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I mean, this is against the rules!

Most USB OTG devices claiming to be host capable (most Android smartphones and tablets) have a simple micro-B connector. Shouldn't they have a micro-AB connectors? Micro-B are supposed to be for devices unable to act as hosts.

It actually leads to a proliferation of USB OTG cables like this one:

enter image description here

Which are forbidden by the USB specs (micro-USB specs chapter 3.4: Compliant Cable Assemblies). We clearly see it is a micro-B, not micro-A plug (there are chamfers), and it is associated with a standard-A receptacle. I wonder how they are even allowed to print the USB logo on this, by the way.

So, why do manufacturers do this (both phone manufacturers and cable manufacturers)? How is it allowed by the USB consortium?

I'm asking this because I am actually designing a USB OTG device. I don't plan to make it certified by USB anyway (given the costs), but I'd like to know whether I should strictly follow the standards, or screw up with them like everyone else (the fact is - because of this - everybody is used to micro-B, not micro-A, and it's true the chamfers help prevent trying to put the connector the other way around).

dim
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    Purely mechanical reason: You can destroy an micro-AB port by forcing the cable in backwards - which is not possible with a micro-B port. – Turbo J Jun 24 '16 at 09:56
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    @TurboJ Is that *really* the reason? It would mean the USB consortium really screwed up, then. They define connectors that we don't really know in which way to insert, *and* that can break when inserted the wrong way? – dim Jun 24 '16 at 10:07
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    The micro-AB port on my Sony Mini Pro (5+ years old) did not break, but I often try to put the plug in the wrong way - which is rather annoying. You need a *bit* more force to be able to destroy the port. – Turbo J Jun 24 '16 at 10:16
  • When I look at USB 3.0 connectors I seriously doubt their capability of looking ahead. C'mon, its just an ugly hack added to the USB 2.0 connectors! (and then USB type C which is nice, but still another connector.. what a mess..) I'm less surprised by non B-AB type compliance now if compared to those other facts. – Wesley Lee Oct 27 '16 at 10:42
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    @WesleyLee I agree, but what bothers me is that people are not conforming to the specs. The mess could be acceptable if it's a weel-specified and agreed-upon mess. But I don't know what to choose, here. – dim Oct 27 '16 at 10:49
  • I do believe in this case the [de facto](https://en.wikipedia.org/wiki/De_facto_standard) standard is good, since it makes clear some problems of the specs. – Wesley Lee Oct 27 '16 at 11:02
  • What you have shown is an adapter cable which is necessary for those OTG products that used the wrong connector. Non-adapter cables that I have seen seem to follow the standards. Have you seen any micro-B to USB-B cables? That would be disturbing, and if they are not common, it would be a strong argument to follow the micro-AB standard for OTG. Personally, I follow the standard for my designs and the correct cables are readily available. The responsibility for incorrect usage is then on somebody else. – Tut Oct 27 '16 at 14:01
  • Oh and by the way, won't that adapter still work in a micro-AB receptacle? I don't really see where you would go wrong using the correct receptacle for your OTG design. I haven't seen any problems with broken receptacles due to wrong plugging, but I must admit that we don't have a huge number in use. – Tut Oct 27 '16 at 14:15
  • @Tut Of course it works in a micro-AB receptable. But the problem is that it works in a micro-B receptable too, which should be used for *devices* only. So, because on the other end there is a A recaptacle, it means that with such an adapter, you can ptentially connect a device to a device (no hosts). That is why the USB consortium forbids it. And why I am reluctant on using a micro-B receptacle for my design (which is both device and host-capable), since it would then rely on the existance of such evil adapters to connect a device to it. – dim Oct 27 '16 at 14:21
  • I agree and I don't really see the down-side to following the standard. BTW, I have yet to run any host applications using my OTG port. I asked above if you had seen any micro-B to USB-B cables, but in all fairness, I have also not seen any micro-A to USB-B cables. It seems that an adapter may be required either way. – Tut Oct 27 '16 at 14:32
  • @Tut That's the point: there is apparently no down-side of following the standard. So my question is: *why nobody seems to follow it?* Regarding the cable ending with USB-B, I don't agree however. Since there are some peripherals that have their cable directly attached (without connector) to them, you'd better have a cable ending with a A receptacle (and think about all the USB keys/dongles - they all have A plugs). – dim Oct 27 '16 at 14:37
  • "I wonder how they are even allowed to print the USB logo on this" That's not the USB Logo: https://www.logicsupply.com/explore/io-hub/wp-content/uploads/2016/04/USB-Comparison-Chart.jpg – endolith Apr 27 '17 at 18:32
  • @endolith Mmh, you're actually right, the symbol at the root of the tree seems to be a square on my image, whereas it should be a round. I fell for it. At least, this somewhat confirms this isn't allowed by the USB consortium, but I'm still wondering why most device manufacturers didn't want to follow the standard. – dim Apr 27 '17 at 21:07
  • @dim No, I mean it's not the "logo" (which requires USB certification), it's the "trident symbol" (which I believe does not) – endolith Apr 27 '17 at 21:16
  • @endolith Ok, I didn't understand, sorry. Indeed, the basic trident symbol isn't even mentioned in their current [logo usage guildeines](http://www.usb.org/developers/logo_license/USB-IF_TLA_and_Logo_Usage_Guidelines_FINAL_March_13.2017.pdf), (although specified in the USB 2.0 spec), so you may actually be right. – dim Apr 28 '17 at 10:30
  • The fact of device being USB-IF certified or not depends on how manufacturer declares the device. If it is declared as "device only", and no OTG functionality is listed (strictly for proprietary service), then the device could be well certifiable. – Ale..chenski Feb 08 '18 at 04:09
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    @Ali Chen I have been thinking about it and I think it is the real reason. Because certifying as OTG certainly adds constraints, and in practice, few people will use a phone as a a host. So certifying as device-only allow them to claim USB compliance, and they offer OTG, uncertified, as an unofficial bonus. You should put it as an answer, if you can back that a bit with facts. – dim Feb 08 '18 at 06:57
  • Yes, OTG certification is a pain, it is so convoluted and very conditional. So you might be right. Unfortunately, it is impossible to back up this motivation, since it is deeply internal decision for a manufacturer. There are few other arguments why they do u-B only, but I was badly beaten by Android Nazis for expressing them, https://electronics.stackexchange.com/a/354577/117785 – Ale..chenski Feb 08 '18 at 14:10
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    @AliChen Not sure the downvotes have to do with nazism. Maybe more because it only answers a part of the original question you linked to, and because the *"for the purpose of debug function and initial partitioning/re-partitioning and configuration"* isn't accurate, according to others. Anyway, I upvoted it, because it gave me some clues. But it would be more fit as an answer to my own question here, actually. – dim Feb 08 '18 at 14:21
  • Also Turbo_J has a very good argument. Symmetrical shape of u-A receptacle doesn't provide a good guide for end user which way to plug a u-cable (A or B, doesn't matter). Which increases chances to break the receptacle with a bit of force if one gets too impatient. So it looks like there are few factors together contributing to industry-wide decision to use only u-B receptacles. In any case, the whole idea of micro connectors was a failure, they are too fragile, and now are replaced by Type-C. Switch to Type-C in your design. – Ale..chenski Feb 08 '18 at 14:26
  • I think Android police is over-reacting, expressing their limited involvement into root development. I probably should restrict my answer just describing the bootleg ID connection in u-B plug, and leave the possible motivation out. – Ale..chenski Feb 08 '18 at 14:29
  • @AliChen I'd like to use type C. But it seems type C with OTG requires some additional logic. I'm using regular STM32 MCUs and want to keep the design simple. I didn't really checked that yet, but if I need to add more components to handle that, it may not be worth it. It is already complicated enough with just providing 5V when acting as a host. – dim Feb 08 '18 at 14:31

2 Answers2

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The more I think about it, the more I'm convinced the reason is not technical. It likely is just to avoid the burden of certifying a device with dual-role OTG capability.

Apparently (see here), testing a device for OTG compliance costs almost twice as much as testing a simple high-speed device. There is also much more work in the design phase, and much more risk of failing the compliance, having to make design iterations, and going through testing again. Since the OTG capability isn't used by many consumers, manufacturers don't feel the need to advertise full OTG compliance.

Instead, they simply certify the phone/tablet/whatever/... for "USB device-only" usage, then use clever marketing wording (claiming "OTG-compatibility", for example), and call it a day.

Because they don't test for dual-role OTG compliance, they are not allowed to use micro-AB connector (or they would fail the simple device-only compliance). So they simply put a micro-B receptacle, and expect cable manufacturers to provide adapters such as the one shown above (although such adapters are officially forbidden by the USB spec - but cable manufacturers don't really care).

Also note that the logo on the cable above is not the USB logo that is subject to strict licensing. It is the "trident" logo, whose usage is much more relaxed. This logo, for example, does not imply that the item has passed the USB compliance tests.

dim
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I believe this is because of the cost of the devices. USB Micro-AB have the same soldering footprint as Micro-B. Micro-B is much wider used hence the connector is much cheaper due to the economy of scale.

Maxthon Chan
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    Yes, but if they used micro-AB instead of micro-B on all phones, wouldn't micro-AB be much more widely used and hence as cheap as micro-B? That looks like a chicken and egg problem. – dim Jun 25 '16 at 07:17
  • @dim Micro-B is also used where OTG is NOT needed, and that is a much wider market than places where OTG is needed. Still, economy of scale. – Maxthon Chan Jun 25 '16 at 08:11
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    Is it really "much cheaper"? I believe B and AB connectors are roughly the same price. – Dmitry Grigoryev Jun 29 '16 at 08:25
  • How close is roughly? 1 cent? for products with 1M production runs it is a substantial amount. – Lior Bilia Oct 27 '16 at 10:56