USB Type-C, the Swiss Army Knife of Electronics, adds HDMI

A design worth a closer look.

USB Type-C™, also known as USB-C™, is the latest Universal Serial Bus connector standard, handling up to 10Gbps and 100 watts of power (at 5V, 12V, and 20V). USB-C connectors and cables allow a single source for data, video, audio, and power while offering a vertically symmetrical connector that can be connected with far less fumbling in the dark required than prior USB connectors. At just under 3mm in height, USB Type-C connectors are another product of the tightly specified USB standards. The USB Type-C connector standard helps designers deliver products that work well together.

It’s important to note that USB Type-C connectors are a separate standard from USB 3.1, which means a USB-C cable does not have to support 10Gbps. Another separate standard that enables up to 100W of power over the Type-C cable is the USB-Power Delivery standard. Again, USB-C cables do not have to support the highest speeds or wattages. See Table 1 for the various modes of operation supported by USB Type-C .

TABLE 1: USB standards and modes of operation.

TABLE 1: USB standards and modes of operation.

Check the logo on the connector or manufacturer’s claims (and reviews) to determine if a USB Type-C cable supports what you require. Only compliance with USB 3.1 and/or USB-PD will guarantee compliance with those standards. USB Type-C is just a connector, albeit an amazingly versatile one.

Figure 1: USB has done some tidying up, designing one connector to replace several prior models. (Image source: Microchip).

Figure 1: USB has done some tidying up, designing one connector to replace several prior models. (Image source: Microchip).

Over the past 20 years, USB has become the most widely used computing interface to date. USB Type-C, as a cable/connector standard, is compatible with devices utilizing either USB 2.0, USB 3.1, and/or USB Power Delivery (USB-PD), Display Port (DP), High Definition Multimedia Interface (HDMI), Thunderbolt ™, Mobile High-Definition Link (MHL), and Peripheral Component Interconnect Express (PCIe).


The design of the USB Type-C connector is rather clever and worth investigating. The heavy lifting is done by designers of both USB-C cables and USB-C equipped products. USB Type-C connectors have a total of 24 pins, although not all are necessarily connected. A standard USB Type-C cable can use 16 wires, including 4 ground lines and 4 twisted differential pairs that create two lanes each of transmitting and receiving pairs. In reality, USB 3.1 is either transmitting or receiving at 5Gbps over two pairs simultaneously to achieve 10Gbps. This accounts for twelve wires, and the remaining lines are associated with power delivery, detecting the use of the cable for alternate mode (ditching USB altogether for another technology such as HDMI), and maintaining backward compatibility with USB 2.0.

How does USB-C determine orientation?
The symmetrical property of the connector that enables us not to bother with the orientation or direction (host- or client-facing) of the plug is due to four ground lines and more than one set of lines called Configuration Channels (CC1 and CC2). The abundance of wires allows for determining orientation as the connector is plugged in. Wires comprising the configuration channels determine cable orientation, the type of attachment, role detection, and current-mode (power delivery) detection. The CCs determine host- or client-facing orientation, each via a pull-down resistor that reveals the current sourcing capability of the USB-C port based on the voltage that is created on one side of the connector. A Downstream Facing Port (DFP) faces clients downstream; thus a DFP is a host of some kind. The designation Upstream Facing Port (UFP) is a fancy term for connection to a peripheral and the pull-down resistors for the UFP path are fixed. UFP pull-down resistors monitor for orientation; DFP pull-down resistors monitor for connection requirements, as in connection-related demands based on what voltage is created at the pull-down resistors. The versatility does not end at supporting USB to 10Gbps with backward compatibility to speeds covered by USB 2.0, however.


HDMI Alternate Mode
HDMI is a digital interface technology that transports uncompressed high-definition video, multiple channels of audio, and CEC over one cable. The alternate mode, defined in the USB-PD standard, allows USB Type-C cables to become HDMI, once alternate mode is detected. The USB Type-C connector specification concerning HDMI Alt-mode was released in September of 2016. HDMI is a digital video and audio standard found on most personal computers and audio/visual (A/V) equipment (including 100% of HD and 4K TVs produced in 2016). HDMI connectors use 19 pins. These pins allow uncompressed audio and video using TMdS (8b/10b) encoding. There are 6 pins dedicated to TMDS and two more pins for the TMDS clock. A hot-plug-detect pin monitors power/plug events. Two Display Data Channel (DDC) pins are used for data, infoframe/Metadata and HDCP communications. A single Consumer Electronics Control (CEC) pin enables remote control of CEC enabled devices over the HDMI cable. There is also one 5V pin and four grounding pins used for shielding the TMDS pins. If this sounds complicated to you, then it’s rather more amazing that the USB Type-C connector with 16 pins under a different protocol can re-configure itself as a native HDMI cable in a matter of seconds. (Native means that full-featured HDMI is supported—including HDMI-specific features.)

Figure 3: HDMI Interface Block Diagram. A total of 19 pins is brought out in an HDMI connector. (Image source: USB Dev Days.)

Figure 3: HDMI Interface Block Diagram. A total of 19 pins is brought out in an HDMI connector. (Image source: USB Dev Days.)

The USB-C HDMI Alt-mode allows HDMI-equipped sources with USB-C connectors to directly connect to HDMI displays. This enables a smartphone to connect to a projector with an HDMI port, for example. Since annual worldwide shipments of HDMI-equipped 4K TVs are expected to grow to 263 million units per year by 2019, this looks good for USB-C connectivity. Alternate mode makes the USB Type-C connector highly flexible. Alt-mode requires a handshake via vendor-defined messages (VDM) to detect, configure, and enter or exit an alternate mode.

Figure 4: Until now, many devices required converters, adapters, or dongles. (Source adapted from USB Dev Days presentation, 2016).

Figure 4: Until now, many devices required converters, adapters, or dongles. (Source adapted from USB Dev Days presentation, 2016).

Devices sourcing the HDMI signal can take advantage of native HDMI features via a single USB-C to HDMI cable. In other words, no adapters are needed to connect an HDMI-enabled source device that has a USB-C port (e.g. tablet, laptop, PC, and smartphone) to a device with an HDMI input (e.g., HD and 4K enabled displays, TVs, PC monitors, projectors and digital signs.) The recent HDMI Alt-mode standard supports native HDMI 1.4b. USB-C-to-HDMI cables must be certified just like HDMI cables and will comply to the Alt Mode USB Type-C specification, as well.

HDMI has evolved into a de-facto standard in display technology, as it is found in more devices than Display Port or Digital Visual Interface (DVI). Worldwide, nearly 6 billion HDMI-equipped devices have been shipped. USB-C connectors are projected to have rapid growth in the next couple of years, expanding to over 2 billion USB Type-C device shipments per year by 2019, according to IHS Markit. HDMI is incredibly successful in display markets, but thanks to USB-C is no longer too large for many consumer electronics devices. The recently released USB-C Alt-mode complies to the HDMI 1.4b specification, which covers resolutions up to 4K including surround sound, deep color, Audio Return Channel (ARC), and 3D support (HD and 4K). HDMI 1.4b, and therefore USB Type-C connectors connected in HDMI Alt-mode, also include High-Bandwidth Digital Content Protection (HDCP 1.4 and 2.2).

How the USB Type-C Cable Becomes an HDMI Cable
Connecting a smartphone to an HDMI display will, of course, require a USB Type-C to HDMI cable. HDMI Alt-mode for USB-C cables auto-detects HDMI Alt-mode source devices (e.g., a smartphone) and HDMI-enabled displays (e.g., a projector). Since multiple protocols are supported in Alt-mode, not just HDMI, multiplexing is used to connect data, audio, and video to the correct destination when in Alt-mode. The protocol for negotiating alternate mode starts when the USB Type-C end of the cable is plugged in. VBUS and ground pins are longer than communication pins so that they make first contact. Through these pins, a default power is immediately available on the VBUS pin at 5V and 500 mA. As the USB Type-C connector is seated, it’s detected by a configuration channel. The battery charging capability (specified in Battery Charging 1.2, device class document) or USB-PD can be used to get additional power on the VBUS. USB-PD, however, is needed to implement vendor-defined messages as a true starting point in the Alt-mode handshaking process. (USB-PD acts as a node to handle all R/W traffic from the HDMI source.) USB enumeration initiates and once the HDMI Alt-mode negotiation is final, HDMI link training begins to establish the HDMI link. Even the source device’s output is AC coupled for HDMI data and clock lanes. Finally, the USB and HDMI channels are ready for video, audio and data transfer over the USB Type-C to HDMI cable.

What’s Next?
Manufacturers of source devices can now use the small form factor USB Type-C connector for full-featured HDMI, which means that more devices will be able to incorporate HDMI technology with a single direct-connect cable. Presenting at a conference room will only require the presenter’s smartphone, not a laptop. Display signage can be changed or visuals downloaded via smartphone from a smartphone (if not wirelessly). In essence, the mobility and ubiquity of the smartphone get transferred to HDMI-enabled displays of all kinds.

The caveat in this new scenario is that not all smartphones, especially existing ones, are USB Alt-mode compliant. Many adapters made for connecting devices equipped with USB Type-C ports to displays with Display Port (DP) did not work after consumers purchased the adapter/dongle. Thus, the complexity of the wonder-connector that is USB Type-C flows downstream somewhat to end-users. Prices should (eventually) be lower for the direct connect cable than the existing adapters/dongles, as there are no extra parts in the direct-connect cable; all the heavy lifting and negotiation is handled by the DFP and UFP. In fact, USB Type-C to HDMI cables will be required to get certification from both the USB-IF and HDMI compliance entities.

An additional burden is placed on the source device manufacturer to support Alt-mode for HDMI, but with a large installed base of HDMI displays, it would not be a surprise to see new smartphones supporting the HDMI Alt-mode as an incentive for consumers to purchase an upgraded phone. In the absence of Wi-Fi connectivity (or bandwidth), consumers may welcome the ability to watch video on large display screens with a single plug-and-play type cable, as will those in the work-world who might make video presentations. The one downside is the length of the direct cable, which with Alt-mode using all 16 lanes such as USB 3.1 (at 10Gbps), is limited in length to one meter. There’s no doubt, however, that the additional security of connecting to a display without negotiating hackable wireless in a bandwidth-strapped locale will be welcome to many.

LynnetteReese_115Lynnette Reese is Editor-in-Chief, Embedded Intel Solutions and Embedded Systems Engineering, and has been working in various roles as an electrical engineer for over two decades. She is interested in open source software and hardware, the maker movement, and in increasing the number of women working in STEM so she has a greater chance of talking about something other than football at the water cooler.

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