DesignWare Technical Bulletin

HDMI and MHL IP for Mobile and Digital Home Connectivity

By Manmeet Walia, Sr. Product Marketing Manager, Synopsys

HDMI and MHL PHY & Controller IP solutions enable chip designers to build next-generation multimedia SoCs for a broad range of digital home and mobile multimedia platforms. In addition to lowering BOM cost with wide variety of advanced, value-added features, the solutions also simplify software and driver development efforts for seamless system integration and faster time-to-market.

With 18 Gbps bandwidth and performance/simplicity/reliability as its key advantages, HDMI is the standard multimedia interface for digital home devices and continues to rapidly grow its install base. In fact, HDMI has continued to accelerate its reach beyond the digital home and into an increasingly broad array of applications such as digital office, industrial and more. HDMI's global acceptance as the connectivity technology of choice is largely due to its ability to deliver the highest quality audio/video signal over a single cable. With the ratification of HDMI version 2.0, we see further adoption of HDMI in 4K digital televisions (DTVs) and other high end digital home devices.

The Mobile High-Definition Link (MHL) interface is optimized for mobile platforms and leverages HDMI technology. MHL requires five signal/connector pins versus the 19 signal/connector pins required by HDMI protocols and can map itself to existing popular connectors in the ecosystem (i.e., USB 2.0 in cell phones and HDMI in DTVs). With support for a maximum bandwidth of 6.0 Gbps and power delivery of up to 10 Watts, MHL offers an enhanced multimedia interface technology for mobile devices such as cell phones, phablets and tablets. MHL continues to prevail over competing wireless interface technologies by virtue of features like power delivery, high performance, low latency and security.

Multimedia Connectivity

While HDMI is the standard interface in digital home applications, MHL is gaining traction in mobile platforms. As digital home and mobile platforms evolve, there is a strong user need for HDMI and MHL technologies to co-exist. Let us consider the use case examples for sink (display) and source (transmit) applications where these technologies would need to co-exist.

Sink/Display Devices

Today's mobile phone devices offer very powerful technologies that handle more than voice communications. Hence there is a strong need for interface technology to connect the mobile device to a bigger screen like a DTV or a monitor. MHL is an optimized interface protocol that provides wired high-definition audio/video connectivity between a mobile device and a larger display device, while providing battery charging capabilities to power mobile devices when connected to display devices.

Meanwhile, traditional digital home devices like set-top boxes, Blu-ray players and game consoles will continue to be connected to display devices via HDMI ports. Hence there is an emerging requirement for display devices, especially high-end DTVs, to offer a single HDMI/MHL input port that can optionally be connected to digital home devices or mobile devices.

Figure 1: MHL protocol technology enables simultaneous charging and media delivery 

Source/Transmit Devices

Source devices like set-top boxes or streaming media devices (Chromecast, Roku) continue to shrink in size. We have recently seen advent of such devices – sometimes referred to as 'cord cutters' or 'streaming sticks' – in form factor of a USB thumb drive. These are typically WiFi-enabled Android devices that plug directly into the DTV's HDMI port and download the content through a WiFi network. Currently the streaming sticks are powered through an additional USB port that connects to a DTV's USB input, or alternatively to a power outlet. MHL eliminates the need for extra wires and an extra USB port to deliver power. We continue to see a myriad of streaming sticks with HDMI and MHL output connectivity that directly plugs into DTVs' single HDMI and MHL input port. 

IP Design Considerations

HDMI and MHL PHY
With ratification of HDMI version 2.0 and MHL version 3.0 standards, HDMI and MHL IP must support the maximum bandwidth of 6 Gbps/channel. The HDMI link consists of 3 Transition-Minimized Differential Signaling (TMDS) data channels plus a separate clock channel. In contrast, the MHL link only requires a single TMDS data channel to transfer the audio/video content. The MHL clock is transmitted as common-mode signal of the differential data signal.

The TMDS channel 0 can be modified to support both HDMI and MHL protocols. The PLL in the HDMI PHY needs to support additional MHL data rates. Additionally, the PHY ensures robustness with special protection circuitry to prevent any damage to the MHL source from shorting of any combination of signals on the connector. To support the MHL functionality, the PHY also has to support cable-detect and power capabilities.

HDMI and MHL Controller
Up until version 2.x, the MHL protocol controller presented many similarities to the HDMI protocol controller with one exception of 1 TMDS data channel vs. 3 TMDS data channels, respectively. Accordingly, an HDMI and MHL controller combination solution is beneficial, as most of the parallel video and audio interface going into the ASIC side is re-used. Other blocks, including color space converter, audio packetizer, I2C interfaces, HDCP blocks, and memory interfaces are also common between HDMI and MHL. The key difference is that HDMI data channels need to be time-domain multiplexed (or-de-multiplexed) into a single channel before being TMDS encoded (or decoded). The other key difference between the HDMI and MHL protocols is the command and control blocks. While HDMI uses consumer electronic control (CEC) technology, MHL uses Control Bus (C-BUS) technology. However with version 3.0, MHL has diverged from being a frame-based technology to a packet-based technology. This presents a more significant change not only to the command and control blocks, but also to the MHL data path.

System Level Considerations

MHL signaling only requires 5 pins and can map itself to the popular connectors in the eco-system. Figure 2 shows an example of how MHL signaling can be mapped on to the HDMI connector in DTVs. MHL data signals are mapped on to the HDMI's TMDS Data Channel 0. The VBUS (for power) of MHL is mapped on to the +5V of HDMI and C-BUS of MHL is mapped to the Hot Plug Detect (HPD) of HDMI. 

Figure 2: HDMI connector with HDMI and MHL signals

The combined HDMI and MHL systems also need to support cable detect capabilities. This is accomplished with a simple pull up/pull down mechanism. For example, when MHL source is plugged into the DTV's HDMI/MHL input, the HDMI/MHL cable embeds 3.3k resistor between pin #2 and pin #15. When connected, the CD_SENSE line is pulled up by cable resistor and on-board pull up. This allows the sink to distinguish whether the source is MHL or HDMI.

Figure 3: HDMI/MHL RX cable detect

Summary

With little to no area overhead, the HDMI PHYs can be modified to incorporate the required MHL functionality. The PHY power consumption in MHL mode can be minimized by turning off the un-used TMDS channels, while a single controller design simplifies the verification and software development effort. HDMI/MHL combination solutions can be implemented with careful IP and system level considerations. IP solutions that support both HDMI and MHL protocols can reduce both area and BOM cost for system designers.