Operation Overview; Display Control; Lcd Screen Format - Motorola DragonBall MC68328 User Manual

4.4 OPERATION OVERVIEW

4.5 DISPLAY CONTROL

The LCDC signal drives single-screen monochrome STN LCD panels with up to 1024x512
pixels in the gray-scale mode at a refresh rate of 60-70 Hz. In any case, the best efficiency
is achieved when the screen width is a multiple of the DMA controller's 16-bit bus width.
Because of LCD driver-technology limitations, large screens, such as 640x480, are usually
organized in spilt-screen format, which the MC68328 processor does not support. The
actual limit is the number of rows that require high driving voltage. The MC68328 processor
4-bit LCD interface will drive up to 240 rows with a maximum of 1024 columns.

4.5.1 LCD Screen Format

The screen width and height of the LCD panel are software-programmable. Figure 4-4
shows the relationship between the portion of a large graphics file displayed on screen vs.
the actual page. All units are measured in pixel counts in this figure.
The screen width (XMAX) and screen height (YMAX) registers specify the LCD panel size.
The LCD will start scanning the display memory at the location pointed to by the screen
starting address (SSA) register. Therefore, the LCD panel will display the shaded area in
Figure 4-4.
The virtual page width (VPW) and virtual page height (VPH) parameters specify the maxi-
mum page width and height, respectively. By changing the screen-starting address (SSA)
register, a screen-sized window can be vertically or horizontally scrolled (panned) anywhere
inside the virtual-page boundaries. The software must position the starting address (SSA)
properly so that the scanning logic's system memory pointer (SMP) does not stretch beyond
VPW nor VPH. Otherwise, strange artifacts will display on the screen. The programmer uses
the VPH only for boundary checks. There is no VPH register internal to the LCDC.
MOTOROLA MC68328 DRAGONBALL PROCESSOR USER'S MANUAL
Virtual Page Width (VPW)
ScreenStarting Address
(SSA)
Screen Width (XMAX)
Cursor X Position
(CX0)
Cursor Width
(CW)
Figure 4-4. LCD Screen Format
LCD Controller
4-7