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Randomizing Circuit
If the LCD driving voltage is left applied to a LCD cell for any length of time, the liquid crystal molecules become permanently polarized so that they can no longer assume their light transmitting orientation. This is called "DC-ing" a panel.
The solution is to reverse the polarity of the LCD drive voltage hundreds of times per second. This is called randomizing because from the point of view of a liquid crystal molecule the polarity is random so that a permanent charge is not acquired. From the point of view of the electronic logic, this switching is determinate. It counts some number
rows, then reverses the polarity.
On all TFT LCDs, on all V-series STN LCDs and on larger
legacy STN LCDs, this randomizing function is performed automatically. In section 2 of each
STN specification is a line titled Additional Circuit. Most often this line lists Randomizing circuit to indicated that the user need not be concerned with this requirement.
On 4.3 inch and smaller legacy STN LCDs, there is no included randomizing circuit. Instead the DF signal line is provided, where the controller must provide an AC signal to control the internal drive voltage polarization. The user still provides the LCD drive voltage as a DC voltage, or as a series of biased DC voltages. The DF signal forces the driver ICs to reverse the polarity.
Section 13b of the specifications of LCDs requiring a DF signal will give examples of randomizing circuits that accept the LOAD signal and presets as inputs and produce the DF signal as output. The circuits use a count down IC to count down LOAD signals from the preset number, then flip the state of the DF signal at zero. Here is an example.
However, users probably will not implement this circuit because controller ICs can perform this function. The presets are replaced by a controller register that the BIOS must load when the controller IC is initialized. Sometimes this randomizing signal is called "M" or "DRDY". Sometimes it can be provided by the general purpose I/O pins.
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