Add graphics without employing dedicated graphics controller(3)

If the on-chip resources are insufficient to contain the entire frame buffer, the designer may be able to use external memory, such as an SRAM module, to hold the frame buffer. An advantage of using an internal controller is that the interface between the microcontroller and graphics controller has been taken care of by the microcontroller supplier.

Another advantage is that swapping out the LCD panel does not mean the controller has to be changed, so the application software may not need significant updates to reflect a different LCD panel. On the other hand, the flexibility of selecting which graphics controller a design can use is limited to the choice made by the microcontroller supplier.

Figure 4: An internal controller simplifies the board design by hosting the frame buffer and graphics controller within the microcontroller package.

Without controller
Adopting a controllerless configuration means a designer can keep using the same microcontroller they're already using for the system, and may be able to add graphics to the design without additional components or having to use a different microcontroller.

In general, a system using a controllerless configuration needs to send a frame of pixel information to a "dumb" LCD display panel at the refresh rate of the display—typically around 60Hz. To accomplish this, the system must continuously send frame data to the LCD panel. This could consume too much of the CPU processing bandwidth, and in that case using one of the three other configurations that rely on a dedicated graphics controller would be preferable.

However, a microcontroller that includes a DMA controller and a PMP peripheral can transfer the necessary pixel data to the LCD panel without consuming significant portions of the microcontroller's CPU bandwidth (figure 5). The DMA controller can manage the transfer of the frame buffer's contents to the LCD panel without requiring constant intervention from the CPU. Depending on the size of the frame buffer, it can reside completely within the microcontroller's on-chip memory, or the frame buffer could reside within an external SRAM chip.

Figure 5: A controllerless configuration simplifies the board design by hosting the frame buffer and graphics controller within the microcontroller package.

The system sets up the DMA controller to transfer one line of the frame-buffer data at a time through the PMP (each line consisting of many pixels). A PMP interrupt request is used to trigger the DMA to perform the next line transfer. In microcontrollers with non-persistent interrupts, a timer can be used as the DMA trigger source. During the DMA data transfer, the PMP strobes a read or write signal after each pixel transfer.

Figure 6: Timing signals enable the LCD panel to know the proper frame position at all times.

The read/write strobes of the PMP peripheral act as the pixel clock for the display glass. At the end of transferring each line of the frame buffer, the DMA controller interrupts the CPU to update the required timing signals, such as HSYNC (horizontal sync), VSYNC (vertical sync), and DEN (data enable), which enable the LCD panel to know the proper frame position at all times (figure 6). This process is repeated continuously until an entire frame buffer has been transferred.

The HYSNC signal tells the LCD panel when the data is at the start or end of a line. The VSYNC signal tells the LCD panel when the data is at the start or end of a frame. TFT LCD panels need the DEN signal to enable it to set up the panel to properly place the pixel locations. The PCLK signal is the clock source for the whole system. One clock pulse from the PCLK updates the LCD panel, and all of the other clock signals need to be synchronized with the PCLK to enable the LCD panel to produce a proper image output.

There are many different types of LCD panels from which to choose. The controllerless graphics method outlined here was designed to work with TFT (Thin Film Transistor) LCD panels, but can also work with CSTN (Color Super-Twist Nematic) or MSTN (Monochrome Super Twisted Nematic) panels, with minor modifications. LCD panels that do not contain all of the highlighted signals, such as HSYNC and VSYNC, can still benefit from using a controllerless configuration.