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M16C/28 Group
Under development Preliminary specification
Specifications in this manual are tentative and subject to change.
7. Clock Generation Circuit
Rev.0.60 2004.02.01
page 35 of N
REJ09B0047-0060Z
b7
b6
b5
b4
b3
b2
b1
b0
RW
CM20
CM21
Oscillation stop detection register (Note 1)
Symbol
Address
After reset
CM2
000C16
0X0000102
Bit name
Function
Bit symbol
System clock select bit 2
(Notes 2, 3, 6, 8, 11, 12 )
0: Oscillation stop, re-oscillation
detection function disabled
1: Oscillation stop, re-oscillation
detection function enabled
0: Main clock or PLL clock
1: Ring oscillator clock
(Ring oscillator oscillating)
Oscillation stop, re-
oscillation detection bit
(Notes 7, 9, 10, 11)
Note 1: Write to this register after setting the PRC0 bit of PRCR register to “1” (write enable).
Note 2: When the CM20 bit is “1” (oscillation stop, re-oscillation detection function enabled), the CM27 bit is “1”
(oscillation stop, re-oscillation detection interrupt), and the CPU clock source is the main clock, the
CM21 bit is automatically set to “1” (ring oscillator clock) if the main clock stop is detected.
Note 3: If the CM20 bit is “1” and the CM23 bit is “1” (main clock not oscillating), do not set the CM21 bit to “0”.
Note 4: This flag is set to “1” when the main clock is detected to have stopped or when the main clock is
detected to have restarted oscillating. When this flag changes state from “0” to “1”, an oscillation stop,
reoscillation restart detection interrupt is generated. Use this flag in an interrupt routine to discriminate
the causes of interrupts between the oscillation stop, reoscillation detection interrupts and the watchdog
timer interrupt. The flag is cleared to “0” by writing a “0” in a program. (Writing a “1” has no effect. Nor is
it cleared to “0” by an oscillation stop or an oscillation restart detection interrupt request acknowledged.)
If when the CM22 bit = 1 an oscillation stoppage or an oscillation restart is detected, no oscillation
stop, reoscillation restart detection interrupts are generated.
Note 5: Read the CM23 bit in an oscillation stop, re-oscillation detection interrupt handling routine to determine
the main clock status.
Note 6: Effective when the CM07 bit of CM0 register is “0”.
Note 7: When the PM21 bit of PM2 register is “1” (clock modification disabled), writing to the CM20 bit has no
effect.
Note 8: When the CM20 bit is “1” (oscillation stop, re-oscillation detection function enabled), the CM27 bit is “1”
(oscillation stop, re-oscillation detection interrupt), and the CM11 bit is “1” (the CPU clock source is PLL
clock), the CM21 bit remains unchanged even when main clock stop is detected. If the CM22 bit is “0”
under these conditions, oscillation stop, re-oscillation detection interrupt occur at main clock stop
detection; it is, therefore, necessary to set the CM21 bit to “1” (ring oscillator clock) inside the interrupt
routine.
Note 9: Set the CM20 bit to “0” (disable) before entering stop mode. After exiting stop mode, set the CM20 bit
back to “1” (enable).
Note 10: Set the CM20 bit to “0” (disable) before setting the CM05 bit of CM0 register.
Note 11: The CM20, CM21 and CM27 bits do not change at oscillation stop detection reset.
Note 12: When the CM21 bit = 0 (ring oscillator turned off) and the CM05 bit = 1 (main clock turned off),
the CM06 bit is fixed to “1” (divide-by-8 mode) and the CM15 bit is fixed to “1” (drive capability High).
CM22
CM23
Oscillation stop, re-
oscillation detection flag
0: Main clock stop,or re-oscillation
not detected
1: Main clock stop,or re-oscillation
detected
0: Main clock oscillating
1: Main clock not oscillating
XIN monitor flag
(Note 4)
CM27
0: Oscillation stop detection reset
1: Oscillation stop, re-oscillation
detection interrupt
Nothing is assigned. When write, set to “0”. When read, its
content is indeterminate.
Operation select bit
(when an oscillation stop,
re-oscillation is detected)
(Note 11)
RW
RO
(b6)
(Note 5)
Reserved bit
(b5-b4)
Must set to “0”
RW
00
Figure 7.5. CM2 Register