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參數資料
| 型號： | M3851 |
| 廠商： | Mitsubishi Electric Corporation |
| 英文描述： | Single Chip 8 Bits Microcomputer(8位單片機) |
| 中文描述： | 單芯片8位單片機（8位單片機） |
| 文件頁數： | 37/61頁 |
| 文件大?。?/td> | 904K |
| 代理商： | M3851 |
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37
3851 Group
SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER
MITSUBISHI MICROCOMPUTERS
CLOCK GENERATING CIRCUIT
The 3851 group has two built-in oscillation circuits. An oscillation
circuit can be formed by connecting a resonator between X
IN
and
X
OUT
(X
CIN
and X
COUT
). Use the circuit constants in accordance
with the resonator manufacturer’s recommended values. No exter-
nal resistor is needed between X
IN
and X
OUT
since a feed-back
resistor exists on-chip. However, an external feed-back resistor is
needed between X
CIN
and X
COUT
.
Immediately after power on, only the X
IN
oscillation circuit starts
oscillating, and X
CIN
and X
COUT
pins function as I/O ports.
Frequency Control
(1) Middle-speed mode
The internal clock
φ
is the frequency of X
IN
divided by 8. After re-
set, this mode is selected.
(2) High-speed mode
The internal clock
φ
is half the frequency of X
IN
.
(3) Low-speed mode
The internal clock
φ
is half the frequency of X
CIN
.
I
Note
If you switch the mode between middle/high-speed and low-
speed, stabilize both X
IN
and X
CIN
oscillations. The sufficient time
is required for the sub-clock to stabilize, especially immediately af-
ter power on and at returning from the stop mode. When switching
the mode between middle/high-speed and low-speed, set the fre-
quency on condition that f(X
IN
) > 3f(X
CIN
).
(4) Low power dissipation mode
The low power consumption operation can be realized by stopping
the main clock X
IN
in low-speed mode. To stop the main clock, set
bit 5 of the CPU mode register to “1.” When the main clock X
IN
is
restarted (by setting the main clock stop bit to “0”), set sufficient
time for oscillation to stabilize.
The sub-clock X
CIN
-X
COUT
oscillating circuit can not directly input
clocks that are generated externally. Accordingly, make sure to
cause an external resonator to oscillate.
Oscillation Control
(1) Stop mode
If the STP instruction is executed, the internal clock
φ
stops at an
“H” level, and X
IN
and X
CIN
oscillation stops. When the oscillation
stabilizing time set after STP instruction released bit is “0,” the
prescaler 12 is set to “FF
16
” and timer 1 is set to “01
16
.” When the
oscillation stabilizing time set after STP instruction released bit is
“1,” set the sufficient time for oscillation of used oscillator to stabi-
lize since nothing is set to the prescaler 12 and timer 1.
Either X
IN
or X
CIN
divided by 16 is input to the prescaler 12 as
count source. Oscillator restarts when an external interrupt is re-
ceived, but the internal clock
φ
is not supplied to the CPU (remains
at “H”) until timer 1 underflows. The internal clock
φ
is supplied for
the first time, when timer 1 underflows. This ensures time for the
clock oscillation using the ceramic resonators to be stabilized.
When the oscillator is restarted by reset, apply “L” level to the
RESET pin until the oscillation is stable since a wait time will not
Fig. 44 Ceramic resonator circuit
Fig. 45 External clock input circuit
X
CIN
X
COUT
X
IN
X
OUT
C
IN
C
OUT
C
CIN
C
COUT
Rf
Rd
X
CIN
X
COUT
X
IN
X
OUT
C
CIN
C
COUT
Rf
Rd
Open
External oscillation
circuit
Vcc
Vss
be generated.
(2) Wait mode
If the WIT instruction is executed, the internal clock
φ
stops at an
“H” level, but the oscillator does not stop. The internal clock
φ
re-
starts at reset or when an interrupt is received. Since the oscillator
does not stop, normal operation can be started immediately after
the clock is restarted.
To ensure that the interrupts will be received to release the STP or
WIT state, their interrupt enable bits must be set to “1” before ex-
ecuting of the STP or WIT instruction.
When releasing the STP state, the prescaler 12 and timer 1 will
start counting the clock XIN divided by 16. Accordingly, set the
timer 1 interrupt enable bit to “0” before executing the STP instruc-
tion.
I
Note
When using the oscillation stabilizing time set after STP instruction
released bit set to “1”, evaluate time to stabilize oscillation of the
used oscillator and set the value to the timer 1 and prescaler 12.
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