R8C/1A Group, R8C/1B Group 10. Clock Generation Circuit
Rev.1.30 Dec 08, 2006 Page 66 of 315
REJ09B0252-0130
10.2 On-Chip Oscillator Clocks
These clocks are supplied by the on-chip oscillators (high-speed on-chip oscillator and a low-speed on-chip
oscillator). The on-chip oscillator clock is selected by the HRA01 bit in the HRA0 register.
10.2.1 Low-Speed On-Chip Oscillator Clock
The clock generated by the low-speed on-chip oscillator is used as the clock source for the CPU clock,
peripheral function clock, fRING, fRING128, and fRING-S.
After reset, the on-chip oscillator clock generated by the low-speed on-chip oscillator divided by 8 is selected as
the CPU clock.
If the main clock stops oscillating when bits OCD1 to OCD0 in the OCD register are set to 11b (oscillation stop
detection function enabled), the low-speed on-chip oscillator automatically starts operating, supplying the
necessary clock for the MCU.
The frequency of the low-speed on-chip oscillator varies depending on the supply voltage and the operating
ambient temperature. Application products must be designed with sufficient margin to allow for the frequency
changes.
10.2.2 High-Speed On-Chip Oscillator Clock
The clock generated by the high-speed on-chip oscillator is used as the clock source for the CPU clock,
peripheral function clock, fRING, fRING128, and fRING1-fast.
After reset, the on-chip oscillator clock generated by the high-speed on-chip oscillator stops. Oscillation is
started by setting the HRA00 bit in the HRA0 register to 1 (high-speed on-chip oscillator on). The frequency
can be adjusted by registers HRA1 and HRA2.
Since there are differences in delay among the bits in the HRA1 register, make adjustments by changing the
settings of individual bits.
The high-speed on-chip oscillator frequency may be changed in flash memory CPU rewrite mode during auto-
program operation or auto-erase operation. Refer to 10.6.5 High-Speed On-Chip Oscillator Clock for details.
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