Challenges in migrating to an LCD-based design

The age of smartphones and tablets is here and has changed the way users interact with their devices. Today, users expect smartphone-like usability with high-resolution, touch screen displays. To satisfy this expectation, embedded system engineers are faced with updating User Interfaces (UIs) from segment Liquid Crystal Display-based (LCD-based) displays to color touchscreen LCD displays, which poses many challenges in both hardware and software.

Segment LCD-based displays typically use 8-, 16-, or 32-bit microcontrollers (MCUs). These devices have integrated flash and RAM that enable low-cost, low-power solutions. However, for applications using Thin-Film Transistor (TFT) LCD displays – especially those that require VGA-or-higher resolution – system engineers will need to migrate to microprocessor-based (MPU-based) designs. MPUs typically do not have integrated flash or RAM. They store code in external flash, load the code into external RAM, and then execute from external RAM. This process creates a long boot time for applications that ranges from hundreds of milliseconds to a few seconds. Since the entire application code needs to be transferred to RAM, the memory requirement of these applications is large and includes the size of the application code plus any work area needed to accommodate video buffers. To adapt to these larger requirements, system designers need to add a significant amount of RAM to the system, which also increases power consumption.

External RAM in display applications is typically DDR2 or DDR3 SDRAM technology. DDRx-based SDRAM has become popular due to widespread use in the PC world, which has driven down costs, but as the PC industry continues to use more RAM, DDR technology has evolved from DDR2 to DDR3 to DDR4. The sweet spot of the DDR technology now moves in lockstep with the PC industry, and with the technology changing every 3-4 years embedded applications with longer lifetimes will find implementing DDR technology more expensive as time progresses.

DDRx-based designs also require dedicated power management chips to handle the multiple voltages needed by the SDRAM, as well as the sequencing requirements on the voltage rails. The busses needed to interface the MPU to the SDRAM have between 16 and 32 signals that need to be routed carefully with picosecond-level skew matching. This routing challenge takes multiple iterations to get right, and also necessitates the use of multiple routing layers on the board, which adds cost. The many external components used in this MPU-based design increase the Bill of Materials (BOM) cost – in many cases to a multiple of the standalone MPU cost.

In addition to hardware changes, embedded system designers face many challenges on the software front. Many LCD-based designs do not use a Real-Time Operating System (RTOS) and have simple connectivity requirements. A TFT LCD-based design will need an OS or RTOS and graphics design package, and while system designers often turn to open-source OSs like to reduce R&D costs, open-source OSs pose many hurdles to getting a design to production – some open-source OSs have specific licensing requirements that require designers to reveal their source code, and many OSs have multiple revisions that require developers to make sure code stays current to ensure software fixes. A full OS also has huge memory demands (>10 MB), which adds power and system cost, Finally, some open-source graphics packages require extensive coding in C to create the graphics elements.

To address these hardware and software challenges, Renesas offers an embedded architecture for graphics applications, the ARM Cortex-A9-based RZ/A family equipped with an on-chip 10 MB SRAM. The large on-chip SRAM can drive displays without need for external DDRx memory, reducing system power and BOM cost. The on-chip SRAM also reduces the complexity of system designs so that RZ/A-based system designs get to market more quickly. Procurement stability concerns are alleviated by the long life offered by Renesas on its products.

In order to efficiently use the internal memory of the RZ/A, system designers can leverage RTOS and middleware support through Renesas partners such as Express Logic. The small-footprint memory of Express Logic RTOS and middleware (~100 KB) leaves enough memory for video buffers needed for graphics applications, and an Express Logic Graphical User Interface (GUI) framework called GUIX allows developers to design UIs graphically on a desktop computer and automatically generate code for RZ/A MPUs. This helps embedded system designers attain similar initial outlays as with open-source software without the time to market delays.

When implementing touch screen and Human-Machine Interface (HMI) functionality into embedded applications, having optimized hardware and software solutions helps embedded engineers transition to add high-resolution, touchscreen LCD displays, while the addition of on-chip memory enables low-cost, low-power display solutions.

Renesas Electronics America, Inc. Varadarajan.Devnath@renesas.com www.renesas.com

Topics covered in this article