EP72xx

The EP72xx LCD Controller provides all the necessary control signals to interface directly to a single-panel, multiplexed LCD module. The EP72xx uses the Universal Memory Architecture (UMA) for storing the video frame buffer. It shares the main memory bus with the core processor (i.e., the ARM720T). The total frame buffer size can be programmed up to 128 kbytes. The EP72xx panel size is programmable and can basically support any panel size available,including the support for full VGA sizes. Any width (line length) from 32 to 1024 in 16-pixel increments can be programmed. The total number of lines (rows) supported is solely determined by the total frame buffer size programmed, divided by the panel width and color depth programmed.

The EP72xx controller can also be programmed to provide 1-, 2-, or 4-bits-per-pixel color depth. The use of these bits is up to the application. They can be used to support a monochrome, gray-scale, or color display.As an example: If a 1/2 VGA display is used, and 4 bpp is desired, the Video Buffer Size field in the LCD Control register (LCDCON) will have to be programmed to equal 640 x 240 x 4 = 614,400 bits = 76,800 bytes. The Line Length field in the register will have to be programmed to equal 640, and the color depth field will have to be programmed to equal 4. These three settings will result in the support for the 240 lines.

To support the various possible colors and gray-scale levels, the LCD Controller has two 32-bit palette registers.These EP72xx palette registers are broken down into eight addressable nibbles each. This makes a total of sixteen nibbles.The nibbles are addressed by the data in the frame buffer. When the LCD Controller is configured to support 4 bpp, each four bits of data in the frame buffer is used to represent one pixel. Each of these nibbles addresses one of the sixteen nibbles in the two palette registers.

For example, if a nibble in the frame buffer contained the decimal value of ten, EP72xx would point to the tenth nibble in the two palette registers. This addressing scheme is used to map the data value in the frame buffer to the actual gray-scale level that will be supplied to the display interface. When 4 bpp mode is configured, all sixteen nibbles in the palettes registers are used for the mapping. This of course, is due to the fact that four bits can provide sixteen different values. When 2 bpp mode is configured, only the lowest addressable four nibbles are used. When in 1 bpp mode, only the lowest two nibbles are used.

Each EP72xx palette register nibble can be programmed with a value from 0 to 15. These sixteen different values correspond to 16 different color depth levels. When the value programmed into each nibble matches its nibble address, the frame buffer data corresponds exactly with the gray-scale levels. This is the usual approach. It is called Direct Bit Mapping.

When the value programmed into any of the nibbles does not match its nibble address, then interesting patterns can be generated on the display. For example, if it is desirable to toggle the display's image between normal and reverse video, this can easily be achieved by simply inverting the values in the nibbles of the palette registers. This is much quicker and easier than having to invert all of the data in the frame buffer.

The LCD Controller also contains a nine-word deep FIFO. It is used as an intermediate storage buffer for the frame data. An integrated DMA controller is used to fetch display data from the frame buffer memory, and refill the FIFO. Thus, once the LCD Controller is configured and the frame buffer data is stored, the EP72xx can continue executing other tasks without having to service the LCD Controller.

Note: This DMA controller is dedicated to servicing the LCD Controller. It cannot be used for any other purpose.