The display controller is capable of transmitting images with up to 24 bpp (bits per pixel) and a resolution of 1600 x 600 pixels at a frame rate of 100 Hz. By using RSDS (Reduced Swing Differential Signaling), symmetrical transmission ensures a good level of signal quality at the display, even under problematic electromagnetic conditions. If this is not required, then the images can also be transmitted asymmetrically. The controller’s own electromagnetic emissions can be reduced by outputting the images using subpixel decimation in dual pixel mode. All this is quite straightforward: By doubling the bandwidth, the frequency can be halved. Display addressing can be performed either indirectly by using the horizontal and vertical synchronization signals or directly via the row and column driver provided by the TCON (timing controller).
Commercially, direct addressing is considerably more appealing, since no additional external control logic is required. For touch displays, an external controller transmits the contact point coordinates via I2C to the ‘Indigo2’, which processes them internally before forwarding them to the control unit. Typically, control of the display’s background lighting is managed by pulse width-modulated signals. For this and similar applications, ‘Indigo2’ provides 16 PWMs (pulse width modulators), whose waveform can be defined using registers 16 bits wide. Alongside the main display, other display instrumentation can be controlled using the MB88F332 ‘Indigo2’ peripheral circuitry. Integrated servo-motor controllers (SMCs) offer an interface for up to six dial instruments, which can be used in hybrid in-vehicle displays to show parameters such as speed, engine RPM and temperature. Tell-tales and LEDs can also be used as additional display components via the GPU’s GPIOs.
Internal memory is kept relatively modest. The 32 kB of ROM and 64 kB of RAM are available to store command sequences for interface configuration, as well as a limited number of small bitmaps. If required, however, storage can be expanded externally and connected up via the high-speed SPI interface. To reduce storage requirements in general, graphics can also be stored compressed, using RLE (random length encoding).
The ‘Indigo2’ works according to the line-buffer principle, by which the final image is composed line by line. The advantage of using this method is that no external frame buffer storage is required, which reduces the system’s material costs. Icons can be stored indirectly as a color palette at 1, 2, 4 and 8 bpp or directly at 16 bpp and 24 bpp, compressed or uncompressed, in internal or external memory. Following the layout and overlaying of the video stream with icons and a background image, the final image is transmitted to the display at a maximum bus speed of 144 MHz, where it is displayed in resolutions such as 1600 x 600 at 100 MHz and 18 bpp or 24 bpp.
Image processing functionality further improves the image quality. Dithering and gamma correction are the techniques applied here: these create a greater apparent color depth and a linear perception of brightness on the display. One further component, namely the Signature Unit, serves to maintain the level of functional security defined by the various requirements and standards, such as the Automotive Safety Integrity Level (ASIL) for in-vehicle electronic components. The Signature Unit monitors the content of predefined screen areas and signals the application if deviations are identified.