The USB type-c interface may become the only data interface for most future laptops and smartphones, but these devices that only support USB interfaces still have to interact with non USB devices, such as monitors, televisions, etc. Therefore, designers need to consider how to convert USB and other high-speed interfaces within a single connector, which involves issues such as switching pin functions, providing external transient protection such as ESD, and maintaining signal quality. The USB type c standard meets these requirements by defining an Alt Mode, which can dynamically change the functionality of pins to support non USB data transmission protocols.

This article introduces various standards that enable USB type C to connect to HDMI or other non USB data interfaces. This article also includes the main issues to consider when adding HDMI backup mode to the USB type c interface.

Introduction to USB Specification

The HDMI Forum released the USB type c standby mode specification at the end of 2016, and this latest version of the USB standard includes the following three parts:

The USB type C interface specification has undergone significant modifications based on the well-known type C and type B specifications. For ordinary users, there are two changes that need to be noted:

The size of the type C interface is 8.3mm x 2.5mm, which is much smaller than the USB type a and type b, but it contains 24 pins, compared to the previous version with only 4 pins.

The type c interface supports forward and backward insertion because it adopts a symmetrical structure, where all signal pins are in the same relative position regardless of which side is on.

USB type C can also interact with traditional USB 2.0 systems through D+/D - and VBUS/GND pins. Its pin layout also includes new functional pins defined in the other two specifications (including backup mode). Figure 1 shows the type c interface standard and backup mode pin mapping.

Figure 1: Layout of USB type c pins displaying standby mode mapping

The USB type 3.1 specification has updated the electrical performance of the USB, specifying a data transfer rate of up to 10Gbps (referred to as SuperSpeed+in the specification). This type of interface requires two dedicated high-speed data differential pins for TX and RX, and the power supply standard is increased to 5V/150mA.

The USB Power Supply Specification (USB PD) specifies the working mode in standby mode, which can support up to 100W of charging power and greatly improve the power supply capacity. When used with a USB type C cable, the USB PD allows for bidirectional charging between two devices. Configure communication on the channel (CC) pin according to type c, and even change the charging direction at any time.

Although these three specifications are independent of each other, USB systems that support HDMI must comply with both type c and USB PD specifications. In addition, each remapped pin must be able to support the data transmission rate of its corresponding HDMI 1.4 protocol.

HDMI 1.4 has six data channels running at four different data rates:

HDMI Ethernet and Audio Return Channel (HEAC): A high-speed bidirectional data communication channel that supports 100Base TX (100Mbps) Ethernet. HEAC includes streaming audio components that comply with IEC 60958-1 standards.

TMDS (Minimum Transmission Differential Signal): Three differential channels used for high-speed video and data transmission. The maximum data throughput of HDMI 1.4 is 10.2Gbps or 3.4Gbps for a single channel.

DDC (Display Data Channel): A communication channel based on the industry standard I2C protocol, with a standard rate of 100 Kbps, capable of allowing the source device to recognize supported audio/video formats.

CEC (Consumer Electronic Control): A low-speed data channel that allows users to control up to 15 compatible devices. This data channel complies with the CENELEC EN 50157-1 specification.

The HDMI pin mapping standard HDMI Type-A interface is shown in Figure 2. Figure 3 shows the new USB type c connector interface pin definition that supports HDMI standby mode. It maps three TMDS pin pairs and their clock signals to eight USB TX/RX pins, two SBU pins are connected to the HEAC channel, and the CC pin is used to transmit low-speed CEC signals. Additionally, it should be noted that the D+/D-pin pair is not affected by this conversion, so the USB 2.0 data channel can coexist peacefully with HDMI.

Figure 2: The HDMI Type-A interface has a total of 19 pins, with three high-speed data channels used as shielded twisted pairs

Figure 3: Pin mapping initialization in HDMI mode in USB type c standby mode interface. The HDMI standby mode USB PD specification defines the sequence of operations required to enter standby mode. When a user connects an active type C cable between two USB PD ports, a series of negotiations are performed on the CC channel (Figure 4) to determine whether to use USB mode or backup mode, and which backup mode standard to apply. A specific set of vendor defined messages (VDMs) are used to determine the standard to be used.

Figure 4: When the USB PD port first recognizes the presence of another USB PD port, it will negotiate to determine which transmission protocol and data format to use. This negotiation process also involves other USB PD functions, such as determining the required power level and power transmission direction, but HDMI operation does not require these functions. Once the initialization sequence determines that HDMI is the required protocol, these two ports will remap pins as needed and enter HDMI standby mode.

HDMI standby mode architecture

What hardware components do you need to add to support HDMI with the USB type c interface? Figure 5 shows the structure diagram of the USB PD interface and identifies the necessary components for standby mode. Please note that even if the application does not specify the power level of the USB PD interface, enabling standby mode still requires negotiation through the CC line, so the USB PD PHY and PD manager must be included:

The standby mode physical layer device (PHY) receives video information from the high-end graphics processing unit (GPU) and encodes it for transmission over three TMDS differential data lines.

The standby mode multiplexer (MUX) supports switching between HDMI standby mode and USB mode. For HDMI applications, it transmits HDMI signals to the corresponding type c interface pins; For USB 3.1 applications, it connects RX/TX signals and switches according to the data transmission direction.

Figure 5: Implementing standby mode through the USB type C connector interface requires two additional modules, as shown in the green section of the figure. The actual implementation method

The HDMI standby mode specification is brand new, so chipsets specifically designed for this type of application are still in the development stage. However, a DisplayPort backup mode component has been introduced and can be used in conjunction with an HDMI converter. Figure 6 shows the USB type c port diagram that supports both USB, HDMI standby mode, and the complete USB PD specification.

Figure 6: USB type c/HDMI port block diagram

This design consists of the following two basic components:

The first one is Texas Instruments' TPS65982 standalone USB type c and PD controller, used to perform multiple tasks as follows:

Check the insertion status and plug direction of the USB type C cable

Negotiate power supply characteristics and pass information to the microcontroller unit through the I2C protocol to determine which operating mode to use

Configure the backup mode settings of the multiplexer to transmit USB or HDMI signals to the corresponding target devices

During operation, TPS65982 also manages and controls the USB power supply path

The second model is Texas Instruments' HD3SS460, which is a 4x6 channel high-speed bidirectional passive multiplexer/demultiplexer switch that can switch between standby mode and USB mode, while supporting interface flipping.

In addition, there is a video converter for converting DisplayPort signals to HDMI format.