Phones typically use micro-B instead of micro-AB or mini-AB, meaning that micro-A and mini-A cables can't be connected to them. Since these cables use an additional, grounded pin to identify which device is the host on the micro-A / mini-A end, how would a phone with a micro-B receptacle determine the host? Or do phones lose the ability to act as a host by use of the micro-B port?
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1\$\begingroup\$ The micro-B receptacle still have the ID pin. \$\endgroup\$– Ale..chenskiCommented Feb 8, 2018 at 17:35
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\$\begingroup\$ @Sparky256, your comment is incorrect. A phone with Type-C receptacle is whatever it is, usually a device. If it is a dual-role device, it advertises the dual-role function by periodically toggling (alternating) sink-source signature on CC lines. When a Type-C "link partner" is plugged in, they end up with correct roles. electronics.stackexchange.com/a/255772/117785 \$\endgroup\$– Ale..chenskiCommented Feb 8, 2018 at 17:47
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2 Answers
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You're confused. USB Mini-A, Mini-B, Micro-A, and Micro-B plugs -- as well as the ports they plug into -- all have five pins, including one ID pin. They are all electrically identical.
The only difference between Micro-B and Micro-AB is the physical shape of the port -- a USB Micro-A connector cannot be inserted into a Micro-B port. As Micro-A (as well as Mini-A) connectors are almost never seen, this has no effect in practice.
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\$\begingroup\$ Thanks for the response - I'm aware that micro-B also has 5 pins, but my understanding was that micro-B cables won't have their ID pin grounded on either side, meaning that neither device identifies as host - am I wrong? I thought you'd only find the ID pin grounded on the micro-A side of a cable. \$\endgroup\$ Commented Feb 7, 2018 at 2:15
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\$\begingroup\$ What do you have in mind when you say "micro-B cables"? Typically, a cable will only have micro-B on one end. The other end is either type A male (for micro-B acting as a device) or type A female (for micro-B acting as a host). \$\endgroup\$– user39382Commented Feb 7, 2018 at 2:17
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3\$\begingroup\$ I've never seen such a cable, and I suspect they're prohibited by the standard. But I don't see any reason you couldn't make one by strapping the ID pins on each Micro-B end appropriately, then printing HOST and DEVICE on the respective ends. :) \$\endgroup\$– user39382Commented Feb 7, 2018 at 2:23
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1\$\begingroup\$ This is not what is available or not. There is no uB - uB cables because the only u-cable is defined as uB - uA. The other legal cable is uB to Type-A plug. The third one is uA captive cable. There is also uA - Type-A receptacle adapter. That's it, only four official types. This is all in the specifications. \$\endgroup\$ Commented Feb 7, 2018 at 3:14
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1\$\begingroup\$ @VortixDev, you are correct, B-cable overmolds must have ID pin floating. However, industry is somewhat in disregard with specifications, and uses uB receptacles instead of u-AB on phones and tablets. As result, there are bootleg uB - Type-A repectacle assemblies called in marketplace as "OTG Adapters", with ID pin grounded. \$\endgroup\$ Commented Feb 7, 2018 at 3:19
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Most (if not all) phones/tablets with a single micro-B port are in fact dual-role devices. They (nearly) all have the capability to act as host, mainly partially for the purpose of debug function and initial partitioning/re-partitioning and configuration. Usually one can connect a hub, and have keyboard, mouse, and flash drive to perform this function in phone bootloader (if it is unlocked). But all these functions are not for the end user to have, so manufacturers resorted to a trick limit this use - they don't use officially specified u-AB receptacles on the devices. Instead they use micro-B receptacle, and, in cases of emergency/necessity, rely on a theoretically illegal "OTG Adapter", which is a short cable with Type-A receptacle on one end, and u-B plug with ID pin grounded instead of u-A plug (which is technically in violation of specifications, since only u-A plug should have the ID pin grounded). More, having a phone to behave as a host to a bunch of peripherals would drain the device battery faster than customer's expectations, so this mode is not desirable.
In short, illegally grounded ID pin in u-B plug makes a phone to assume the role of USB host. This is how most phones manage their USB host function.
answered Feb 7, 2018 at 4:38
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4\$\begingroup\$ Lots of wildly imaginative ignorance wrapping a tiny shred of truth. There's no secret trick or attempt to lock end users out the USB host mode, in fact it's pretty much only for end users and no small number of phones ship with the adapter cable. In contrast, manufacturing and development systems strictly operate the phone in device mode as the programming and test systems are the host. Knowledge of these systems and the software itself is very widespread and common among people who actually work on them - it's even right there in the public Android documentation. \$\endgroup\$ Commented Feb 7, 2018 at 4:52
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1\$\begingroup\$ This makes no sense, what do the manufacturers stand to gain from stopping end users from using their device as a USB host? Why do you have to make this a conspiracy theory? A plausible reason why nobody uses micro-AB is because it sucks mechanically. See electronics.stackexchange.com/questions/242619/… \$\endgroup\$– jmsCommented Feb 7, 2018 at 4:57
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2\$\begingroup\$ @GAttuso actually neither answers the question. The potential host detects the presence of a USB slave device by checking the sense pin (pin 4) of the connector - if it's grounded, it's a slave device (i.e. an OTG adapter) and the phone acts as a host, if it's floating, it's a regular USB cable and the phone acts as a slave. If there's a 124k resistor to ground, it's an externally powered slave device which can deliver power to the phone, so the phone simultaneously charges and acts as a host. \$\endgroup\$– jmsCommented Feb 7, 2018 at 8:21
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4\$\begingroup\$ @AliChen - on the contrary, dealing with phone bootloaders is a substantial part of my job, which is why I'm pointing out your outright and imaginative lies on the subject. Phone bootloaders operate in USB device mode, so that code can be loaded from a development system. Having to use a USB device like a USB stick for that purpose is extremely disadvantageous in a production or software development environment, which is why the people who designed phones and their SoC chose device mode as the primary interface for manufacturing, test, and development. \$\endgroup\$ Commented Feb 7, 2018 at 16:47
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3\$\begingroup\$ I, too, have never heard of a device that operated this way. It wouldn't make sense. Operating as a USB host is significantly more complex (and, thus, costly) than operating as a device; there's no reason a manufacturer would include that functionality just for debug and provisioning. \$\endgroup\$– user39382Commented Feb 7, 2018 at 22:42