Demystifying USB-C… a guide on buying USB type-C cables.

The Promise of USB-C

World peace…. one ring to rule them all… one connector for everything. Fits right in doesn’t it? Unfortunately, things are never as simple as they may seem. USB Type-C, or USB-C as it is commonly called, promises a lot, but also brings along a lot of compromises, confusion, and catches.

For starters, USB-C is just a connector. That’s all. It does not indicate any particular charging rate, any particular data transfer speed, and it does not guarantee that any two objects with USB-C connectors will be compatible. Most of the confusion starts with the cables, so that is why I have written this article.

Let’s break it down, and talk about each important aspect, one by one.

The Connector Itself

If you have ever inserted a USB connector the wrong way around, then you will like this. The USB-C connector offers reversibility. It doesn’t matter which way you insert the cable, it works both ways around.

Power Charging Speed

Note about power:
Power (Watts) = Voltage (Volts) times Current (Amps)
e.g. 15W = 5V x 3A

All USB-C cables, should be constructed to enable 3A of power transfer, at a minimum. This means that any USB-C cable should be able to charge a USB-C device from a USB-C power supply at a minimum of 15W (5V x 3A = 15W)  if the device requests it. This is a big step up from the 5W maximum under USB 3.0.

Fast Charging

To deliver even more power, people have defined fast-charging protocols, which exceed the normal USB-C standards.  Unfortunately, there are a number of different fast charging protocols out there, and fast charging is only possible if both the device and the power supply support the same protocol.

In order to transfer more power through a cable, there are two basic options: higher current, or higher voltage. For higher current to be possible,  it requires physically thicker cables, as well as support for handling this on the devices. On the other hand, higher voltage is possible with regular cables, but still requires support from both the charger and device to be designed to handle the higher voltage. For this reason, higher voltage is more commonly used for fast charging.

Qualcomm Quick Charge (QC) is likely the most common fast-charging existing protocol, and operates using higher voltages up to 20V, which enables power transfer up to 60W (20V x 3A). QC is possible using both USB-C and older high-current USB 2.0 cables, which means that it can be used

USB-C introduces a new optional protocol called USB Power Delivery (USB-PD). In an attempt to reduce the number of competing standards, USB-PD is an official USB-C exclusive standard for fast charging, with any normal USB-C cable, or up to 100W with high-current 5A rated cables.

100W Charging

For charging with 100W to be possible over USB-PD,  this requires special cables that can handle a higher 5A of current. These 100W cables are not just made with thicker wire, but under the USB-PD standard they also must have a chip inside them, known as an “E-Marker”. This “E-Marker”, or “E-Mark” communicates to both the charger and device to say “hey, I’m a high power cable, and I can safely handle up to 5A”.  Even if a cable is made to physically handle 5A of current, it will not deliver more than 3A unless it has an E-mark. The E-mark chip is also used by some other proprietary fast charging protocols to identify specially supporting cables.

Data Transfer Speeds

USB has increased in speed exponentially over the years. USB 2.0 delivered 480Mbps, while USB 3.1 Gen 1 (previously known as USB 3.0) increased this to 5Gbps, and cables operating at this speed can be found in both the old USB Type-A and new USB Type-C plug formats. The more recent USB 3.1 Gen 2 standard raised this to 10Gbps, using the USB-C connector exclusively. The current speed champion however is Intel’s Thunderbolt technology, which also uses the USB-C port, and pushes this to two speeds of 20Gbps and 40Gbps.

The five different supported USB-C speeds are:

  1. USB 2.0: 4 wires 480Mbps (cheap, most basic charging USB-C cables use this speed)
  2. USB 3.1 Gen 1: 9 wires 5Gbps (an E-mark is supposed to be required, but often omitted)
  3. USB 3.1 Gen 2: 13 wires 10Gbps (USB-C exclusive, and requires an E-mark)
  4. Thunderbolt 3: 13 wires 20Gbps (USB-C exclusive, and requires an E-mark, backwards compatible with USB 3.1 Gen 2)
  5. Thunderbolt 3: 13 wires 40Gbps (USB-C exclusive, requires additional active circuitry in the cable, not backwards compatible with USB 3.x*)

*The incorporation of active circuitry in 40Gbps Thunderbolt cables means that when they are used with Non-thunderbolt USB devices, these cables can only operate at USB 2.0 speeds.

USB-C cables can be manufactured to meet each of these five speed specifications, but each increase in speed also raises the cost to make the cable. For higher speeds, a greater number of wires need to be used in the cable, and the quality of the electrical shielding and cable winding becomes more and more critical for maintaining the signal integrity (avoiding data transfer errors) through the cable.

Alternate Modes

To increase the flexibility of the USB-C connector, a standard was defined to allow the pins to be used for a range of different purposes, such as audio or video transfer. These profiles are known as “alternate modes”, and require support both from the cable and the connected devices. Here the cable plays an important role as well, as it is the role of the E-mark chip to communicate which alternate modes the cable supports carrying to the devices that are connected.

Brief note: Adapters & USB 2.0

The 3A current rating of USB-C is a lot higher than the old USB 2.0 standard of 0.5A. While you can buy adapters for USB 2.0 cables to adapt them to USB-C ports, because of the higher current carrying capabilities required it may not be safe to use USB 2.0 cables for powering your USB-C device.

Final Word:

I hope this clarifies a little of the confusion around USB-C connectors and cables. If there’s something I have missed, please let me know in the comments!


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