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11. Clarification to the term "rainbow" effect:
This refers to a red and green circle or rainbow on the LCD glass. The LCD panel
under uneven pressure causes this problem from the bezel. It is very common in LCD
modules and normally it will not affect the performance or the appearance of the
display when operational.
12. Pin assignments for a Character module:
Example of a standard 14-pin character module:
PIN 1: Vss
PIN 2: Vdd
PIN 3: Vo
PIN 4: RS
PIN 5: R/W
PIN 6: Enable
PIN 7 ~ 14: DB0 ~ DB7
13. What is temperature compensation and why is required
A LCD operating voltage varies at different temperatures. The operating voltage must
rise as temperature lowers or the contrast will degrade. Conversely, the operating
temperature must fall as the temperature rises or the contrast will degrade. For this
reason it is often a requirement, with graphics modules, to control the input voltage
accordingly. The temperature compensation circuit is the circuit that controls the input
voltage as the temperature changes. This temperature compensation circuit can be
located on the LCD module or on the customer's motherboard.
14. Troubleshooting a LED backlit module in which the display is turning
dark:
This problem is more than likely caused by the temperature rise from the LED
backlight. In this case the LED backlight has consumed too much of the power. When
the temperature rises, the VLCD becomes lower causing the input voltage to be too
high. The result is a poor contrast and the display becoming too dark. The solution
would be to lower the power consumption of the LED. This can be accomplished by
raising the value of R8 or R9 to reduce the current to the LED backlight.
15. How to control the LED backlight on a 14-pin module:
Short J2, the Vdd is controlling the input to the LED backlight. In addition, it is
necessary to place a current limiting resistor to lower the voltage from 5V to 4.2V.
Note: If the LED is drawing too much current, it may cause the Vdd † Vo too low and
the contrast becomes poor. If this should occur increasing the value of R9 should
decrease the current draw to the LED backlight or another approach would be to
increase the voltage input to the LCD by decreasing the value of R7.
16. Examples of the current consumption of an LED backlit, EL backlit and
the LCD for the following modules:
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11/10/2004
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