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M16C/5L Group, M16C/56 Group
28.5
Notes on Clock Generator
28.5.1
Oscillator Using a Crystal or a Ceramic Resonator
To connect a crystal/ceramic resonator follow the instructions below:
•
The oscillation characteristics are tied closely to the user's board design. Perform a careful
evaluation of the board before connecting an oscillator.
•
Oscillator structure depends on a crystal/ceramic resonator. The M16C/5L Group, M16C/56 Group
MCUs contain a feedback resistor, but an additional external feedback resistor may be required.
Contact the manufacturer of crystal/ceramic resonator regarding circuit constants, as they are
dependent on the a crystal/ceramic resonator or stray capacitance of the mounted circuit.
•
Check output from the CLKOUT pin to confirm that the clock generated by the oscillator is properly
transmitted to the MCU.
The procedure for outputting a clock from the CLKOUT pin is listed below. Set the clock output from the
CLKOUT pin to 25 MHz or lower.
Outputting the main clock
(1) Set the PRC0 bit in the PRCR register to 1 (write enabled).
(2) Set the CM11 bit in the CM1 register, the CM07 bit in the CM0 register, and the CM21 bit in the
CM2 register all to 0 (main clock selected).
(3) Select the clock output from the CLKOUT pin (see the table below).
(4) Set the PRC0 bit in the PRCR register to 0 (write disabled).
Table 28.3
Output from CLKOUT Pin When Selecting Main Clock
PCLKR register
PCLK5 bit
1
0
0
Outputting the sub clock
(1) Set the PRC0 bit in the PRCR register to 1 (write enabled).
(2) Set the CM07 bit in the CM0 register to 1 (sub clock selected).
(3) Set the PCLK5 bit in the PCLKR register to 0, and bits CM01 to CM00 in the CM0 register to 01b
(fC output from CLKOUT pin).
(4) Set the PRC0 bit in the PRCR register to 0 (write disabled).
Figure 28.3
Oscillator Example
R01UH0127EJ0110 Rev.1.10
Sep 01, 2011
Bit Setting
CM0 register
Bits CM01 to CM00
00b
10b
11b
MCU
XIN
XOUT
VSS
Output from the CLKOUT Pin
Clock with the same frequency as the main clock
Main clock divided by 8
Main clock divided by 32
CIN
Ceramic resonator
Rf
or crystal
feedback
resistor
Rd
COUT
damping resistor
28. Usage Notes
Page 763 of 803
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