Add graphics without employing dedicated graphics controller

For example, a QVGA display with a 16 BPP color depth would require a frame buffer of at least 320 x 240 x 16 bits, or 153,600B. In contrast, a WQVGA display with a 24 BPP color depth would require a frame buffer of at least 432 x 240 x 24 bits, or 311,040B. The size of the frame buffer may contain more than the minimum number of bytes to support windowed scrolling functions, double or triple buffering, or to simplify drawing bitmaps of pixels near the display edges. Details about how the frame buffer is used to refresh the LCD are explained later in this article (details about implementing window scrolling, multiple buffers, and drawing bitmaps is beyond the scope of this article).

The display refresh rate is defined in Hertz, and it is the rate at which the entire LCD panel frame is redrawn per second. The refresh rate affects not just how animations/updates on the display will appear to the user, but in a controllerless implementation, the refresh rate affects how much processing load on the microcontroller is necessary to keep the LCD refreshed.

The pixel clock (PCLK) signal enables the LCD panel to synchronize the sampling of incoming color data; the clock signal needs to be faster for higher refresh rates, as well as higher-resolution displays, so that all of the pixels in the frame can be properly clocked. The pixel throughput is the speed at which a pixel can be redrawn on the LCD screen. The time to draw an entire frame is the pixel throughput multiplied by the display resolution.

Controller configurations
To better understand how to implement a virtual graphics controller, it is helpful to understand how graphics controllers are used with LCD panels. There are three basic configurations for using a dedicated graphics controller. A "smart glass" configuration consists of a LCD panel that has a graphics controller integrated within the panel (figure 2).

This configuration simplifies the integration effort for the designer because the frame buffer and graphics controller have already been matched to the display panel. The application software may make changes to the contents of the frame buffer through Application Programming Interface (API) calls that are based on the specific controller contained within the LCD module. If the designer needs to change the LCD panel, there may need to be changes made to the application code unless the new module supports the same controller APIs.

Figure 2: A smart-glass configuration integrates the frame buffer and graphics controller within the LCD module.

While a smart-glass configuration always includes an external graphics controller that has been integrated within the LCD module, a "dumb glass" configuration may be used with an external graphics controller located on the system board or with an internal controller that has been integrated into the microcontroller package. Using a dumb-glass configuration means that the system designer is responsible for the integration of the controller with the LCD screen.

Using an external controller is similar to the application software used with the controller that is integrated in the smart-glass configuration, in that the microcontroller communicates with the frame buffer and controller through a parallel interface and API calls (figure 3).

If the design needs to change the LCD panel or controller, the designer has the most control and flexibility in how to make the change with an external-controller configuration. On the other hand, using an external controller involves managing additional components on the system board, which all the other approaches avoid.

Figure 3: An external controller affords the most flexibility, but it requires an additional component for the controller.

Using an internal-controller configuration, the graphics controller and possibly the frame buffer are integrated within the microcontroller (figure 4). The frame buffer may or may not be able to reside entirely within the on-chip memory resources, depending on the display resolution and color depth chosen by the designer.

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