R8C2C datasheet(128/615 Pages) RENESAS

128 / 615 page

R8C/2C Group, R8C/2D Group

10. Clock Generation Circuit

Rev.2.00

Dec 05, 2007

Page 107 of 585

REJ09B0339-0200

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 FRA01 bit in the FRA0 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, fOCO, and fOCO-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 XIN clock stops oscillating when bits OCD1 to OCD0 in the OCD register are set to 11b, 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 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, fOCO, fOCO-F, and fOCO40M.

To use the high-speed on-chip oscillator clock as the clock source for the CPU clock, peripheral clock, fOCO,

and fOCO-F, set bits FRA20 to FRA22 in the FRA2 register as follows:

• All divide ratio mode settings are supported when VCC = 3.0 V to 5.5 V 000b to 111b

• Divide ratio of 4 or more when VCC = 2.7 V to 5.5 V

010b to 111b (divide by 4 or more)

• Divide ratio of 8 or more when VCC = 2.2 V to 5.5 V

110b to 111b (divide by 8 or more)

After reset, the on-chip oscillator clock generated by the high-speed on-chip oscillator stops. Oscillation is

started by setting the FRA00 bit in the FRA0 register to 1 (high-speed on-chip oscillator on). The frequency can

be adjusted by registers FRA1 and FRA2.

Furthermore, frequency correction data corresponding to the supply voltage ranges VCC = 2.2 V to 5.5 V is

stored in FRA6 register. To use separate correction values to match this voltage ranges, transfer them from the

FRA6 register to the FRA1 register.

The frequency correction data of 36.864 MHz is stored in the FRA7 register. To set the frequency of the high-

speed on-chip oscillator to 36.864 MHz, transfer the correction value in the FRA7 register to the FRA1 register

before use.

Since there are differences in the amount of frequency adjustment among the bits in the FRA1 register, make

adjustments by changing the settings of individual bits. Adjust the FRA1 register so that the frequency of the

high-speed on-chip oscillator clock will be 40 MHz or less.

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