Motorola MPC823e Reference Manual page 1000

The time it takes to refresh a row is called the duty cycle. In a simple refresh policy, one row
is refreshed at a time. However, this creates large voltage spikes on the LCD electrodes.
Another technique, called active addressing, refreshes groups of rows and smoothes the
applied voltage that each pixel receives over approximately half a frame cycle. This
significantly improves the contrast and other characteristics of a simple LCD display. The
drawback of active addressing is that it is fairly complex and requires that you have a
dedicated chip to perform it on. This chip holds a large display buffer (typically equivalent to
a quarter of a VGA display per circuit). The active addressing algorithm is done on entire
columns and it usually fits in the LCD display path and appears to the system as a thin film
transistor (TFT) interface.
It is important to avoid long-term DC bias in the voltages applied to the LCD screen because
it causes defects in the polarization, as well as raindrop-like patterns (raindrop effect), to
appear on the display. To overcome this problem, the controller changes the field polarity
every few frames.
18.1.2 Types of LCD Interfaces
Basic LCD panels require a clock, one or more data input lines, and horizontal and vertical
syncs. These signals are usually provided by an LCD controller that includes a frame buffer
RAM for display memory. An example of a complete LCD subsystem is illustrated in
Figure 18-2.
CPU
MOTOROLA
LCD
CONTROLLER
FRAME
BUFFER
RAM
Figure 18-2. LCD Subsystem
MPC823e REFERENCE MANUAL
LCD Controller
LCD PANEL
X-DRIVERS
LCD ARRAY
DC-DC CONVERTER
18-3