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Understanding graphics memory needs of wearables

Posted: 08 Dec 2015     Print Version  Bookmark and Share

Keywords:Internet of Things  IoT  embedded RAMs 

High definition media consumption is going through a two-fold growth—one is an increase in number of consumers and the other is a transition towards even higher definition content. This is driven by more widespread and faster Internet access combined with an explosion of mobile devices (cell phones, tablets, wearable devices, etc). As a consequence, many wearable devices are now coming equipped to handle HD media consumption.

The demand for Internet of Things (IoT) and wearable devices is estimated to grow three-fold by 2020, even by the most conservative estimates. This means there will be 50 billion devices. This will create demand for a new family of display drivers and frame buffers – a memory option that is unlike those used in legacy displays. While embedded RAMs could suffice in early generation wearable devices, today's high definition and large wearable displays require significantly larger frame buffer memories. These requirements differ from the traditional large displays of PCs and televisions by virtue of being battery operated and built with power efficiency as a primary design constraint. A majority of the latest wearable devices will be so space- and power-efficient that they will need to be able to run days and possibly even weeks on a single charge, all the while performing complex operations. This is why we will require a new family of display drivers.

To understand frame buffer requirements for wearable devices, let us first explore the architecture of graphics systems. Every graphics system consists of 3 components – hardware, graphics library, and an application that utilizes it.

While the library and the application are software-controlled, the hardware is controlled by a frame buffer, a contiguous high throughput memory. Each element of a frame buffer corresponds to a single pixel on the screen. The intensity of that pixel is decided by its voltage.

A display's resolution is determined by: Number of scan lines, Number of pixels per line and Number of bits per pixel.

For example, consider a 1024x768 24-bit image, the most widely used screen resolution for PCs.

1024 X 768 X 24 = 18.9Mb

This is the minimum size of frame buffer required to support such a display. However, simply having a memory of this size won't suffice if it is a dynamic display with video capabilities. This brings up the throughput requirement for frame buffers.

For a 30 frame per second (fps) video of this resolution, the maximum throughput would be: 18.9 x 30 = 566Mbps

As described earlier, every memory cell in a frame buffer corresponds to a single pixel. In the case of an n-bit color display, each of these n bits is a separate bit plane (e.g. 24-bit color will have 24 bit planes). N cells will store the state for each pixel. The binary values from each of the n bit planes is loaded into corresponding positions in a register. The resulting binary number is interpreted as an intensity level between 0 to 2n – 1. This is then converted into an analog voltage between 0 and the maximum voltage by a digital-to-analog converter, hence enabling 2n intensity levels.

There are two factors that decide the type of frame buffer used for a display – size and throughput. Increasing the resolution of an image requires more memory while increasing the fps of the video requires higher throughput. There are two ways to meet this requirement – minimizing frame buffer size and maximizing throughput or maximizing frame buffer size to minimize throughput (e.g. one doubled while the other is halved). By increasing the frame buffer size (essentially having multiple frame buffers within a single chip), we can reduce throughput because the chip has to go through the input-output cycle fewer times. For example, by doubling the size, two frames can be stored simultaneously in a single buffer, meaning the buffer is called/referenced half the number of times in a given timeframe, thus allowing for lower throughput. Memory options are hence split into two types – high density and high throughput. This aspect will be discussed later in this article.

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