USB: Decipher the signaling, connector, and power delivery differences

Article By : Brian Dipert

Get up to speed on USB-C, along with its alternatives and predecessors, with this review of interface signaling, physical connector characteristics, and power delivery capabilities.

As mentioned in a recent post, one of the motivations for my acquisition of a Google Chrome OS-based Pixelbook is that it’ll enable me to do a hands-on evaluation of its latest-generation external interface:

The Pixelbook is USB-C-based, which will enable me to test the transfer bandwidth and power distribution capabilities of this newest interface standard both in an absolute sense and relative to USB generational precursors and Thunderbolt alternatives.

Strictly speaking, to be clear, the Pixelbook doesn’t represent my first personal exposure to USB Type-C (“USB-C”, for short). My two Google Pixel smartphones, along with my wife’s Pixel XL, all include it as a successor to the micro-USB (in my case, Lightning in her case) connectors used with precursor handsets. But in all of these cases, USB-C has to date exclusively been leveraged as a charging port (more on that topic later in this post).

The Google Pixelbook, which includes two USB-C connectors, one on either side, also leverages USB-C for charging. But since it provides no other wired interface ports whatsoever, unless you count the analog audio jack, it also uses USB-C for peripheral (external display, memory card reader, mouse, etc.) and network (wired Ethernet, etc.) connectivity purposes. And since there are only two USB-C ports, and one of them is commonly employed for an AC (-sourced, DC-converted) power feed, you can imagine that multi-port expansion hubs will also commonly be in the mix.

In researching USB-C, along with its alternatives and predecessors, I’ve come across an abundance of incorrect co-mingling of various aspects of the standard, along with plenty of other inaccuracies; interface signaling (and associated transfer rate speeds), physical connector characteristics, and power delivery capabilities, for example. The intent of this blog post is to briefly take a stab at sorting these topics out. For far more detail, see the USB Implementers Forum website; Wikipidia’s entry for USB is also a good jumping-off point to dive into the deep end of the details.

USB 1.0 was publicly launched in January 1996; yes, I feel as old while writing those words as you do when reading them. Its modestly enhanced, more broadly adopted successor, USB 1.1, arrived on the scene two and a half years later. A four-wire half-duplex (arbitrated by the host) bus, it came in two peak claimed transfer rate options (although real-life implementations notably undershot this): 1.5 Mbps Low Speed (intended for mice, joysticks, and the like) and 12 Mbps Full Speed (intended for hard drives, etc.).

USB 2.0, released in April 2000, is backwards compatible with USB 1.x and adds a 480 Mbps High Speed transfer rate mode. Smaller-sized connector options (keep reading for more details later) unveiled in conjunction with USB 2.0 also optionally supported the five-wire OTG (On-The-Go) spec variant, which allows for two USB devices to directly communicate with each other without need for an intermediary USB host.

USB 3.0, released in November 2008, is once again backwards compatible with USB 1.x and USB 2.0 from a transfer rate mode(s) standpoint. It broadens the pin count to a minimum of nine wires, with the additional four implementing the two differential data pairs (one transmitter, one receiver, for full duplex support) harnessed to support the new 5 Gbps SuperSpeed transfer mode. It’s subsequently been renamed USB 3.1 Gen 1, commensurate with the January 2013 announcement of USB 3.1 Gen 2, which increases the maximum data signaling rate to 10 Gbps (known as SuperSpeed+) along with reducing the encoding overhead via a protocol change from 8b/10b to 128b/132b.

Even more recently, in the summer of 2017 to be exact, the USB 3.0 Promoter Group announced two additional USB 3 variants, to be documented in the v3.2 specification. They both leverage multi-lane operation over existing cable wires originally intended to support the Type-C connector’s rotational symmetry. USB 3.2 Gen 1×2 delivers a 10 Gbps SuperSpeed+ data rate over 2 lanes using 8b/10b encoding, while USB 3.2 Gen 2×2 combines 2 lanes and 128b/132b encoding to support 20 Gbps SuperSpeed+ data rates.

Speaking of connectors, the standard Type-A (host) and Type-B (device) “male” plugs and associated “female” receptacles are, more than 20 years after USB’s initial release, now ubiquitous. They coexist in part to prevent two power-supplying devices (Type-A) from inadvertently being connected directly to each other. Cables labeled as intended for USB 1.x are generally also usable in USB 2 systems. USB 3.x-compliant versions of these standard plugs and receptacles are blue in color, to indicate their higher pin count and commensurate SuperSpeed support.

With the emergence of compact digital audio players, smartphones, tablets, portable hard drives, and the like also came the need for smaller form factor USB connectors. The mini-USB (released in April 2000, in conjunction with USB 2.0) and follow-on micro-USB (January 2007) standards responded to this demand. In both cases, GND moved from pin 4 (on standard Type-A and Type-B) to pin 5, with pin 4 now used to support optional On-The-Go support. The Type-A variants of both of these smaller USB connector formats have seen little adoption; more generally, micro-USB has largely replaced mini-USB in new product designs. Micro-USB’s SuperSpeed-compliant version is roughly twice as wide and subdivided into two sections, to leave room for the additional five pins (two differential data pairs plus GND).

The most recent USB connector version, alluded to earlier, is Type-C (commonly referred to by its USB-C shortcut). It addresses one longstanding user complaint about USB, that the plug-and-receptacle mating was unidirectional (i.e. you couldn’t insert a plug into a receptacle “upside down”). Apple had addressed this complaint with its proprietary, reversible Lightning standard, and USB-C (developed at roughly the same time as the USB 3.1 signaling standard, albeit distinct from it) does as well. To quote Wikipedia, “the 24-pin double-sided connector provides four power-ground pairs, two differential pairs for USB 2.0 data bus (though only one pair is implemented in a Type-C cable), four pairs for SuperSpeed data bus (only two pairs are used in USB 3.1 mode), two “sideband use” pins, VCONN +5 V power for active cables, and a configuration pin for cable orientation detection and dedicated biphase mark code (BMC) configuration data channel.”

Apple MacBook USB-C port
2015 Apple MacBook USB-C port. Source: Maurizio Pesce, flickr

[Continue reading on EDN US: Power delivery]

Brian Dipert is Editor-in-Chief of the Embedded Vision Alliance, and a Senior Analyst at BDTI and Editor-in-Chief of InsideDSP, the company’s online newsletter.

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