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SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER
7531 Group
MITSUBISHI MICROCOMPUTERS
30
Clock Generating Circuit
An oscillation circuit can be formed by connecting a resonator be-
tween X
IN
and X
OUT
, and an RC oscillation circuit can be formed by
connecting a resistor and a capacitor.
Use the circuit constants in accordance with the resonator
manufacturer's recommended values. No external resistor is needed
between X
IN
and X
OUT
since a feed-back resistor exists on-chip.
Set the constants of the resistor and capacitor when an RC oscillator
is used, so that a frequency variation due to LSI variation and resis-
tor and capacitor variations may not exceed the standard input fre-
quency.
G
Oscillation control
Stop mode
When the STP instruction is executed, the internal clock f stops at
an “H” level and the X
IN
oscillator stops. At this time, timer 1 is set
to “01
16
” and prescaler 12 is set to “FF
16
” when the oscillation sta-
bilization time set bit after release of the STP instruction is “0”. On
the other hand, timer 1 and prescaler 12 are not set when the
above bit is “1”. Accordingly, set the wait time fit for the oscillation
stabilization time of the oscillator to be used. f(X
IN
)/16 is forcibly
connected to the input of prescaler 12. When an external interrupt
is accepted, oscillation is restarted but the internal clock f remains
at “H” until timer 1 underflows. As soon as timer 1 underflows, the
internal clock f is supplied. This is because when a ceramic oscilla-
tor is used, some time is required until a start of oscillation. In case
oscillation is restarted by reset, no wait time is generated. So apply
an “L” level to the RESET pin while oscillation becomes stable.
Wait mode
If the WIT instruction is executed, the internal clock f stops at an “H”
level, but the oscillator does not stop. The internal clock restarts if
a reset occurs 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 interrupts will be received to
release the STP or WIT state, interrupt enable bits must be set to
“1” before the STP or WIT instruction is executed.
When the STP status is released, prescaler 12 and timer 1 will start
counting clock which is X
IN
divided by 16, so set the timer 1 inter-
rupt enable bit to “0” before the STP instruction is executed.
Note
For use with the oscillation stabilization set bit after release of the
STP instruction set to “1”, set values in timer 1 and prescaler
12 after fully appreciating the oscillation stabilization time of the
oscillator to be used.
G
Switch of ceramic and RC oscillations
After releasing reset the operation starts by starting a built-in ring
oscillator. Then, a ceramic oscillation or an RC oscillation is selected
by setting bit 5 of the CPU mode register.
The bit 5 can be rewritten only once after releasing reset. However,
after rewriting it is disable to write any value to the bit.
G
Double-speed mode
When a ceramic oscillation is selected, a double-speed mode can be
used. Do not use it when an RC oscillation is selected.