Mismatch between Controller and Display Interfaces: A Developer’s Guide

The choice of Display is becoming an increasingly important element in the embedded development process. For a new generation of users who have grown up with smartphone touchscreen interfaces, the fixed-function buttons, knobs and switches of traditional industrial equipment interfaces, and basic status indicator LEDs seem like a throwback to the dark ages.

By Pawel Kaczynski, Manager, Center of Excellence for Embedded Systems

The choice of Display is becoming an increasingly important element in the embedded development process. For a new generation of users who have grown up with smartphone touchscreen interfaces, the fixed-function buttons, knobs and switches of traditional industrial equipment interfaces, and basic status indicator LEDs seem like a throwback to the dark ages.

As a result, embedded developers around the world often approach new design projects with the expectation that they will need to design larger and more graphically rich display interfaces than the previous generation.

This has important implications not only for the specifications of the display itself, but also for the choice of microcontroller or application processor for embedded systems. This is because a display may have one of a number of interfaces to a host controller or processor that is not universally supported by the MCUs and application processors most commonly used in embedded systems.

This means that it is more likely than ever that a designer’s development plan will be frustrated by a mismatch between the display and the host controller. To help designers avoid this risk, this article describes the most common interfaces used by LCDs and how well they are supported by popular MCU and processor families.

Multiple Display Interface Technologies

The problem with matching a designer’s preferred MCU or application processor to their preferred display is that while display manufacturers use many interfaces, the MCU or processor typically supports only one or two.

Fortunately, display manufacturers’ choice of interface is not random: low-frequency, low-data-rate interfaces are typically used for smaller, simpler displays; faster interfaces are typically used for larger displays measuring more than 10 inches diagonally. display, as shown in Figure 1. Embedded designers often want to specify a low-end MCU that supports a low-speed interface to control a system with a small display, and a high-speed processor that supports a high-speed interface to control a system with a large display.

Mismatch between Controller and Display Interfaces: A Developer’s Guide
Approximate relationship between monitor interface and monitor size (Image source: Future Electronics)

However, issues often arise when migrating, for example, when upgrading a system design with a new, larger graphics display, while keeping the existing MCU. The MCU may have enough power to drive the expected display output, but does it have the correct onboard interface?

At this point, it is important to understand the full range of interfaces that may be used in an embedded display. The most common are:

・ RGB: Parallel interface. The full version of the RGB interface transmits 24 bits of data per pixel (24 bpp), or 8 bits of data per color. Lite versions are RGB565 (16 bpp) and RGB332 (8 bpp). In addition to display data signals, the interface also transmits control signals: row and column pointers (VSYNC and HSYNC), and clock signals that control the refresh rate.

・Serial Peripheral Interface (SPI): In embedded systems, SPI is most commonly used for communication between peripherals such as sensors, data converters, memories, and transceivers and the host MCU. However, a small low-resolution LCD can also be connected to the MCU via SPI.

・ MCU parallel interface: Several versions of this interface are in use. The 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 Signaling (LVDS): A high-speed signal interface. In smaller LCDs of 15 inches, manufacturers use a single-channel LVDS interface with 4 or 6 channels, while in LCD units larger than 15 inches, a dual-channel LVDS interface is used. The LVDS interface has high immunity to EMI and low power consumption. But its high-speed operation requires considerable PCB layout expertise.

・ MIPI Display Serial Interface (MIPI-DSI): Similar to LVDS, a high-speed signal interface, but mainly used in mobile devices such as cell phones and tablets, as well as in automotive and IoT devices. It consists of a differential signal for clock and at least one differential pair for data (usually with two or four channels). MIPI-DSI runs through complex protocol software, performing high-speed data transfers while consuming very little power. Unlike the LVDS interface, it supports bidirectional communication. But like LVDS, it requires advanced PCB layout techniques.

In fact, the complexity of circuit board design involving high-speed display interfaces is considerable. Unlike SPI or RGB24, which only need to control single-ended trace impedance, for MIPI-DSI and LVDS interfaces, developers need to control differential trace impedance. Strict rules governing the processing of features in high-speed signal systems need to be followed, including differential pair stacking. It is important to take these difficulties into account when planning to implement designs that include large displays.

It should also be said that, in addition to the aforementioned interfaces widely used in embedded systems, displays may also support multimedia interfaces used in consumer devices such as TVs and computer monitors: HDMI, DisplayPort, and Embedded DisplayPort (eDP). Some LCD Modules for embedded designs also support these interfaces. For example, Winstar makes LCD modules as small as a 5-inch WF50BTIFGDHTV with an HDMI interface, intended for use in Raspberry Pi™ board-based development projects.

MCU/MPU specifications require careful study

Display interface diversity is an issue for embedded developers, not only because the preferred MCU or processor may only support a single display interface, but it may not be the interface in the selected display. Even more challenging than this: As shown in Figure 2, some device manufacturers only support a limited number of interfaces across their entire product line. Many OEMs only develop on a single MCU platform: this means they can only choose from a limited range of displays that support the same interface as that platform.

How leading MCU and embedded processor manufacturers are supporting display interfaces in their products (Image credit: Future Electronics)

In general, high-end MCUs provide dedicated support for graphics, including interfaces such as parallel RGB24 supporting various color depths, or MIPI-DSI interfaces with two or four channels.

At the very high end, dedicated processors for graphics applications can even provide integrated HDMI or eDP interfaces.

Of course, SPI is a standard feature of any MCU or processor, so for small, low-resolution displays, the choice of host controller or processor is virtually limitless. Developers only need to pay attention to applications that contain many peripherals connected via SPI: Here, the designer needs to ensure that the controller or processor has enough pins and board space to connect the chip select (CS) signal to the display because Each SPI device (including the display) requires its own CS signal.

How to handle interface mismatch

Therefore, this article suggests that the preferred MCU or processor may not be compatible with the preferred display Model or size. Instead of compromising by choosing a device or display that is less suitable for the application, developers can build a bridge between two incompatible interfaces.

The easiest way to implement this bridging function is to use an off-the-shelf IC to perform the necessary conversion operations. An example is the CrossLink family of interface bridges from Lattice semiconductor, shown in Figure 3. These devices are actually function-specific FPGAs: they convert signals between MIPI-DSI, LVDS, and RGB24 formats, supporting all data types and any number of channels.

Conversion capabilities of the Lattice CrossLink family of products (Image source: Lattice Semiconductor)

Furthermore, CrossLink devices perform additional functions to offload tasks from the host controller, such as LCD initialization, control, and sequencing. Lattice provides application-specific IP for FPGA hardware, so designers do not need to develop FPGA code in VHDL or Verilog.

The eight products in the CrossLink family include a component in a chip-scale package as small as 2.5 mm x 2.5 mm. Performance in MIPI-DSI interface mode is sufficient to support 4K UHD resolutions and provide data rates up to 12 Gbps.

While the FPGA-based CrossLink family offers the flexibility to support multiple data formats in a single hardware platform, ROHM also offers a range of fixed-function IC solutions. For example, the BU90T82 serializer IC performs RGB24 to LVDS conversion, while the BU90R102 performs conversion in the opposite direction.

Sometimes the situation is not in favor of the developer and the use of a discrete bridge IC is not possible, such as when the main controller or processor board design is finalized but a last-minute change in marketing specifications requires the use of a new, higher performance or larger when the LCD is displayed.

If the production run rate is high enough, some display manufacturers such as Tianma offer display customization, providing unique displays that support the interface required by the customer’s motherboard.

Obviously, the issue of display selection is complicated by limited display interface support in the host MCU or application processor. Manufacturers such as NXP Semiconductors, Renesas, STMicroelectronics, and Microchip are steadily increasing the range of professional graphics display controllers and processors they offer in response to growing market demands, so compatibility between embedded devices and displays Sex will be improved.

But in the case of an interface mismatch, an interface conversion solution from Lattice or Rohm Semiconductor provides a solution that allows developers to retain the preferred display and host controller or processor.

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