TK-90
30
Display Section
The display section consists of a MCU for the panel and
the peripheral circuit, an LCD display, and LEDs for the back-
light. Communication with the main MCU is performed with
the TXD and RXD lines. In order to avoid voltage drop, buffer
ICs are inserted into both the TX-RX unit and the Display unit
on the TXD and RXD lines.
■
Panel MCU Circuit
The 8D power supply sent from the main unit is converted
to 5V by the 5V AVR (IC1) to use as the power supply for the
digital section. For the MCU clock (CLK), 14.756MHz (X1) is
used. The reset signal is performed by the reset IC (IC5).
■
LCD Driver
LC75810 is the LCD driver. Because of the IC specifica-
tion, the 8V power supply for the LCD needs to be applied
after VDD (5V). Therefore, switching circuits for turning the
8V power supply on after 5V was emitted (Q4 and Q5) are
provided.
■
LED Section
12 LEDs are used for the LCD backlight which can be con-
trolled with 3 settings: Bright, Low, Off. The keypad light
uses a tact switch with the LEDs and is controlled with 2
settings: On and Off. On is used with both the Bright and
Low settings, and Off is used when the backlight is off.
When Busy or TX, the LED used is D7, which includes 2
LEDs. The MCU controls the on/off setting of these LEDs.
CIRCUIT DESCRIPTION /
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TX
Busy
BLC1
DIMM
LED
SW
IC2
IC3
MCU
RXD
TXD
TX-RX unit
DM
HK
PTT
BLC
MIC
IC5
Reset IC
X1
14.7456MHz
IC4
LCD driver
LCD
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Fig. 13
Display circuit /
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