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Chapter 15
LCD Controller/Driver
Preliminary User’s Manual U14913EE1V0UM00
15.5 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 VLCD). The light
goes off when the potential difference becomes VLCD 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 15-5, and operations are
repeated with these as the cycle.
Table 15-5: COM Signals
(2)
Segment signals
Segment signals correspond to a 40-byte LCD display data memory (3FFF720H to 3FFF746H).
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) (S39 to S0 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.
(3)
Common signal and segment signal output waveforms
The voltages shown in Table 15-6 are output in the common signals and segment signals.
The ±VLCD 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 15-6: 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
-1/3 V
LCD
, +1/3 V
LCD
Non-Select Level
V
LC2
, V
LC1
-1/3 V
LCD
, +1/3 V
LCD
-1/3 V
LCD
, +1/3 V
LCD
COM Signal
COM0
Time Division
4-Time Division
COM1
COM3
COM2
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