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CHAPTER 18 LCD CONTROLLER/DRIVER
18.6 Common Signals and Segment Signals
An individual pixel on an LCD panel lights when the potential difference of the corresponding common signal and
segment signal reaches or exceeds a given voltage (the LCD drive voltage V
LCD
).
As an LCD panel deteriorates if a DC voltage is applied in the common signals and segment signals, it is driven by
AC voltage.
(1) Common signals
For common signals, the selection timing order is as shown in Table 18-4 according to the number of time
divisions set, and operations are repeated with these as the cycle. In the static mode, the same signal is output
to COM0 through COM3.
With 2-time-division operation, pins COM2 and COM3 are left open, and with 3-time-division operation, the
COM3 pin is left open.
Table 18-4. COM Signals
(2) Segment signals
Segment signals correspond to a 40-byte LCD display data memory (FA58H to FA7FH). Each display data
memory bit 0, bit 1, bit 2, and bit 3 is read in synchronization with the COM0, COM1, COM2 and COM3 timings
respectively, and if the value of the bit is 1, it is converted to the selection voltage. If the value of the bit is 0,
it is converted to the non-selection voltage and output to a segment pin (S0 to S39) (S24 to S39 have a dual
function as input/output port pins).
Consequently, it is necessary to check what combination of front surface electrodes (corresponding to the
segment signals) and rear surface electrodes (corresponding to the common signals) of the LCD display to be
used form the display pattern, and then write bit data corresponding on a one-to-one basis with the pattern to
be displayed.
In addition, because LCD display data memory bits 1 and 2 are not used with the static method, bits 2 and 3
are not used with the 2-time-division method, and bit 3 is not used with the 3-time-division method, these can
be used for other than display purposes.
Bits 4 to 7 are fixed at 0.
COM signal
Time division
Static
2-time division
3-time division
4-time division
Open
Open
Open
COM0
COM1
COM2
COM3