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CHAPTER 12 APPLICATIONS OF LCD CONTROLLER/DRIVER
(d) Common and segment signals
Each pixel on an LCD panel lights when the potential difference between the corresponding common and
segment signals reaches to a specific level (LCD drive voltage V
LCD
).
Because an LCD panel degrades if DC voltages are applied as common and segment signals, it is driven by
AC voltages.
<1> Common signal
The common signal is selected as shown in Table 12-2 according to the set number of time divisions
and repeatedly operates in the cycles shown in the table. In the static mode, the same signal is output
to COM0 through COM3.
In the 2-time division mode, open the COM2 and COM3 pins. Open the COM3 pin in the 3-time division
mode.
Table 12-2. COM Signal
COM0
COM1
COM2
COM3
Static
2 time divisions
Open
Open
3 time divisions
Open
4 time divisions
<2> Segment signal
Segment signals correspond to a 40-byte LCD display data memory (FA58H through FA7FH). Bits 0,
1, 2, and 3 of the display data memory are read in synchronization of COM0, COM1, COM2, and COM3,
respectively. If the content of each bit is 1, the corresponding segment signal is converted to a select
voltage and is output to the segment pin (S0 to S39). If the content of a bit is 0, the segment signal is
converted to an unselect voltage. (Note that S24 through S39 are multiplexed with input/output port pins.)
Therefore, confirm what combination of the front panel electrode (corresponding to a segment signal)
and rear panel electrode (corresponding to a common signal) of the LCD panel generates a display
pattern, and write the bit data corresponding to the pattern to be displayed on a one-to-one basis to the
LCD display memory.
In the static mode, bits 1, 2, and 3 of the LCD display data memory are not used for LCD display. In
the 2- and 3-time division modes, bits 2 and 3, and bit 3 are not used for LCD display, respectively. These
bits therefore can be used for any other purposes.
Bits 4 through 7 are fixed to 0.
COM Signal
Number of Time
Divisions