IN THE KNOW | Trends in USB PCB Interface
USB Standard Brings Forward Interface Upgrades
For this year, USB-IF is expected to introduce USB Type-C for connectors, USB PD for power supply and USB4 for data communications.
U

niversal Serial Bus (USB) 1.0 was introduced in 1996. In the first 10 years since introduction, the USB standard spread mainly as a data communication standard for PCs. In the following years, USB became the standard for charging mobile devices, such as smartphones. It is anticipated that USB will continue to be popularly used as a data communication and charging interface in 2020 and thereafter.

It is expected that in 2020 the USB standard will evolve along three keywords: USB Type-C®, USB Power Delivery (USB PD), and USB4TM (Figure 1). These keywords are vital to three key technologies, including connector, power supply, and data communication. To put it simply, USB Type-C relates to connector technology, USB PD relates to power supply technology, and USB4 relates to data communication technology. Rather than being independent, the three technologies are closely related to each other.

Key USB technologies
Figure 1: Key USB technologies
USB Type-C
USB Type-C is used in most smartphone models, which are the most familiar devices for common consumers, and therefore is rapidly spreading. In PCs, USB Type-C is adopted mainly in high-end notebook PCs (Photo 1), and the number has been increasing. On the other hand, low-priced PCs have yet to integrate USB Type-C connectors, but this may change from 2020. USB Type-C is also used in solid state drives, which require high-speed data communication, and in monitors that support DisplayPort Alternate Mode.

The most important feature of USB Type-C is its reversibility. Unlike conventional USB connectors (called legacy connectors), USB Type-C is very easy to use because it can be connected in any way. In addition, USB Type-C standardizes a single type of connector, and therefore consumers can use the same cable for different kinds of device, including smartphones, PCs, and other products (Photo 2). In terms of functionality, Type-C is a universal connector. In fact, it can support not only USB standards, such as USB PD and USB4, but also Thunderbolt 3 and DisplayPort. It can also pass analog audio signals, as many people are probably using smartphones that are not equipped with earphone jacks.

USB Type-C
Photo 1: USB Type-C
USB Type-C product portfolio
Photo 2: USB Type-C product portfolio
On the other hand, it is difficult for consumers to understand that the supported standards differ depending on products even with the same USB Type-C. For example, even if USB4 is supported on a PC and a USB hub, the feature of USB4 cannot be performed unless the cable supports the standard (In this case, it will be connected as USB 3.2 instead of USB4). Vendors need to advice the consumers on whether their products support USB PD, USB4, or Thunderbolt 3.
Table 1: Summary of power operation specifications of USB Type-C
Table 1: Summary of power operation specifications of USB Type-C
USB PD
A typical smartphone charging technology is the Quick Charge of Qualcomm. The latest-generation Quick Charge 4.0 (QC4) is a superset of USB PD. Therefore, smartphones using QC4 could automatically support USB PD. As a result, USB PD has become widespread for charging smartphones. As for PCs, the support of USB PD has become more and more popular mainly in high-end notebook PCs. Thunderbolt 3 is becoming popular in PCs, thus contributing to the spread of USB PD.

USB PD supports up to 100W of power supply, which is sufficient for charging smartphones and laptops and also for charging small desktop PCs and monitors. Another feature of USB PD is the function called Alternate Mode, which is intended for using USB Type-C in modes other than USB. Popular Alternate Modes include Thunderbolt 3 and DisplayPort. Nowadays, Windows PC/Mac have removed a single-purpose power-supply connector or DisplayPort connectors by adopting USB Type-C with USB PD.

USB PD can only be used with USB Type-C. Unfortunately, USB PD cannot be used with USB legacy connectors.

USB4
USB4 is the latest generation USB standard announced by Universal Serial Bus-Implementers Forum (USB-IF) in Sept. 2019. The data transmission speed in USB4 is doubled in comparison to the previous USB 3.2. USB4 is based on Thunderbolt 3, which originated from Intel’s standard. Due to the circumstances related to the establishment of the standard, USB4 is expected to be adopted first in Windows PC/Mac and their peripheral devices.

Table 2: Summary of data operation specifications of USB Type-C

Table 2: Summary of data operation specifications of USB Type-C
The year 2020 is likely to be the period of preparation of USB4 for vendors. The USB4 certification program provided by USB-IF is expected to become available in mid-2020 at the earliest, or possibly at the end of 2020. Therefore, products with USB4 certification will be introduced on the market in 2021 or later. However, USB is not a standard that requires certification, and therefore it is possible that non-certified USB4 products will appear earlier.
Table 3: Summary of USB Type-C passive cable
Summary of USB Type-C passive cable
As a feature of USB4, data of various protocols pass through the bus at 40Gbps (maximum). Even the conventional USB standard supports various device classes, such as Human Interface Device (HID) Class and Mass Storage Class (MSC), and USB4 is the standard that has expanded support capabilities for such device class. Specifically, USB4 can handle PCI Express and DisplayPort signals like in Thunderbolt 3. When a high-resolution DisplayPort is used, for example, PCI Express cannot send data at maximum bandwidth because it shares the same bus. However, USB4 can support various standards with a single cable. PCI Express can be converted into Serial-ATA or Ethernet, while DisplayPort can be converted into High-Definition Multimedia Interface (HDMI) or D-Sub. Therefore, by supporting both PCI Express and DisplayPort, it could support a very large number of interfaces although some conversion is necessary.
USB4 uses USB PD Alternate Mode. Therefore, support of USB4 would automatically require that of USB PD and USB Type-C.

Finally, active cables should be mentioned as another remarkable topic in the USB4 generation. The passive cable compatible with USB4 Gen3 is the one with the length less than 0.8m. USB-IF is discussing active cables in order to develop longer cables. The certification program will be available in the future.

Notes:
Allion Labs, Inc. (www.allion.com) has participated in the development of a variety of standards including Type-C, providing various standard certifications ranging from certifications for connectors, cables, USB devices; and charging standards for USB-PD, QC4, among others; to Thunderbolt3 and DisplayPort using Alternate Mode.

USB Type-C®, USB-C® and USB4TM are trademarks of the Universal Serial Bus Implementers Forum (USB-IF).

ThunderboltTM is a trademark of Intel Corporation.

Quick Charge is a trademark of Qualcomm Incorporated.

About This Article:
The author, Kei Tanaka, is Technical Supervisor at the Engineering Service Department of Allion Labs, Inc.