
8. Clock Generation Circuits
8.1.1
Main Clock
Main clock oscillation circuit generates the main clock. The main clock becomes the clock source for the CPU
clock and peripheral function clock.
The main clock oscillation circuit is configured by connecting an oscillator or resonator between the XIN and
XOUT pins. The circuit has an on-chip feedback resistor. The feedback resistor is disconnected from the
oscillation circuit in stop mode to reduce power consumption. The main clock oscillation circuit may also be
configured by feeding an externally generated clock to the XIN pin.
Figure 8.10 shows examples of main clock
circuit connection. Circuit constants vary depending on each oscillator. Use the circuit constant recommended
by each oscillator manufacturer.
The main clock divided-by-eight becomes the CPU clock source after reset.
To reduce power consumption, set the CM05 bit in the CM0 register to 1 (main clock stopped) after the sub
clock or on-chip oscillator clock is selected as the CPU clock source. In this case, the XOUT pin becomes high
(“H”). The XIN pin is pulled up by the XOUT pin via the feedback resistor which remains on. When an external
clock is input to the XIN pin, do not set the CM05 bit to 1.
Figure 8.10
Main Clock Circuit Connection
XIN
XOUT
Oscillator
CIN
COUT
Rd(1)
MCU
(On-chip feedback resistor)
XIN
XOUT
MCU
(On-chip feedback resistor)
Open
External clock
VCC
VSS
NOTE:
1. Insert a damping resistor if required. Resistance values vary depending on the oscillator setting. Use the resistance values
recommended by the oscillator manufacturer.
If the oscillator manufacturer recommends that a feedback resistor be added to the chip externally, insert a feedback
resistor between XIN and XOUT following the instructions.