R8C/18 Group, R8C/19 Group
10. Clock Generation Circuit
Rev.1.30
Apr 14, 2006
Page 61 of 233
REJ09B0222-0130
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
10.6.4 High-Speed On-Chip
Oscillator Clock
for details.
