
338
μPD780948, μPD78F0948, μPD780949, μPD78F0949
19.6 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 19-3, and operations are
repeated with these as the cycle.
Table 19-3: 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) (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.
Bits 4 to 7 are fixed at 0.
(3) Common signal and segment signal output waveforms
The voltages shown in Table 19-4 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 19-4: LCD Drive Voltage
COM Signal
COM0
COM1
COM2
COM3
Time Division
4-Time Division
Segment
Select Level
V
SS1
, V
LC0
-V
LCD
, +V
LCD
-1/3 V
LCD
, +1/3 V
LCD
Non-Select Level
V
LC1
, V
LC2
-1/3 V
LCD
, +1/3 V
LCD
-1/3 V
LCD
, +1/3 V
LCD
Common
Select Level
Non-Select Level V
LC2
, V
LC1
V
LC0,
V
SS1