As the title suggests, it’s expected that USB Type-C will become the sole data port for many future laptops and smart phones, but these USB-only devices will still have to interface to non-USB monitors or TVs.
The USB Type-C standards accommodate this reality by defining an Alternate Mode (AM) method of operation, which dynamically changes the function of pins in the USB Type-C connector to allow the use of non-USB data protocols. In September 2016, the HDMI Forum released their specification governing the AM operation of HDMI 1.4 in a USB Type-C environment.
Adding the ability to switch between USB and other high-speed formats in a single connector presents challenges to the designer, including switching between pin functions, providing protection from external transients such as ESD, and maintaining signal quality.
This article explores the various standards that came together to enable USB Type-C connectors to stream HDMI and other non-USB formats, and offers key design considerations for adding HDMI Alternate Mode capability to a USB Type-C interface.
Overview of the New USB Specifications
The latest set of USB standards has three parts. Although they are closely intertwined, they are released separately, and the design goals will dictate which specifications are supported by a particular USB implementation.
- The USB Type-C connector specification introduces some significant changes to the familiar Type-A and Type-B versions. To the casual observer, two features stand out: the Type-C measures 8.3 mm x 2.5 mm, much smaller than the USB Type-A and -B connectors, but contains 24 pins versus only four in the earlier versions; and the Type-C connector is reversible and operates equally in any orientation. To accomplish this, the connector pinout is symmetrical; regardless of which row is on top, all the signals are in the same relative positions.
Figure 1: The USB Type-C connector pinout (Source: Texas Instruments)
The USB Type-C specification can communicate with legacy USB 2.0 systems via the D+/D- and VBUS/GND pins. The pinout also includes pins for the new functions (including Alternate Mode) that are defined in the other two specifications. Figure 1 shows the standard and AM mappings for the Type-C connector.
- The USB Type 3.1 specification updates the electrical performance of USB and increases the data rate to 10Gbps (called SuperSpeed+ in the specification). This requires two differential TX and RX pairs dedicated to high-speed data. The specification also boosts the baseline power delivery capability to 5V at 150mA.
- The USB Power Delivery specification (USB PD) defines Alternate Mode operation, but it also increases the deliverable power to 100W and dramatically expands the range of power delivery options available to the designer. When used with an active USB Type-C cable, USB PD adds bi-directional power flow between two devices; the power flow can even reverse in real time courtesy of a communication channel carried on the Type-C configuration channel (CC) pins.
Although these three specifications are separate, an HDMI-capable USB system must support both Type-C and USB PD specifications. In addition, each remapped pin must support the data rate of its corresponding HDMI 1.4 function.
HDMI 1.4 Data Rates
HDMI 1.4 has six data channels operating at four different speeds. These channels are:
- TMDS (Transition Minimized Differential Signaling): three differential channels for high-speed video and data transmission. HDMI 1.4 has a maximum throughput of 10.2 gigabits per second (Gbps), or 3.4Gbps per channel.
- DDC (Display Data Channel): communication channel based on the industry-standard I2C protocol at a standard rate of 100kbps; it enables the source device to identify the supported audio/video formats.
- CEC (Consumer Electronics Control): a low-speed (channel that allows the user to control up to 15 compatible devices. This channel conforms to the CENELEC EN 50157-1 standard.
- HDMI Ethernet and Audio Return Channel (HEAC): a high-speed bi-directional data communication that supports 100Base-TX (100Mbps) Ethernet. HEAC includes a streaming audio component that conforms to the IEC 60958-1 standard.
Figure 2 shows the standard HDMI Type-A connector. For AM use, an HDMI-to-USB cable maps the HDMI pins to the corresponding pins on the USB-C connector.
Figure 2: The HDMI Type A connector has 19 pins and includes three high-speed data channels as shielded twisted pairs (Image Source: HowStuffWorks.com)
HDMI Alternate Mode Pin Mapping
Figure 3 shows the new pin definitions of the USB Type-C connector in HDMI Alternate Mode. The implementation maps the three TMDS pairs and their clock signal to the eight USB TX/RX pins. The two SBU pins now carry the HEAC channel and the CC pin carries the low-speed CEC signal.
Note that the D+/D- pair is not affected by this transformation, so a USB 2.0 data channel can still operate in parallel with HDMI.
Figure 3: Pin mapping for HDMI in USB Type-C Alternate Mode (Image Source: HDMI.org)
HDMI Alternate Mode Initialization
The USB PD specification defines the sequence of events needed to enter Alternate Mode. When the user connects an active Type-C cable between two USB PD-enabled ports, a series of negotiations (Figure 4) takes place over the CC line. The negotiations establish whether USB or Alternate Mode is to be used, and which AM standard applies: a specific set of vendor-defined messages (VDMs) identifies the standard to be used.
Figure 4: When a USB PD-capable port first recognizes the presence of another USB PD port, negotiations take place to determine the power delivery protocol and the data format to be used. (Image Source: Texas Instruments)
Although not needed for HDMI operation, the negotiation sequence also includes other USB PD functions such as the required power level and the direction of power flow.
Once the initialization sequence has established HDMI as the desired protocol, both ports remap their pins as needed, and HDMI AM operation begins.
HDMI Alternate Mode Architecture
What hardware components are needed to add HDMI to a USB Type-C port? Figure 5 shows a block diagram of a USB PD port with the Alternate Mode components highlighted.
Note that even if the application does not specify USB PD power levels, agreeing on the Alternate Mode requires negotiation over the CC line, so a USB PD PHY and PD manager must still be included. For more information on the other components, consult this useful resource.
There are two added parts:
- The Alternate Mode Physical Layer Device (PHY) takes the video information from a high-end Graphics Processing Unit (GPU) and encodes it onto the three TMDS differential data lines.
- The Alternate Mode Multiplexer (MUX) allows switching between HDMI AM and USB implementations. For HDMI applications, it connects the HDMI signals to the correct Type-C connector pins; for USB 3.1 applications, it connects up to the RX/TX signals, and also swaps them depending on the cable orientation.
Figure 5: Implementing Alternate Mode over USB Type-C requires two extra blocks, shown in green. (Image Source: Texas Instruments)
Practical Implementation
The HDMI AM specification is new, so chipsets designed specifically for this application are still in development. DisplayPort AM parts are readily available, though, and can be used with the addition of an HDMI format converter.
Figure 6 shows the block diagram of a USB Type-C port that supports USB, HDMI Alternate Mode, and the full USB PD specification.
Figure 6: USB Type-C/HDMI Port Block Diagram (Image Source: Texas Instruments)
Two devices form the basis of the design:
The Texas Instruments’ TPS65982 stand-alone USB Type-C and PD controller performs multiple tasks:
- It detects the insertion of a USB Type-C cable and its plug orientation.
- It negotiates the power delivery features and passes the information via I2C to a supervisory microcontroller that determines the mode of operation.
- It configures AM settings for the multiplexer to route the USB or HDMI signals to their proper destinations.
- During operation, the TPS65982 also manages the USB power routing and control.
The Texas Instruments’ HD3SS460 high-speed bi-directional passive 4-by-6 multiplexer/demultiplexer switches between AM and USB mode and accommodates connector flipping.
The final building block is a video converter to switch from DisplayPort to HDMI formats.
Design Considerations
In addition to the main blocks discussed above, three items in particular deserve careful consideration: the first two protect components against electrical overstress, and the third boosts the overall system performance.
Pay Attention to Protection
Because the USB port is connected to the outside world, the design must provide protection against potential ESD strikes when users insert or remove cables, but different ESD solutions are needed for different pins.
At gigabit data rates, the designer must take special precautions to maintain signal integrity. Any extra circuitry added to a high-speed data channel, such as an ESD protection device, must add minimal capacitance to the line; it must also maintain impedance matching throughout the signal path, since any mismatch can cause reflections that can increase jitter and reduce signal quality. Pins that carry lower-speed data, such as SBU and CC, are not as sensitive to added capacitance or impedance mismatch.
The eight TX/RX pins carry the high-speed data channels in both USB and HDMI modes: the USB 3.1 channels for USB operation, and the three TMDS channels plus clock for HDMI AM operation.
The Texas Instruments’ TPD4E02B04 transient voltage suppression (TVS) device protects the high-speed data pins. It is a four-channel bi-directional ESD protection diode array with only 0.25pF I/O capacitance per channel, and uses the industry- standard USON-10 package with flow-through routing for matched trace impedances.
The ESD protection for lower-speed pins is included in another device, discussed in the next section.
Guard Against Shorts to VBUS
The pin pitch of the USB Type-C connector is only 0.5 mm. This increases the likelihood of a pin-to-pin short compared to the earlier Type-A connector. Pins adjacent to the VBUS pins—SBU and CC—are particularly at risk, especially if the USB/HDMI port supports the full USB PD specification as it allows VBUS pin to carry up to 22V. Not only could this sustained voltage appear on an adjacent pin in the event of a short, it could also give rise to a ringing voltage of up to 44V during a short-to-VBUS hot-plug insertion.
Figure 7: The TPD8S300 block diagram shows the inline FETs and control circuitry that protect the SBU and CC pins against a short-to-VBUS event, plus four additional channels of ESD protection (D1 – D4). (Image Source: Texas Instruments)
Texas Instruments’ TPD8S300 USB Port Protector (Figure 7) provides overvoltage protection against short-to-VBUS events on the CC and SBU pins, plus protection against voltage ringing. Although these and other pins aren’t as sensitive to added capacitance as the high-speed pins discussed earlier, they still need ESD protection. The TPD8S300 protects the SBU and CC pins, and also includes additional ESD protection for the two USB2.0 D+/D- pairs.
Compensate for Signal Degradation
Adding components to protect against ESD and VBUS shorts has an effect on a high-speed HDMI or USB signal: despite the best efforts of the designer, the quality of the signal inevitably suffers as it propagates through the board. IC pin parasitics, PCB traces, and vias all play their part in degrading the signal before it reaches the output pin.
Including a redriver in the signal chain immediately before the Type-C connector is a cost-effective solution to maintain good signal quality at high data rates. A redriver boosts the signal output and includes linear equalization to compensate for channel losses. In a USB Type-C system it can help pass compliance tests and increase device interoperability when used with low-quality or extremely long cables.
Figure 8: The TUSB1046 redriving switch configured for an HDMI AM application: the part can also support USB 3.1 SuperSpeed+ in normal mode. (Image Source: Texas Instruments)
Texas Instruments’ TUSB1046 (Figure 8) combines the Alternate Mode multiplexer and redriver into a single device. The part contains a linear redriver that can support a data rate of up to 8.1Gbps per AM channel, more than enough for HDMI 1.4’s 3.4Gbps.
Although the TUSB1046 was originally designed for DisplayPort use, it is protocol-agnostic. Figure 8 shows the DisplayPort four-channel setting repurposed to three HDMI TMDS channels plus the TMDS clock.
Future Trends
Designers can expect to see other popular video standards join DisplayPort, Thunderbolt, MHL, and HDMI, jumping on the Alternate Mode bandwagon. The HDMI AM specification is new but only supports HDMI 1.4, a revision that dates back to 2009. HDMI 2.0 (2013) still uses three channels, but the 2017 revision, HDMI 2.1, adds a fourth TMDS channel. The HDMI Forum has not commented on when they might release an updated AM definition.
Since HDMI is such a popular standard, expect to see HDMI-specific Alternate Mode chipsets make their appearance soon. Regardless of the particular circuit blocks, though, many of the issues discussed in this article are fundamental engineering problems that must be solved for any system with similar performance.
Conclusion
USB Type-C, the latest revision of the popular standard, is becoming the standard for high-speed data communication in consumer devices such as laptops and smartphones. Taking advantage of USB’s Alternate Mode capability, HDMI is the latest high-speed data standard to issue a specification defining its use in a USB Type-C environment.
This article has discussed the new USB specifications that make this advance possible. Although designers are still waiting for a chipset optimized for HDMI, this article has reviewed the architecture of a practical approach using available parts and detailed some of the design considerations needed for a successful solution.
