NEC PD78F9488 - Manual (Page 258)

CHAPTER 13 LCD CONTROLLER/DRIVER
258
User’s Manual U15331EJ4V1UD
13.6 Common and Segment Signals
Each pixel of the LCD panel turns on when the potential difference between the corresponding common and
segment signals becomes higher than a specific voltage (LCD drive voltage, V
LCD
). It turns off when the potential
difference becomes lower than V
LCD
.
Applying DC voltage to 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) Common signals
Each common signal is selected sequentially according to the specified number of time slots at the timing
listed in Table 13-4. In the static display mode, the same signal is output commonly to COM0 to COM3.
In the three-time-slice mode, leave the COM3 pin open.
Table 13-4. COM Signals
COM Signal
Number of Time Slices
COM0 COM1 COM2 COM3
Three-time-slice mode Open
Four-time-slice mode
(2) Segment signals
The segment signals correspond to LCD display data memory. Bits 0, 1, 2, and 3 of each byte are read in
synchronization with COM0, COM1, COM2, and COM3, respectively. If the contents of each bit are 1, that
bit is converted to the select voltage, and if 0, it is converted to the deselect voltage. The conversion results
are output to the segment pins.
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.
Bit 3 of the LCD display data memory is not used for LCD display in the three-time-slice mode. So this bit
can be used for purposes other than display.
LCD display data memory bits 4 to 7 are fixed to 0.
(3) Output waveforms of 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 13-7 shows the common signal waveforms, and Figure 13-8 shows the voltages and phases of the common
and segment signals.