
8. Clock Generation Circuits
8.1.2
Sub Clock
Sub clock oscillation circuit generates the sub clock. The sub clock is used for the clock source for the CPU
clock and the count source for timer A and timer B. The same frequency, fC, as the sub clock can be output
from the CLKOUT pin.
The sub clock oscillation circuit is configured by connecting a crystal oscillator between pins XCIN and
XCOUT. 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 sub clock oscillation circuit may also be
configured by feeding an externally generated clock to the XCIN pin.
Figure 8.11 shows examples of sub clock
circuit connection. Circuit constants vary depending on each oscillator. Use the circuit constant recommended
by each oscillator manufacturer.
The sub clock stops after reset. The feedback resistor is disconnected from the oscillation circuit. To start
oscillating the sub clock oscillation circuit, set both bits PD8_7 and PD8_6 in the PD8 register to 0 (input
mode), the PU25 bit in the PUR2 register to 0 (no pull-up), and then the CM04 bit in the CM0 register to 1
(XCIN-XCOUT oscillation function). To input the external clock to the XCIN pin, set the PD8_7 bit to 0, the
PU25 bit to 0, and the CM04 bit to 1. A clock input to the XCIN pin becomes the sub clock.
When the CM07 bit in the CM0 register is set to 1 (sub clock) after the sub clock oscillation stabilizes, the sub
clock becomes the CPU clock source.
Figure 8.11
Sub Clock Circuit Connection
XCIN
XCOUT
Oscillator
CCIN
CCOUT
RCd(1)
MCU
(On-chip feedback resistor)
XCIN
XCOUT
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 XCIN and XCOUT following the instructions.