The lcd module interface port – Cirrus Logic AN179 User Manual
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AN179REV2
Copyright 2001 Cirrus Logic (All Rights Reserved)
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EP72xx and EP73xx
Another characteristic of displays is their Refresh Rate. The Refresh Rate is a number that states how frequently the
entire frame of data can be rewritten to the display. If the data is written too slowly, the display’s Response time
will affect the quality of its appearance. Too fast, and the display’s Response time will not be able to keep up with
the changing pixel drive states. Most displays today recommend rates around 70 – 80 Hz.
The LCD Controller has a fixed-clock source, which is used to generate the frequency that the pixels are written to
the display. When the EP72xx’s PLL is used to create its clocks, the LCD Controller is provided with a 36.864 MHz
clock. When the EP72xx is supplied a 13 MHz external clock, the LCD Controller is supplied with a 13 MHz clock.
These fixed clock sources place limits on the maximum rate that the pixels can be written to the display. Thus there
is a limit on the maximum Refresh Rate.
The EP72xx device has a field in the LCD Control Register (LCDCON) called the “Pixel Prescale”. The Pixel
Prescale field is a 6-bit field that sets the pixel rate prescale. When the EP72xx PLL is used to create its clocks, the
pixel rate is derived from
Equation 1. Pixel Rate (MHz) = 36.864/(Pixel prescale + 1)
The Pixel Prescale value can be expressed in terms of the LCD size by
.
Equation 2. Pixel Prescale = (36864000/(Refresh Rate x Total pixels in display)) – 1
The value should be rounded down to the nearest whole number and zero is illegal and will result in no pixel
clock.
For a ¼ VGA color display, the maximum Refresh Rate can be calculated by using the equations below.
Rearranging
to solve for the maximum Refresh Rate results in
Equation 3. Refresh Rate = 36864000/ (Total pixels in display) x (Pixel Prescale + 1)
Since, the minimum Pixel Prescale value is 1, we use this value in
to calculate the maximum Refresh
Rate, as shown in
Equation 4. Maximum Refresh Rate = 36864000/((320 (x3) x 240) x (1 + 1)) = 80 Hz
The LCD Controller can provide a maximum Refresh Rate of 80 Hz, for a ¼ VGA color LCD display. This is perfect
for most displays.
If color is desired, the maximum recommended display size is a ¼ VGA color LCD module. This is solely due to
their minimum required Refresh Rate. If a 1/2 VGA color display size is desired, the below restrictions must be
taken into consideration:
1). The LCD Controller can only support a 320 x 480 configuration, not a 640 x 240. If a color display with a 640
x 240 configuration were chosen, it would require 640 x 3 = 1920 pixels/line, which is larger than the
maximum programmable line width of 1024.
2). The LCD Controller can only provide a maximum Refresh Rate of 40 Hz (per
). This is too slow
for most displays.
If a monochrome display is chosen, the LCD Controller can support a full VGA display with a Refresh Rate of
60 Hz; along with support for up to 2 bpp, or 4 gray-scale levels.
Note:
The Refresh Rate is completely independent of the number of bits per pixel selected.
The LCD Module Interface Port
The LCD module interface port built into the EP72xx device, contains the following signals:
DD[0:3]
FRM
CL1