Dsc Circuit; Power Supply Circuits; Logic Circuits; Port Allocations - Icom IC-M502 Service Manual

4-4 DSC CIRCUITS (DSC UNIT)
4-4-1 DSC ENCODE CIRCUIT
The DSC signal created at CPU (IC16), is passed through
the buffer amplifier (Q33) and applied to the analog switch
(IC12, pin 1). The analog switch (IC12) is a modulation
switch that switches between microphone audio signal and
DSC signal.
4-4-2 DSC DECODE CIRCUT
The AF signals from FM IF IC (IC1, pin 9) are filtered at the
bandpass filter (IC4) with +18 dB/octave characteristics.
IC4b functions as a low-pass filter to suppress unwanted
higher noise signals and IC4a functions as a high-pass filter
to suppress lower noise signals. The filtered signals are con-
verted analog signals into digital signals at DSC decoder IC
(IC5), and are then applied to the CPU (IC16) after shaping
waveform at IC6.

4-5 POWER SUPPLY CIRCUITS

4-5-1 VOLTAGE LINE (MAIN UNIT)
LINE
The voltage from the connected DC power sup-
HV
ply.
Same voltage as the HV line which is passed
HVS
through the [PWR] switch (LOGIC unit; S1).
Same voltage as the HVS line which is passed
VCC
through the power controller (RL1).
Common 8 V converted from the VCC line at the
8V
8V regulator circuit (IC8).
Common 5 V converted from the 8V line at the
+5V
+5V regulator circuit (IC9).
Transmit 8 V controlled by the T8 control circuit
T8
(Q20, Q21) using the SEND signal from CPU.
Receive 8 V controlled by the R8 control circuit
(Q22, Q23) using the RCV signal from CPU. The
R8
controlled voltage is applied to the receiver cir-
cuits.

4-6 LOGIC CIRCUITS

4-6-1 MAIN UNIT
• MPU
IC16 is a 16 bit multifunction micro-computer and contains
FLASH memory, serial I/O, timer, A/D converter, D/A con-
verter, programmable I/O, ROM and RAM.
• SYSTEM CLOCK CIRCUIT
X3, X4 are crystal oscillators and oscillate 7.9872 MHz and
32.768 kHz system clocks for the MPU (IC16) respectively.
• RESET CIRCUIT
IC15 is a reset IC. When turn power ON, IC15 outputs a
reset signal ("LOW" pulse) to MPU (IC16, pin 75).
• LOW BATTERY DETECTOR
VCC voltage is divided by R204, R205 and is applied to the
low battery detector section in the MPU (IC16, pin 42).
DESCRIPTION
4-6-2 LOGIC UNIT
• CPU
IC1 is an 8 bit single chip micro-computer and contains LCD
driver, serial I/O, timer, A/D converter, programmable I/O,
ROM and RAM.
• SYSTEM CLOCK CIRCUIT
X1 is a ceramic oscillator and oscillates a 4.91 MHz system
clock for the CPU (IC1).
• LCD DRIVER
IC2 is a LCD driver for a dot matrix LCD.
• DIMMER CIRCUIT
CPU (IC1) and Q2, Q3, Q8 are dimmer circuit and control
the LCD backlight (LED).
• CONTRAST CIRCUIT
CPU (IC1) and Q1, Q4 are contrast circuit and control the 8
step display contrast.

4-7 PORT ALLOCATIONS

4-7-1 SUB CPU (LOGIC unit; IC1)
Pin Port
number name
27, LRESET,
28, 29, 30 E, RW, RS
DB7–
31–38
DB0
42 SCAN
43 CHWX
44 CH16
46 SQLV
47 KEYM
DIALA,
51, 52
DIALB
54 SRXD
55 STXD
CONTSEG3–
58–60
CONTSEG1
61 DTRS
62 IC
63 DSC
64 HL
DIM3–
75–77
DIM1
CONDOT3–
78–80
CONDOT1
4 - 4
Description
Output ports for the LCD driver (IC2)
control signals.
I/O port for the LCD driver (IC2) control
signals.
Input port for the [SCAN] key.
Input port for the [CH/WX] key.
Input port for the [16/9] key.
Input port for the squelch volume level.
Input port from the microphone (HM-
126) for remote control signal
Input ports for the [CHANNEL].
Outputs communication data for main
CPU (MAIN unit; IC16).
Input port for the communication data
from main CPU (MIAN unit; IC16).
Output port for the LCD contrast.
Input port for the [DISTRESS] key.
Input port for the [LO/DX] key.
Input port for the [DSC/ENT] key.
Input port for the [HI/LO] key.
Output LCD backlight control signal for
the dimmer circuit (Q2, Q3, Q8).
Output port for the LCD contrast.