SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER
MITSUBISHI MICROCOMPUTERS
38C1 Group
39
CLOCK GENERATING CIRCUIT
The oscillation circuit of 38C1 group can be formed by connecting
an oscillator, capacitor and resistor between XIN and XOUT (XCIN
and XCOUT). To supply a clock signal externally, input it to the XIN
pin and make the XOUT pin open. The clocks that are externally
generated cannot be directly input to XCIN. Use the circuit con-
stants in accordance with the oscillator manufacturer's recom-
mended values. No external resistor is needed between XIN and
XOUT since a feed-back resistor exists on-chip. However, a 10 MΩ
external feed-back resistor is needed between XCIN and XCOUT.
Immediately after reset is released, only the built-in ring oscillator
starts oscillating, XIN -XOUT oscillation stops oscillating, and XCIN
and XCOUT pins function as I/O ports.
Operation mode
(1) Ring oscillator mode
The internal clock φ is the built-in ring oscillator oscillation divided
by 8.
(2) Middle-speed mode
The internal clock φ is the frequency of XIN divided by 8.
(3)High-speed mode
The internal clock φ is half the frequency of XIN.
(4) Low-speed mode
The internal clock φ is half the frequency of XCIN.
After reset release and when system returns from the stop mode,
the ring oscillator mode is selected.
Refer to the clock state transition diagram for the setting of transi-
tion to each mode.
The XIN–XOUT oscillation is controlled by the bit 5 of CPUM, and
the sub-clock oscillation is controlled by the bit 4 of CPUM. When
the mode is switched to the ring oscillator mode, set the bit 3 of
CPUM to “1”.
In the ring oscillator mode, the oscillation by the oscillator can be
stopped. In the low-speed mode, the power consumption can be
reduced by stopping the XIN–XOUT oscillation.
When the mode is switched from the ring oscillator mode to the
low-speed mode, the built-in ring oscillator is stopped.
Set enough time for oscillation to stabilize by programming to re-
start the stopped oscillation and switch the operation mode. Also,
set enough time for oscillation to stabilize by programming to
switch the timer count source .
Note: If you switch the mode between ring oscillator mode,
middle/high-speed mode and low-speed mode, stabilize
both XIN and XCIN oscillations. Especially be careful imme-
diately after power-on and at returning from stop mode. Re-
fer to the clock state transition diagram for the setting of
transition to each mode. Set the frequency in the condition
that f(XIN) > 3•f(XCIN).
When the middle- and high-speed mode are not used (XIN-
XOUT oscillation and external clock input are not
performed), connect XIN to VCC through a resistor.
Fig. 40 Oscillator circuit
Fig. 41 External clock input circuit
Oscillation Control
(1) Stop mode
Set the timer 1 interrupt enable bit to disabled (“0”) before execut-
ing the STP instruction. If the STP instruction is executed, the in-
ternal clock φ stops at an “H” level, and main clock, ring oscillator
and sub-clock oscillators stop.
In this time, “0116” is set to timer 1 and the ring oscillator is con-
nected forcibly for the system clock and the timer 1 count source.
Also, the bits of the timer 123 mode register except bit 4 are
cleared to “0”.
When an external interrupt is received, the clock oscillated before
stop mode and the ring oscillator start oscillating.
However, bit 3 of CPUM is set to “1” forcibly and system returns to
the ring oscillator mode.
Tthe internal clock φ is supplied to the CPU after timer 1
underflows. However, when the system clock is switched from the
ring oscillator to main clock and sub-clock, generate the wait time
enough for oscillation stabilizing by program.
(2) Wait mode
If the WIT instruction is executed, only the internal clock φ stops at
an “H” level. The states of main clock, ring oscillator and sub-clock
are the same as the state before the executing the WIT instruction
and the oscillation does not stop. Since the internal clock φ re-
starts when an interrupt is received, the instruction is executed im-
mediately.
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