MITSUBISHI MICROCOMPUTERS
3822 Group
SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER
CLOCK GENERATING CIRCUIT
Oscillation Control
The 3822 group has two built-in oscillation circuits. An oscillation
circuit can be formed by connecting a resonator between XIN and
XOUT (XCIN and XCOUT). Use the circuit constants in accordance
with the resonator manufacturer's recommended values. No exter-
nal resistor is needed between XIN and XOUT since a feed-back
resistor exists on-chip. However, an external feed-back resistor is
needed between XCIN and XCOUT.
(1) Stop Mode
If the STP instruction is executed, the internal clock φ stops at an
“H” level, and XIN and XCIN oscillators stop. Timer 1 is set to
“FF16” and timer 2 is set to “0116”.
Either XIN or XCIN divided by 16 is input to timer 1 as count
source, and the output of timer 1 is connected to timer 2. The bits
of the timer 123 mode register except bit 4 are cleared to “0”. Set
the timer 1 and timer 2 interrupt enable bits to disabled (“0”) be-
fore executing the STP instruction. Oscillator restarts at reset or
when an external interrupt is received, but the internal clock φ is
not supplied to the CPU until timer 2 underflows. This allows timer
for the clock circuit oscillation to stabilize.
To supply a clock signal externally, input it to the XIN pin and make
the XOUT pin open. The sub-clock XCIN-XCOUT oscillation circuit
cannot directly input clocks that are externally generated. Accord-
ingly, be sure to cause an external resonator to oscillate.
Immediately after poweron, only the XIN oscillation circuit starts
oscillating, and XCIN and XCOUT pins function as I/O ports.
(2) Wait Mode
Frequency Control
If the WIT instruction is executed, the internal clock φ stops at an
“H” level. The states of XIN and XCIN are the same as the state be-
fore the executing the WIT instruction. The internal clock restarts
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.
(1) Middle-speed Mode
The internal clock φ is the frequency of XIN divided by 8.
After reset, this mode is selected.
(2) High-speed Mode
The internal clock φ is half the frequency of XIN.
(3) Low-speed Mode
ꢀThe internal clock φ is half the frequency of XCIN.
ꢀA low-power consumption operation can be realized by stopping
the main clock XIN in this mode. To stop the main clock, set bit 5
of the CPU mode register to “1”.
X
CIN
X
COUT
X
IN
XOUT
When the main clock XIN is restarted, set enough time for oscil-
lation to stabilize by programming.
Rf
Rd
Note: If you switch the mode between middle/high-speed and low-
speed, stabilize both XIN and XCIN oscillations. The
sufficient time is required for the sub-clock to stabilize, es-
pecially immediately after poweron and at returning from
stop mode. When switching the mode between middle/high-
speed and low-speed, set the frequency on condition that
f(XIN) > 3f(XCIN).
C
OUT
C
CIN
C
COUT
CIN
Fig. 42 Ceramic resonator circuit
XCOUT
XCIN
XIN
XOUT
Rf
Open
Rd
External oscillation circuit
CCIN
CCOUT
VCC
VSS
Fig. 43 External clock input circuit
43
MITSUBISHI [ Mitsubishi Group ]
