Renesas R8C/20

R8C/20 Group, R8C/21 Group 10. Clock Generation Circuit
Rev.2.00 Aug 27, 2008 Page 64 of 458
REJ09B0250-0200
10. Clock Generation Circuit
The clock generation circuit has:
• XIN clock oscillation circuit
• Low-speed on-chip oscillator
• High-speed on-chip oscillator
Table 10.1 lists Specifications of Clock Generation Circuit. Figure 10.1 shows a Clock Generation Circuit. Figures
10.2 to 10.7 show clock-associated registers.
NOTES:
1. These pins can be used as P4_6 and P4_7 when using the on-chip oscillator clock as the CPU
clock while the XIN clock oscillation circuit is not used.
2. Set the CM05 bit in the CM0 register to 1 (main clock stops) and the CM13 bit in the CM1 register to
1 (XIN-XOUT pin) when the external clock is input.
3. The clock frequency is automatically set to up to 20 MHz by a driver when using the high-speed on-
chip oscillator as the CPU clock source.
Table 10.1 Specifications of Clock Generation Circuit
Item
XIN Clock
Oscillation Circuit
On-Chip Oscillator
High-Speed On-Chip Oscillator Low-Speed On-Chip Oscillator
Use of Clock • CPU clock source
• Peripheral
function clock
source
• CPU clock source
• Peripheral function clock
source
• CPU and peripheral function
clock sources when XIN clock
stops oscillating
• CPU clock source
• Peripheral function clock
source
• CPU and peripheral function
clock sources when XIN clock
stops oscillating
Clock Frequency 0 to 20 MHz
Approx. 40 MHz
(3)
Approx. 125 kHz
Connectable
Oscillator
•Ceramic
resonator
• Crystal oscillator
−−
Oscillator
Connect Pins
XIN, XOUT
(1)
−
(1)
−
(1)
Oscillation Stop,
Restart Function
Usable Usable Usable
Oscillator Status
After Reset
Stop Stop Oscillate
Others Externally
generated clock
can be input
(2)
−−