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CHAPTER 12 LCD CONTROLLER/DRIVER
155
3
3
+
V
LCD
/-
V
LCD
5
5
Each pixel of the LCD panel becomes on when the potential difference between the corresponding common and
segment signals becomes higher than a specific voltage (2V
LCD
). It becomes off when the potential difference
becomes lower than 2V
LCD
(for details, see
(1)
below).
A segment signal is converted to a select voltage if the contents of the corresponding bit of each display data
memory are 1; if the contents of the bit are 0, they are converted to an unselect voltage and output to a segment pin
(S0 to S39). Note that S32 to S39 can be used also as an I/O port.
Check, with the information given above, what combination of the front-surface electrodes (corresponding to the
segment signals) and the rear-surface electrodes (corresponding to the common signals) forms display patterns in
the LCD display data memory, and write the bit data that corresponds to the desired display pattern on a one-to-one
basis.
Applying DC voltage as the common and segment signals for an LCD panel would deteriorate it. To avoid this
problem, this LCD panel is driven with AC voltage.
(1)
Output waveforms of common and segment signals
Voltages listed in Table 12-3 are output as common and segment signals.
When both common and segment signals are at the select voltage, a display on-voltage of
±
V
LCD
is
obtained. The other combinations of the signals correspond to the display off-voltage. Figure 12-6 shows
an example LCD drive waveform between a segment signal and a common signal.
Table 12-3. LCD Drive Voltage
Select Signal Level
Deselect Signal Level
Segment Signal
Common Signal
V
SS0
/V
LC0
V
LC3
/V
LC2
Select signal level
V
LC0
/V
SS0
+V
LCD
/-V
LCD
Deselect signal level
V
LC4
/V
LC1
1
5
1
5
-
V
LCD
/+
V
LCD
5
5
+
V
LCD
/-
V
LCD
1
1