USB-C for Engineers, Part 1

USB Type-C (USB-C for short) is a new connector that promises wonderful things, including reversibility, 100W of power, and 20 Gbps data transfer. However, I've noticed a lot of confusion about this new standard and how it relates to the rest of the USB ecosystem. I've been having to learn all about USB-C for my day job, and I'd like to share what I've learned and help other engineers design USB-C into their products.

The first point of confusion with USB-C is with the variety of other specs that came out at the same time, including

  • USB 3.1
  • USB SuperSpeed and SuperSpeedPlus
  • USB Type-C
  • USB Power Delivery 2.0

In this post, I'd like to go over what each of these specs are how they interrelate. In future posts, I'll go into more detail on each one.

USB 3.1

USB 3.1 is a specification published by the USB Implementers Forum. This is the overall standard for the next generation of USB devices and cables. Inside is everything you need to connect to the latest USB hosts and devices. It includes by reference the other specs listed above. If you download the spec from the USB-IF website (here), you will receive the following documents in a zip file.

  • USB 3.1 Rev 1.0
  • USB Type-C Rev 1.2 or later
  • USB Power Delivery Rev 2.0 or later
  • USB Port Controller Rev 1.0 or later
  • various other specifications, agreements, and redline versions of the above

USB SuperSpeedPlus

USB SuperSpeedPlus refers the latest USB data bus. In this case it operates at 10 Gbps/lane. SuperSpeedPlus is used to refer the new features that distinguish it from SuperSpeed (not Plus). USB 3.1 uses "Enchanced SuperSpeed" as a general term to refer features common to both. To make matters more confusing, SuperSpeed is also referred to as USB 3.1 Gen 1. Although this is usually more specifically referring to the physical layer.

The blue Type-A plugs you see add five wires to support the new USB 3.1 bus. There is one differential pair each for receive and transmit, e.g. full duplex, as well as an extra ground.

USB Type-C

USB Type-C is the connector itself. This name follows from the USB 2.0 connectors of Type-A (host side) and various forms of Type-B (device side), including standard B, mini-B, and micro-B. With USB Type-C, there is no distinguishing the power and data role based on the physical connector. Both devices and hosts, sources and sinks, will have the USB-C receptacle. USB-C cables have the same plug on both ends.

The spec includes mechanical, electrical, and some functional details on how to use it. Some of these functions include signaling data and power roles, available power, and whether or not a device is an accessory. The USB Port Controller Specification provides a common interface to ICs that are directly connected to a port. The goal of this spec is to ease the development of software.

USB Power Delivery

USB Power Delivery is where most of the revolutionary features are. "Power Delivery" is a bit of a misnomer, because it does so much more. This is the spec that describes how to swap data roles, swap power roles, move Vbus to different voltages, and use the pins on the USB-C connector for other purposes like DisplayPort. This is what lets you plug your laptop into your monitor to present slides while recharging and running a full USB 2.0 hub, all with one cable.

Technically Type-A and Type-B connectors can use Power Delivery, but I've never seen it implemented. The spec that lets your phone charge at high speed if D+ and D- are tied together is the USB Battery Charging Spec. Power Delivery will likely be a de facto replacement for Battery Charging as more phone go to USB-C, but it is not a direct replacement.


I hope this helps clear things up. The important take away is that USB 3.1 is the overall spec, and each of these new features and connectors are parts of that spec. In part 2, I will go into more details on the Type-C spec.