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Add graphics without employing dedicated graphics controller

Posted: 24 Nov 2011 ?? ?Print Version ?Bookmark and Share

Keywords:graphics controller? LCD? microcontroller?

As more flexible and powerful user interfaces are demanded by embedded applications, the need to include a graphical interface in a design puts pressure on the designer trying to keep costs low. A typical system with a graphical display relies on an internal or external graphics controller to drive the display, but this can increase the complexity of the design and add cost. For a simple graphical interface, the extra cost for a graphics controller may not be justifiable. However, some microcontrollers include a mix of peripherals coupled with a specialized graphics library, which enables designers to incorporate simple graphical interfaces without incurring the cost of a dedicated graphics controller.

A low-cost, controllerless approach is to use a microcontroller's peripherals and memory resources as a substitute for a graphics controller, thus creating a virtual graphics controller that can render graphics and continuously update an LCD display, while consuming approximately six percent of the total CPU bandwidth.

A microcontroller only needs Direct Memory Access (DMA) and Parallel Port Master (PMP) peripherals to create a virtual graphics controller. This approach is appropriate for use in cost-sensitive consumer applications such as thermostats, cordless phones, remote controls, coffee machines, washing machines, and ovens. It may also be appropriate for commercial and industrial applications such as ATMs, digital instrument gauges, storage controls, remote terminals, and movie-rental boxes. Medical applications that can benefit from a controllerless graphics display include glucometers, blood-pressure monitors, and portable Electrocardiograms (ECGs).

Figure 1: The display resolution specifies the dimension of the display in pixels, not the actual size of the display.

Graphics basics
A pixel is a fundamental unit that denotes a single dot of color in the display area. The pixel color depth defines how many possible colors a pixel can be drawn, which has a direct impact on the amount of memory a system requires to represent and store each pixel of the display. Color depth is commonly represented as Bits-Per-Pixel (BPP).

A common color depth for LCD displays is 16 BPP, with the color data represented in a 565 RGB color format. The 565 RGB format specifies a pixel color as a 5bit red value, a 6bit green value, and a 5bit blue value that together directly define the blended color. In contrast, a 24 BPP color depth represents the pixel color as three 8bit byteseach byte specifying the 8bit value of the red, green, and blue components of the blended color.

The display resolution specifies the number of pixels contained within the display area, and it is defined by its horizontal and vertical pixel dimensions. There are a number of standard resolutions used for display resolutions. For example, a Quarter Video Graphics Array (QVGA) display is defined to be a rectangle of 320 by 240 pixels. A Wide QVGA (WQVGA) display is defined as a rectangle of 432 by 240 pixels, and it supports wide displays. Note that the resolution specifies the number of pixels, not the actual size dimensions of the display (figure 1). The resolution, combined with the dimensions of the display, affect the size of each individual pixel.

Table: The display resolution and the color depth, taken together, determine the amount of memory needed to contain the contents of the frame buffer.

The frame buffer is a block of volatile memory where all of the pixel-color data for the display is stored. In addition to acting as the work area for changing the contents in the display area for the application code, the frame buffer is a necessary part of the refresh process for the LCD. The size and configuration of the frame buffer directly correlate with the display resolution and the pixel-color depth (table).

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