The New USB Type-C New Specification Is Released, Which Can Directly Support HDMI Output
Jul 26, 2019
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The USB Type-C interface is likely to be the only data interface for most laptops and smartphones in the future, but these devices that only support the USB interface must still interact with devices that are not USB-connected, such as monitors and televisions. Therefore, designers need to consider how to implement USB and other high-speed interface conversions in a single connector, which involves switching pin functions, providing external transient protection such as ESD, and maintaining signal quality. The USB Type-C standard addresses these needs by defining an Alternate Mode (Alt Mode) that dynamically changes the functionality of the pins to support non-USB data transfer protocols.
This article describes various standards. With these standards, USB Type-C can be connected to HDMI or other non-USB-like data interfaces. This article also covers the main issues to consider when adding the HDMI standby mode to the USB Type-C interface.
USB specification introduction The HDMI Forum released the USB Type-C standby mode specification at the end of 2016. The latest USB standard includes the following three parts: USB Type-C interface specification The USB Type-C interface specification has been greatly modified based on the well-known Type-A and Type-B specifications. For the average user, there are two changes that need to be noted: The Type-C interface measures 8.3mm x 2.5mm, which is much smaller than USB Type-A and Type-B, but contains 24 pins. The previous version had only 4 pins.The Type-C interface supports both forward and reverse insertions because the Type-C interface uses a symmetrical structure, and all signal pins are in the same relative position on either side. The USB Type-C can also interact with traditional USB 2.0 systems via the D+/D- and VBUS/GND pins. Its pinout also includes new feature pins (including alternate mode) as defined in the other two specifications. Figure 1 shows the Type-C interface standard and alternate mode pin mapping.
Figure 1: USB Type-C pin layout showing alternate mode mapping (Source: Texas Instruments)
USB Type 3.1 specification
The USB Type 3.1 specification updates the electrical performance of USB by specifying data transfer rates of up to 10Gbps (SuperSpeed+ in the specification). This type of interface requires two dedicated differential pins for high-speed data TX and RX, and the power supply standard is raised to 5V/150mA.
Specification for USB power supply
USB power supply specification (USB PD) specifies the mode of operation in standby mode, which can support up to 100W of charging power and greatly improve the power supply capacity. When used with USB type-c cable, USB PD allows two-way charging of both devices. Communication on channel (CC) pins can be configured according to type-c, and the charging direction can even be changed at any time.
Although these three specifications are independent, USB systems that support HDMI must comply with both the type-c and USB PD specifications. In addition, each remapped pin must be able to support the data transfer rate of its corresponding HDMI 1.4 protocol.
HDMI 1.4 data transfer rate
HDMI 1.4 has six data channels running at four different data rates:
HDMI Ethernet and audio return channel (HEAC) : high-speed two-way data communication channel that supports 100base-tx (100Mbps) Ethernet. HEAC includes streaming audio components that comply with IEC 60958-1.
TMDS (minimize transmission differential signal) : three differential channels for high-speed video and data transmission. HDMI 1.4 has a maximum data throughput of 10.2Gbps or 3.4Gbps per channel.
DDC (display data channel) : a communication channel based on the industry standard I2C protocol, with a standard rate of 100 Kbps, which enables the source device to identify the supported audio/video format.
CEC (consumer electronic control) : low speed data channel, allowing users to control up to 15 compatible devices. This data channel conforms to CENELEC EN 50157-1 specification.
HDMI pin mapping
The standard HDMI type-a interface is shown in figure 2. Figure 3 shows the new USB type-c 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 connected to the HEAC channel, and CC pins used to transmit low-speed CEC signals. Also note that the D+/ d-pin pair is not affected by this conversion, so USB 2.0 data channels can coexist peacefully with HDMI.
Figure 2: HDMI type-a has 19 pins, including three high-speed data channels for shielded twisted pair (source: How Stuff Works.COM)
Figure 3: pin mapping in HDMI mode in USB type-c alternate mode interface (source: HDMI.org)
Initialize HDMI standby mode
The USB PD specification defines the sequence of operations required to enter standby mode. When the user connects an active type-c cable between two USB PD ports, a series of negotiations is performed on the CC channel (figure 4) to determine whether USB mode or standby mode, which standby mode standard to apply, and a specific set of vendor definition messages (VDM) to determine which standard to use.
Figure 4: when a USB PD port first recognizes the existence of another USB PD port, negotiations are conducted to determine which transmission protocol and data format to use (source: Texas Instruments)
Other USB PD features, such as determining the power level required and the direction of power transmission, were also covered in the negotiation process, but they were not required for HDMI operations. Once the initialization sequence determines that HDMI is the required protocol, the two ports will be remapped as needed to the HDMI standby mode.
HDMI alternate 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 indicates the necessary components for alternate mode. Note that even if the application does not specify the power level of the USB PD interface, enabling the standby mode requires negotiation with the CC circuit and therefore must include the USB PD PHY and PD manager:
The standby mode physical layer device (PHY) receives video information of the high-end graphics processing unit (GPU) and encodes it for transmission over three TMDS differential data lines.
Alternate mode multiplexer (MUX) supports switching between HDMI alternate mode and USB. For HDMI applications, it transmits HDMI signals to the corresponding type-c interface pin; For USB 3.1 applications, it connects to RX/TX signals and switches according to the direction of data transmission.
Figure 5: two additional modules are required to implement the standby mode through the USB type-c interface, as shown in green (source: Texas Instruments)
Actual implementation method
The HDMI alternate mode specification is new, so chipsets designed specifically for such applications are still in development. However, the DisplayPort backup mode component is already available and can be used with HDMI converters. Figure 6 shows a block diagram of USB type-c port that supports both USB, HDMI standby mode and full USB PD specification.
Figure 6: USB type-c /HDMI port block diagram
The design consists of the following two basic components:
The first is the TPS65982 independent USB type-c and PD controller of Texas Instruments, which is used to perform the following tasks:
Check the insert state and plug direction of USB type-c cable
Negotiate the power supply characteristics and pass the information to the microcontroller unit through the I2C protocol to determine which mode of operation to use
Configure the multiplexer's alternate mode Settings to transmit USB or HDMI signals to the appropriate target device
During operation, TPS65982 also manages and controls the USB power supply path
The second is Texas Instruments' HD3SS460, which is a 4x6 channel high-speed two-way passive multiplexer/demultiplexer switch that can be switched between alternate and USB modes while supporting interface rolled-over.
In addition, there is a video converter for converting DisplayPort signals to HDMI format.
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