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CHAPTER 16 LCD CONTROLLER/DRIVER
16.6 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
). The light goes off when the
potential difference becomes V
LCD
or lower.
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 16-3, and operations are repeated with these
as the cycle.
Table 16-3. COM Signals
COM signal
Time division
4-time division
COM0
COM1
COM2
COM3
(2) Segment signals
Segment signals correspond to a 20-byte LCD display data memory (FA59H to FA6CH). 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 S19) (S18 to S5 have an alternate
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 panel 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.
Bits 4 to 7 are fixed at 0.
(3) Common signal and segment signal output waveforms
The voltages shown in Table 16-4 are output in the common signals and segment signals.
The
±
V
LCD
ON voltage is only produced when the common signal and segment signal are both at the selection
voltage; other combinations produce the OFF voltage.
Table 16-4. LCD Drive Voltage
Segment
Select Level
Non-Select Level
Common
V
SS1
, V
LC0
V
LC1
, V
LC2
Select level
V
LC0
, V
SS1
–V
LCD
, +V
LCD
–1/3V
LCD
, +1/3V
LCD
Non-select level
V
LC2
, V
LC1
–1/3V
LCD
, +1/3V
LCD
–1/3V
LCD
, +1/3V
LCD
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