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M16C/64C Group
8.3.4
Sub Clock (fC)
The sub clock is supplied by the sub clock oscillator. This clock is the clock source for count sources of
the CPU clock, timer A, timer B, real-time clock, CEC function, and remote control signal receiver.
The sub clock oscillator is configured by connecting a crystal between pins XCIN and
XCOUT. The sub clock oscillator contains a feedback resistor, which is disconnected from the
oscillation circuit in stop mode in order to reduce the amount of power consumed by the chip. The sub
clock oscillator circuit may also be configured by feeding an externally generated clock to the XCIN pin.
Figure 8.4 shows Sub Clock Connection Example.
MCU
(Built-in feedback resistor)
XCIN
XCOUT
VSS
Note:
1.
Insert a damping resistor if required. The resistance will vary depending on a crystal/ceramic
resonator and the oscillation drive capacity setting. Use the value recommended by the
manufacturer of crystal/ceramic resonator.
When the oscillation drive capacity is set to low, check if oscillation is stable in low status. Also,
place a feedback resistor between XCIN and XCOUT if the manufacturer recommends placing
the resistor externally.
Figure 8.4
Sub Clock Connection Example
After reset, the sub clock is stopped. At this time, the feedback resistor is disconnected from the
oscillator.
Follow the steps below to start the sub clock. Refer to 8.2 "Registers" for details on register and bit
access.
(1) Set the PU21 bit in the PUR2 register to 0 (P8_4, P8_6 and P8_7 not pulled high).
(2) Set bits PD8_6 and PD8_7 in the PD8 register to 0 (P8_6, P8_7 function as input ports).
(3) Set the CM04 bit to 1 (XCIN-XCOUT oscillation function). Set the CM03 bit to 1 (XCIN-XCOUT
drive capacity high).
(4) Wait until sub clock oscillation stabilizes (enter the external clock when entering it from the XCIN
pin).
R01UH0092EJ0110 Rev.1.10
Jul 31, 2012
CCIN
Crystal
(1)
RCd
CCOUT
MCU
(Built-in feedback resistor)
XCIN
VCC1
VSS
Open
XCOUT
8. Clock Generator
External clock
Page 99 of 807
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