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M16C/5L Group, M16C/56 Group
8.3.6
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.
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.
Figure 8.4 shows Sub Clock Connection Example.
Note:
1. Insert a damping resistor if required. Resistance values vary depending on a crystal/ceramic resonator and the drive level. Use the
resistance values recommended by the manufacturer of crystal/ceramic resonator.
When setting the oscillation drive level to low, check if the oscillation stabilizes. If the manufacturer recommends to place a feedback
resistor on the chip externally, insert a feedback resistor between the XCIN and XCOUT pins following the instructions.
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 to 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.
R01UH0127EJ0110 Rev.1.10
Sep 01, 2011
MCU
(On-chip feedback resistor)
XCIN
Oscillator
XCOUT
(1)
RCd
VSS
CCIN
CCOUT
8. Clock Generator
Page 107 of 803
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