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The configuration for Liquid Crystal Display Multiplex Drive technique
differs from a Static Drive technique is that it uses more than a single
"backplane" or segment common. With this configuration, each segment control
line can be connected to as many segments as there are backplanes, providing
that each of the segments that it is connected to are tied to a separate
backplanes. This method "Multiplexes" each of the segment control lines and
minimizes the number of interconnects. This is the method used with complex
displays that have limited interconnection surface area or available drive
circuits. This reduction in the number of external connections enhances
device reliability and increases the potential display density. The
liability of a higher multiplex rate will effect display quality,
operational temperature range, and the increased complexity of drive
circuitry (or perhaps microprocessor software) may necessary for their
operation.
The method of drive for multiplexed displays is essentially a time division
multiplex with the number of time divisions equal to twice the number of
common planes used in a given format. As is the case with conventional LCDs,
in order to prevent irreversible electrochemical action from destroying the
display, the voltage at all segment locations must be caused to reverse
polarity periodically so that zero net DC voltage is applied. This is the
reason for the doubling in time divisions: Each common plane must be
alternately driven with a voltage pulse of opposite polarity.
As is the case with non multiplexed displays, the drive frequency should be
chosen to be above the flicker-fusion rate, i.e. >30 Hz. Since increasing
the drive frequency significantly above this value increases current demand
by the CMOS drive electronics, and to prevent problems due to the finite
conductivity of the display segment and common electrodes, an upper drive
frequency limit of 60-90 Hz is recommended. |
