completely surprised by the lack of compliance of the Dynamode kit, they are not exactly a top tier manufacturer are they ?

It is cheap and i'm afraid that this only confirms that you get what you pay for !

Steve

Steve Read wrote:

 

Funny enough, i'm not completely surprised by the lack of compliance of the Dynamode kit, they are not exactly a top tier manufacturer are they ?

Well perhaps this varies by country, but at least in the UK, the fact that a company is not "top tier" does not give it a free pass to make false claims about standards compliance nor about power rating of their enclosed mains adapters.  They have the same duty as everyone else.

UK consumer laws make the buck stop with the retailer, Farnell in this case, and Farnell is definitely a "top tier" company.  It's unfortunate that they were deceived by a poor quality outfit like Dynamode, but now that they know about it, they in turn have a duty to prevent further customers from being deceived by the false advertising.

I don't know how things work behind the scenes in this sector, nor how companies sort out these kinds of mishaps among themselves, but I expect that Dynamode is about to get an unhappy phone call from an irate corporate buyer.  I certainly don't expect Farnell to collude with them and hide the problem.  Their "Best of British" tag would go down the drain pretty fast if they did that.

We'll see what happens.  Keep your eye on the Pi Accessories page.

Morgaine.

Completely agree that Farnell is a *great* British company, no argument there from me !

I guess they can't possibly fully test every bit of kit they hold (Don't they carry 100K + lines?) so it is only down to situations like this that can them aware.

In other news, i've just bought a Gertboard, so they have a bit more of my cash !

Don't they carry 100K + lines?

http://uk.farnell.com/jsp/bespoke/bespoke4.jsp?bespokepage=farnell/en_UK/support/about.jsp

"with fast, easy access to over 480,000 stocked products" - it's hard to imagine the scale of keeping track of that many products.

In other news, i've just bought a Gertboard, so they have a bit more of my cash !

I see they're not showing a lead time on it, what sort of estimated delivery date did they give you ?

Nearly Half a million lines, can't even comprehend that !

Estimated delivery date of w/c 8th October for the Gertboard.

*sigh at the lead-times* It's like the RPi launch all over again...

Hopefully it improves, I've just noticed the front page here says "Shipping begins mid-September". I suppose it'll be the PCB lead times that control it, the rest of the parts look to be fairly standard stuff that you'd expect to have off-the-shelf availability.

Steve Read wrote:

 

I guess they can't possibly fully test every bit of kit they hold (Don't they carry 100K + lines?) so it is only down to situations like this that can them aware.

Absolutely.  It's totally impossible (practically and commercially) for them to test their whole product line even superficially, so they have to rely on the product advertising and specifications from each supplier being truthful.  When the supplier neither tests a product for claimed compliance nor checks that what is in the package is consistent with what's written on the packaging, then top tier distributors like Farnell are presented with an impossible problem.

This would destroy their operation if it became common in the industry, because the buck stops with the retailer for end-user sales,

I hope they nail this one.  Dynamode are clearly breaking at least two different advertising and/or trading regulations on this product.