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87
SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER for VOLTAGE SYNTHESIZER
with ON-SCREEN DISPLAY CONTROLLER
M37225M6–XXXSP, M37225M8–XXXSP
M37225ECSP
MITSUBISHI MICROCOMPUTERS
Rev. 1.0
8.13 CLOCK GENERATING CIRCUIT
The built-in clock generating circuit is shown in Figure 8.13.3. When
the STP instruction is executed, the internal clock
φ
stops at HIGH.
At the same time, timers 3 and 4 are connected by hardware and
“FF
16
” is set in timer 3 and “07
16
” is set in the timer 4. Select f(X
IN
)/16
as the timer 3 count source (set bit 0 of the timer mode register 2 to
“0” before the execution of the STP instruction). Moreover, set the
timer 3 and timer 4 interrupt enable bits to disabled (“0”) before ex-
ecution of the STP instruction). The oscillator restarts when external
interrupt is accepted. However, the internal clock
φ
keeps its HIGH
until timer 4 overflows, allowing time for oscillation stabilization when
a ceramic resonator or a quartz-crystal oscillator is used.
When the WIT instruction is executed, the internal clock
φ
stops in
the HIGH but the oscillator continues running. This wait state is re-
leased when an interrupt is accepted (See note). Since the oscillator
does not stop, the next instruction can be executed at once.
When returning from the stop or the wait state, to accept an interrupt,
set the corresponding interrupt enable bit to “1” before executing the
STP or the WIT instructions.
Note:
In the wait mode, the following interrupts are invalid.
V
SYNC
interrupt
OSD interrupt
Timer 2 interrupt using external clock input from TIM2 pin as count
source
Timer 3 interrupt using external clock input from TIM3 pin as count
source
Timer 4 interrupt using f(X
IN
)/2 as count source
Timer 1 interrupt using f(X
IN
)/4096 as count source
f(X
IN
)/4096 interrupt
Multi-master I
2
C-BUS interface interrupt
A-D conversion interrupt
SPRITE interrupt
A circuit example using a ceramic resonator (or a quartz-crystal os-
cillator) is shown in Figure 8.13.1. Use the circuit constants in accor-
dance with the resonator manufacture’s recommended values. A cir-
cuit example with external clock input is shown in Figure 8.13.2. In-
put the clock to the X
IN
pin, and open the X
OUT
pin.
Fig.8.13.1 Ceramic Resonator Circuit Example
Fig.8.13.2 External Clock Input Circuit Example
X
IN
X
OUT
C
IN
Microcomputer
C
OUT
X
IN
Microcomputer
Vcc
Vss
External oscillation circuit
Fig.8.13.3 Clock Generating Circuit Block Diagram
Interrupt request
Interrupt disable
flag I
Reset
S
Q
R
STP instruction
S
Q
R
WIT
instruction
S
Q
R
STP
instruction
Reset
Internal clock
φ
1/2
1/8
Timer 3
Timer 4
X
OUT
X
IN
TM20
TM22
Selection gate :
Connected to black
side at reset.
TM2 : Timer mode register 2