Both the number and the sizes of screens in modern vehicles are increasing. To integrate them optimally, it is necessary to adjust both the interior concept and the system concept. Security aspects such as the ISO26262 standard and the ASIL level should also be considered.
Currently, in a fully equipped car you can find a good number of screens. The latest trend in dashboard and rear seat entertainment designs is to combine two or even three screens into one large, unified surface. The displays with their interactive features are merging with each other into a single user experience space. For so-called column-to-column designs, a combination of standard panels or extra-wide panels with a horizontal resolution greater than 6k is selected. In addition to the design that comes with many innovations in user interaction, an important aspect is the fundamental need for security features. More than half of the screens are relevant to car safety and driving, where the instrument cluster plays the most important role in communication with the driver.
ROHM Semiconductor offers a wide range of integrated circuits (ICs) specifically designed for automotive display applications. Video streaming, synchronization, power management, control and security functions, but also backlight detection and control are some of the most important aspects.
Single screen architecture
The panel is arguably one of the most important components when it comes to the visual performance of the screen. The following article describes the components/CIs that surround such a panel. Figure 1 illustrates those components. A deserializer IC receives the direct video stream over the high-speed link originating from the domain controller and supplies it for further processing through the display components. 3, 6, or 12 Gbps are typical link speeds for sufficient video transport. The deserializer IC receives the high-speed direct video feed but also provides a side channel for two-way data communication within the vehicle network. The deserializer CI optionally supports daisy chaining to reduce the drawbacks of a point-to-point topology, so that a second or perhaps a third display can be connected on the same link. Each direct video stream is distributed to each screen respectively, using the super frame merging and splitting method or using the multiple video option that makes sense in the transport protocol.
Information overlaid via OSD
In case it is necessary to superimpose error messages, indicators or other graphical elements on the incoming video, an on screen display (OSD) functionality will be required. Extensive checks will need to be applied on the timing and pixel content of the incoming video. These operations must be processed without delay or extensive buffering. A dedicated IC is required with that OSD function, mostly combined with interface adaptation and optional framing and format conversion. Integrity check operations, such as CRC, are applied to the content of the image.
Figure 1. Diagram of components of a car screen. Components/ICs in red color available from ROHM Semiconductor.
This method is the best practice for effectively comparing the actual content displayed against the predefined and stored values. Another important scenario for a fully digital instrument cluster is the ability to switch to a limp mode and display in its OSD layer a subset with the most important information and messages for the driver (figure 2). The synchronization controller (T-Con) is responsible for directing the frame-by-frame source and gate controllers. For larger displays and higher resolution than F-HD, multiple parallel T-Con ICs are often used. At the T-Con outlet and in the case of a wide column-to-column panel, connections longer than 0,5m should be allowed. ROHM has developed RL-Link technology to support such a high-speed connection.
TFT backlight management
When LCD technology is used, an external light source or backlight is required which is located behind the TFT screen. Backlight LEDs are positioned along the edge or evenly across the panel area. The most important prerequisites for selecting an LED driver are to ensure constant current under different conditions such as voltage transients during start-up. Depending on the output voltage requirements, buck, boost or buck-boost topologies can be adopted. In order for the user to benefit from a good experience in all ambient light conditions, different dimming methods are used, including DC or analog dimming, PWM dimming, or a combination of DC and PWM called hybrid dimming.
Figure 2. Instrument cluster display works in limp mode with no background video.
Another important requirement is to interface the LED driver interface with the MCU, either through the SPI or I2C interface. This enables flexible and easy control, and ensures proper monitoring of protections and implementation of diagnostics to ensure functional safety requirements. A dedicated TFT power management IC is required to support the specific panel voltages. Furthermore, functional safety requires monitoring of voltage anomalies. The interfacing of the TFT power management IC with the MCU uses I2C which enables flexible and easy control of protections and diagnostics to ensure functional safety requirements.
Conclusion
Car displays are the most important communication interface technology in modern vehicles. They have to be reliable and secure, but also easy to use and attractive. Innovations in autonomous driving are difficult to achieve without integrating the intuitive environment generated by the screens. Car designers are challenging engineering capabilities as demanded by consumers and marketers to bridge the gap with other markets such as consumer. ROHM Semiconductor offers the most comprehensive and innovative portfolio of dedicated components to meet the high standards of the automotive industry for each display.
