MB87P2020 Datasheet, PDF (212/356 Pages) Fujitsu Component Limited. – Colour LCD/CRT/TV Controller

MB87J2120, MB87P2020-A Hardware Manual
7.5 Limitations for support
7.5.1 No Support due to VCOM Inversion
There were another two displays in the survey, the Fujitsu FLC15ADCAW and the Sharp LQ7BW506,
which suffer from a rather “substrate-oriented” interface. These displays do not have the necessary circuitry
to prevent DC from the liquid crystal. Therefore, they require RGB input voltage and common electrode
voltage (V_COM) be reversed every line. This inversion works as follows: For one line a voltage of e.g. -1.9V
is applied to the common electrode. A voltage of 0.6V on the RGB inputs produces then a white, a voltage
of 4.6V a black display. The next line, the common electrode is clamped to 5.9V. Now, a voltage of 0.6V
on RGB produces black and a voltage of 4.6V produces white. In other words: common electrode voltage
is alternated between -1.9 and 5.9V, while the voltage difference for RGB remains at 4V but with alternating
result (color) for every line. This causes the following problems:
• There is no way to produce the voltages mentioned above on-chip within GDC, since they exceed the
limits of supply voltage by far. Therefore, additional external circuitry is indispensable (Sharp rec-
ommend their dedicated video signal polarity reversing IC IR3Y29A).
• As the spec sheets state, the voltages actually used need to be fine-tuned for every individual display
to obtain optimum contrast (i.e. V_COM DC component (mean level), V_COM AC component (voltage
difference), as well as RGB DC and AC components). This implies external potentiometers to pro-
duce the bias voltages.
7.5.2 Problems due to 5V CMOS Interfaces
There is a number of displays, which demand 5V CMOS levels for their digital interface signals (marked
with “C5” in the respective column). This is problematic since the GDC-ASIC is not able to guarantee the
correct “High”-level at its outputs since the ASIC runs at 3.3V.
Work-around: Either external pull-up resistors or special level shifter ICs are used as e.g. the Philips
74ALVC164245 dual supply translating transceiver.
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