Intel PXA250 and PXA210 Applications Processors
Intel PXA250 and PXA210 Applications Processors
Intel PXA250 and PXA210 Applications Processors
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LCD Display Controller<br />
However, typical transfer rates are considerably less than 83 Mhz. For example, an 800x600 color<br />
active display running at 75 Hz requires a transfer rate of approximately 36 MHz. To determined<br />
this, calculate the number of pixels (800 x 600 = 480,000) <strong>and</strong> multiply by the screen refresh rate<br />
(75 Hz). Since active panels replace 1 pixel of data with every clock cycle this determines the final<br />
transfer rate. Active displays normally do not require refresh rates as high as 75 Hz, so you may use<br />
a lower refresh rate to reduce transmission rates even more.<br />
Passive displays often do require refresh rates greater than 75 Hz, which transfers more pixels each<br />
clock cycle. For instance, a color passive display with 8 data lines transfers 2 2/3 pixels’ worth of<br />
data each clock cycle. This divides the transmission rate by 2 2/3. Further reductions in the transfer<br />
rate come by using dual panel displays which use twice as many data lines to transfer data - halving<br />
the rate again.<br />
Generally, this gives you lower transfer rates to even large displays <strong>and</strong> thus simpler design<br />
considerations <strong>and</strong> fewer layout constraints.<br />
When laying out your design, minimize trace length of the LCD panel signals <strong>and</strong> allow sufficient<br />
spacing between signals to avoid crosstalk. Crosstalk decreases the signal integrity, especially the<br />
data line signals.<br />
LCD system design is not considered to be critical as infrequent or single bit errors are, typically,<br />
not noticed by the user. Also, the errors are transitory, as the old data is constantly being replaced<br />
with new data. Slower panel refresh rates increase the likelihood that a single error is noticed by the<br />
user. However, there is an counteracting effect in that slower refresh rates relax LCD timing <strong>and</strong><br />
therefore result in fewer screen transmission errors. There are other factors related to choosing a<br />
refresh rate for an LCD system, most significant is the impact on system b<strong>and</strong>width.<br />
If you must use excessively long or poorly routed signals, one possible solution is to add buffers<br />
between the <strong>PXA250</strong> applications processor <strong>and</strong> the LCD panel. This helps strengthen the LCD<br />
panel signal levels <strong>and</strong> synchronizes signal timing. However, this is usually not required as the<br />
LCD panel timings are fairly relaxed. Since the LCD display essentially operates asynchronously<br />
from the processor, the propagation delay of the buffers is not a major concern.<br />
When mounting the LCD panel, it is critical to shield the touchscreen control lines, if present.<br />
Noise from the LCD panel <strong>and</strong> its control signals can become injected into the touchscreen control<br />
lines, causing spurious touch interrupts or loss of resolution.<br />
3.5.4 Panel Connector<br />
Most LCD panels are connected to the system board via a connector, instead of being directly<br />
mounted on the system board. This increases flexibility <strong>and</strong> ease of manufacture. Typically the<br />
manufacturer of the panel recommends a particular connector for the panel. Follow the panel<br />
manufacturer’s recommendation.<br />
<strong>PXA250</strong> <strong>and</strong> <strong>PXA210</strong> <strong>Applications</strong> Processor Design Guide 3-9