High performance GUIs to improve wearable devices

4 min read

High-end wearable devices such as smart watches are characterised by a rich functionality, a sophisticated graphical user interface (GUI) with touch operation and communication interfaces to a smartphone, tablet or PC – but also by extremely short battery life times.

Demand for these devices is growing rapidly and much of that growth will be driven by demand for devices that balance cost, form and function. However, research suggests that half of all activity trackers purchased are no longer used, and a further third are discarded within six months of purchase. Among the reasons: a lack of functionality, operating difficulties, and inconvenience, not least in terms of the frequency of required battery charges.

As processor technology advances, code structures become more efficient and sensors reduce in size, delivering more advanced functionality in miniature wearable devices. Convenience often equates to longevity of battery life achieved through improving battery, processor and display technology and intelligent power management schemes. Users want products with operating times measured in weeks and even months without compromising on functionality.

Single function wearable devices, such as fitness bands, use BLE wireless communication to a smartphone for data exchange or device software updates, and Apps then analyse the sensor data that’s gathered.

In the middle of this sector, between the single function devices and the high end wearables, is the next significant growth area for wearable technology. While simpler than smartphones, users increasingly expect devices in this space to have fashionable colour displays and intuitive and flexible user interfaces; the wearable becomes a life-style product and tends to be worn as a classical watch with ‘smart’ functions.

The toughest wearable device design challenge surrounds the area of ergonomics or operator convenience, both of which are defined by the user interface. As functionality increases, there’s not enough space to add a button-based user interface, so designers and users rely on the soft menus that graphics can facilitate.

This has created a dilemma for designers as more advanced graphics meant using more powerful processors with higher clock frequencies with consequently higher power demands.

The TZ1200 processor from Toshiba Electronics Europe’s (TEE) looks to square this. Measuring 8 x 8 x 0.6mm, it is based on a high performance 32bit ARM Cortex-M4F processor with floating-point unit, memory protection unit and flexible interrupt processing making it capable of operating at frequencies of up to 120MHz.

This core, in combination with the on-board power management functionality, delivers an ultra-low power consumption of 70μA/MHz in active mode. With a 350mAh battery, suitable power management software, and a low power colour display, time between battery charges last about one month in watch applications.

Together with 2.2MB of embedded high-speed SRAM, an advanced LCD controller and a total of four 2Dgraphics engines, the TZ1200 is also supported by an integrated MIPI Display Bus Interface and Display Serial Interface. This means it can offer a range of display resolutions of up to HVGA (480x320) at 30fps or QVGA (320x240) displays at up to 60fps.

The 2D Graphics (GFX) Accelerators provide a powerful platform for drawing, rotating, texturing and resizing images on a display. It also performs on-the-fly colour conversion, which almost removes the entire load on the processor, contributing to further power efficiencies.

The use of external sensors and peripheral devices that can monitor activity and movement are supported by integrated USB, UART, SPI and I2C interfaces.

The flexibility built into the device means that designers can better specify the most suitable memory ICs and capacities for their devices. Together with the embedded data compressor, it enables large capacity data storage for long periods of time without frequent uploading of data.

A particularly important element of the processor is the high precision analogue front-end (AFE) that brings together a 24bit delta-sigma ADC, a 12bit ADC, a 12bit DAC and an LED DAC. One of the benefits of the AFE is that it supports direct sensing – analogue sensor outputs can be connected directly to the TZ1200’s high resolution ADC. This offers the potential for significant space and power savings – as well as EMI reduction and simplified design - by eliminating the traditional ‘pre-conditioning’ elements of high pass filter, high gain amplifier and low pass filter. In the direct sensing scenario, these conditioning functions are performed in software running directly on the processor.

Powerful graphics
GFX accelerators provide opportunities to deploy animation, which have tended to be reserved exclusively for smartphones and high-end smart watches using power hungry gigahertz-class processors with full-blown 3D accelerators.

The device contains a total of four hardware graphics engines, each with a dedicated role; blitting, rotation, transformation and drawing. It offers exciting 3D-like graphics animation possibilities, at low CPU load and low power consumption.

The drawing engine draws the shapes necessary to create the user interface - a combination of lines, rectangles and triangles. To deliver the highest quality graphics, the drawing engine calculates the specific transparency of each pixel when applying antialiasing.

The blitting and rotation engines work together to provide Bit BLock Transfer (BitBLT), alpha blending and rasterising, as well as image scaling, shearing and rotation. Using this approach, only the elements of the display that have changed or moved are refreshed, saving processing power and battery life. The transformation engine gives high quality and flexible image transformation through Look Up Table (LUT) mapping. Together with the rotation engine, it creates the 3D perspective transformation, enabling a variety of 3D like animations: menus, lists or coverflows, such as creating a 3D globe from a 2D map of the Earth.

While the GFX provides the functionality, and enables highly sophisticated, feature rich user interfaces, the TZ1200 also comes with a dual-bus architecture. With this approach, the GFX is, in effect, an autonomous sub-system with its own SRAM and display interface. The CPU prepares a command list, based upon the needs of the device and the application software sends this short set of instructions to the GFX. The GFX interprets these high-level instructions into tasks for each of the graphics engines and then executes them directly to the display interface.

Figure 3: The TZ1200 features separate bus matrices for CPU and graphics operations

The GFX accelerator’s power allows it to render realistic graphics displays at high frame rates, which far exceed the needs of most wearable applications. Yet, due to the efficient, segregated, architecture while performing complex graphical manipulation, the load on the main Cortex CPU is minimised, meaning that the CPU retains capacity for application-oriented processing. The actual CPU load depends heavily on the desired GUI and the effectiveness of the GFX usage. As a reference, for typical GUI implementations with 2D and 3D animations, measurements showed a CPU load between 3% and 8%.

This highly efficient architecture allows the TZ1200 to achieve throughput performance often associated with CPUs with significantly higher clock frequencies. This, along with advanced features such as the ability to power down memory banks and on-chip peripherals individually, contributes significantly to the low power consumption.

The world of wearable technology is changing and is extremely competitive. Separating the graphics engine from the general processing demonstrate real benefits in terms of high performance at a reduced clock frequency, allowing designers to meet both the power budget and deliver the high-functionality, eye-catching, graphical user interfaces that will define more compelling wearable products.

Author profile
Luciano Duca is General Manager, Solution Marketing, Toshiba Electronics Europe