UPD78F9418A NEC, UPD78F9418A Datasheet - Page 209

• Manufacturer Part Number: UPD78F9418A
• Description: (UPD7894xxA) 8-Bit Single Chip Microcontrollers
• Manufacturer: NEC

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14.6 Common and Segment Signals
segment signals becomes higher than a specific voltage (LCD drive voltage, V
potential difference becomes lower than V
problem, this LCD panel is driven by AC voltage.
Each pixel of the LCD panel turns on when the potential difference between the corresponding common and
Applying DC voltage to the common and segment signals of an LCD panel causes deterioration. To avoid this
(1)
(2)
Common signals
Each common signal is selected sequentially according to a specified number of time slices at the timing
listed in Table 14-4. In the static display mode, the same signal is output to COM0 to COM3.
In the two-time-slice mode, leave the COM2 and COM3 pins open. In the three-time-slice mode, leave the
COM3 pin open.
Segment signals
The segment signals correspond to 28 bytes of LCD display data memory (FA00H to FA1BH). Bits 0, 1, 2,
and 3 of each byte are read in synchronization with COM0, COM1, COM2, and COM3, respectively. If a bit
is 1, it is converted to the select voltage, and if it is 0, it is converted to the deselect voltage. The conversion
results are output to the segment pins (S0 to S27). Note that S16 to S27 can also be used as I/O port pins.
Check, with the information given above, what combination of front-surface electrodes (corresponding to the
segment signals) and rear-surface electrodes (corresponding to the common signals) forms display patterns
in the LCD display data memory, and write the bit data that corresponds to the desired display pattern on a
one-to-one basis.
LCD display data memory bits 1 and 2, bits 2 and 3, and bit 3 are not used for LCD display in the static
display, two-time slot, and three-time slot modes, respectively. So these bits can be used for purposes other
than display.
LCD display data memory bits 4 to 7 are fixed to 0.
Number of Time Slices
Static display mode
Two-time-slice mode
Three-time-slice mode
Four-time-slice mode
COM Signal
CHAPTER 14 LCD CONTROLLER/DRIVER
LCD
.
Table 14-4. COM Signals
User’s Manual U13952EJ3V0UD
COM0
COM1
Open
LCD
COM2
). The pixels turn off when the
Open
Open
COM3
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