Future Electronics — A Complete Guide to Display Interface Technologies

Mismatch Between a Controller and a Display Interface: A Developer’s Guide

By Pawel Kaczynski, Manager, Embedded Systems Centre of Excellence, Future Electronics

The choice of display is emerging as an increasingly important element of the embedded development process. For a new generation of users who have grown up with a smartphone touchscreen interface, the fixed-function buttons, knobs and switches and basic status indicator LEDs of a traditional industrial equipment interface appear to be a throwback to the Dark Ages.

 

Embedded developers everywhere, then, often approach new design projects with the expectation that they will need to design in a bigger, more graphics-rich display interface than in the previous generation of their product.

 

This has important consequences not only for the specification of the display itself, but also for an embedded system’s choice of microcontroller or applications processor. That is because a display could feature one of a large number of interfaces to a host controller or processor, but this variety of interfaces is not universally supported by the MCUs and applications processors that are most commonly used in embedded systems.

 

This means that there is more scope than ever before for designers’ development plans to be tripped up by a mismatch between the display and the host controller. To help designers to avoid this risk, this article describes the most common interfaces used by LCDs, and the extent to which they are supported by popular lines of MCUs and processors.

 

Multitude of display interface technologies

The problem in matching the designer’s preferred MCU or applications processor with their preferred display is that, while the interfaces used by display manufacturers are numerous, an MCU or processor typically supports only one or two.

 

Fortunately, the display manufacturer’s choice of interface is not random: low-frequency, low data-rate interfaces are generally used in smaller, simpler displays; faster interfaces are used in the larger displays above 10” diagonal size, shown in Figure 1. And embedded designers will tend to want to specify a low-end MCU supporting a low-speed interface to control a system with a small display, and a high-speed processor capable of supporting a high-speed interface with a large display.

 

It is often, however, at the point of migration that problems can arise, for instance, when upgrading a system design with a new, larger graphics display, but while retaining the existing MCU. The MCU might have enough horsepower to drive the intended display output, but does it have the right interface onboard?

The approximate correlation between display interface and display size
The approximate correlation between display interface and display size

This is when it is important to know the full range of interfaces that might be used in an embedded display. The most common are:

    • RGB: a parallel interface. The full version of the RGB interface carries 24 bits of data per pixel (24 bpp), or eight bits per colour. Scaled down versions are RGB565 (16 bpp) and RGB332 (8 bpp). In addition to the display data signals, this interface also carries control signals: row and column pointers (VSYNC and HSYNC), and a clock signal which controls the refresh rate.
    • Serial peripheral interface (SPI): in embedded systems, the SPI is most often used for communications between peripherals such as sensors, data converters, memories and transceivers and a host MCU. A small, low-resolution LCD, however, could also connect to an MCU via SPI.
    • MCU parallel interface: several versions of this interface are in use. An MCU can use 11 signals (8-bit parallel data), 12 signals (9-bit parallel data) or 21 signals (18-bit parallel data).
    • Low-voltage differential signalling (LVDS): a high-speed signal interface. In smaller LCDs up to 15″, manufacturers use a single-channel LVDS interface with four or six lanes, and a dual-channel LVDS interface in LCD units larger than 15”. The LVDS interface offers high immunity to EMI, and consumes little power. But its high-speed operation calls for considerable expertise in PCB layout.
    • MIPI Display Serial Interface (MIPI-DSI): like LVDS, a high-speed signal interface, but intended primarily for use in mobile devices such as phones and tablets, and in automotive and IoT devices. It consists of one differential signal for the clock and at least one differential pair for data (usually in two or four lanes). Operating via complex protocol software, MIPI-DSI performs high-speed data transfers while consuming little power. Unlike the LVDS interface, it supports bi-directional communication. But like LVDS, it calls for advanced PCB layout techniques.

 

In fact, the complexities of board design involving high-speed display interfaces are considerable. Unlike SPI or RGB24, for which only single-ended trace impedance needs to be controlled, with both the MIPI-DSI and LVDS interfaces the developer needs to control differential trace impedance. Strict rules governing the treatment of features of high-speed signaling systems including intra-pair skew, trace-length skew referred to the clock signal, and exotic PCB layer stack-ups need to be observed. It is important to take these difficulties into account when planning to implement a design which includes a large display.

 

It should also be said that, in addition to the interfaces above which are widely used in embedded systems, displays might also support multimedia interfaces used in consumer devices such as TVs and computer monitors: HDMI, DisplayPort and embedded DisplayPort (eDP). Some LCD modules intended for embedded designs also support these interfaces. Winstar, for instance, makes LCD modules as small as the 5” WF50BTIFGDHTV with an HDMI interface, intended for use in development projects based on a Raspberry Pi™ board.

 

MCU/MPU specifications bear close study

The diverse range of display interfaces is a problem for the embedded developer, not just because the preferred MCU or processor might only support a single display interface, and this might not be the interface in the chosen display. It is even more challenging than that: as Figure 2 shows, some device manufacturers only support a limited range of interfaces across entire families of products. Many OEMs develop exclusively on a single MCU platform: this means that they will only be able to choose from a limited range of displays that natively support the same interface(s) as this platform.

 

In general, MCUs at the high end provide dedicated support for graphics displays, including an interface such as parallel RGB24, supporting various color depths, or MIPI-DSI with two or four lanes.

 

At the very high end, specialized processors for graphics applications might even provide an integrated HDMI or eDP interface.

 

And of course, an SPI is a standard feature of any MCU or processor, so for small, low-resolution displays the choice of host controller or processor is practically unlimited. Developers just need to take care in applications which contain many peripherals connected via SPI: here, the designer needs to ensure that the controller or processor has sufficient pins and board space for a Chip Select (CS) connection to the display, because every SPI device, including the display, needs its own CS signal.

How leading MCU and embedded processor manufacturers support display interfaces in their products
How leading MCU and embedded processor manufacturers support display interfaces in their products

How to handle an interface mismatch

This article has shown, then, that a preferred MCU or processor might be incompatible with a preferred display model or size. Rather than compromise by choosing a device or display which is less suitable for the application, a developer can instead implement a bridge between two incompatible interfaces.

 

The simplest way to implement this bridging function is with an off-the-shelf IC which performs the necessary translation operation. An example is the CrossLink family of interface bridges supplied by Lattice Semiconductor, shown in Figure 3. These devices are actually function-specific FPGAs: they convert signals to and from MIPI-DSI, LVDS and RGB24 formats, supporting all data types and any number of lanes.

 

Helpfully, the CrossLink devices also perform additional functions to offload tasks from the host controller, such as LCD initialization, control and sequencing. Lattice supplies application-specific IP with the FPGA hardware, so the designer does not need to develop FPGA code in VHDL or Verilog.

 

The eight products in the CrossLink family include a part in a chip-scale package as small as 2.5 mm x 2.5 mm. Performance in MIPI-DSI interface mode is fast enough to support 4K UHD resolution and to provide a data rate up to 12 Gbits/s.

The translation functions of the CrossLink family of products from Lattice
The translation functions of the CrossLink family of products from Lattice

While the FPGA-based CrossLink family provides flexibility to support multiple data formats in a single hardware platform, a range of fixed-function IC solutions is available from ROHM Semiconductor. For instance, the BU90T82 serializer IC performs RGB24-to-LVDS translation, while the BU90R102 performs translation in the opposite direction.

 

Sometimes, circumstances conspire against the developer, and there is no possibility of using a discrete bridging IC, for instance when the main controller or processor board design has been finished, but a last-minute change in marketing specifications requires the use of a new, higher-performance or larger LCD which has an incompatible interface.

 

If the production run-rate is high enough, some display manufacturers such as Tianma offer display customization, providing a unique display which supports the interface required by the customer’s main board.

 

It is clear, then, that the question of display selection is complicated by the limited support for display interfaces in the host MCU or applications processor. Manufacturers such as NXP Semiconductors, Renesas, STMicroelectronics and Microchip are steadily increasing the range of specialist graphics display controllers and processors that they offer in response to growing demand from the market, so compatibility between embedded devices and displays is only going to improve.

 

But in the event of an interface mismatch, interface translation solutions from Lattice or ROHM Semiconductor provide a solution to enable the developer to retain the preferred choice of display and host controller or processor.

 

Raspberry Pi is a trademark of the Raspberry Pi Foundation.

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