R8C20 datasheet(89/501 Pages) RENESAS

89 / 501 page

R8C/20 Group, R8C/21 Group

10. Clock Generation Circuit

Rev.2.00 Aug 27, 2008

Page 73 of 458

REJ09B0250-0200

10.3

CPU Clock and Peripheral Function Clock

There are two type clocks: a CPU clock to operate the CPU and a peripheral function clock to operate the

peripheral functions. Refer to Figure 10.1 Clock Generation Circuit.

10.3.1

System Clock

The system clock is a clock source for the CPU and peripheral function clocks. The XIN clock or on-chip

oscillator clock can be selected.

10.3.2

CPU Clock

The CPU clock is an operating clock for the CPU and watchdog timer.

The system clock can be the divide-by-1 (no division), 2, 4, 8 or 16 to produce the CPU clock. Use the CM06

bit in the CM0 register and the CM16 to CM17 bits in the CM1 register to select the value of the division.

After reset, the low-speed on-chip oscillator clock divided-by-8 provides the CPU clock.

When entering stop mode, the CM06 bit is set to 1 (divide-by-8 mode).

10.3.3

Peripheral Function Clock (f1, f2, f4, f8 and f32)

The peripheral function clock is operating clock for the peripheral functions.

The clock fi (i = 1, 2, 4, 8, 32) is generated by the system clock divided-by-i. The clock fi is used for timers

RA, RB, RD, RE, serial interface and A/D converter.

When the WAIT instruction is executed after setting the CM02 bit in the CM0 register to 1 (peripheral function

clock stops in wait mode), the clock fi stop.

10.3.4

fOCO

fOCO is operating clocks for the peripheral functions.

The fOCO run at the same frequency as the on-chip oscillator clock and can be used as the source for the timer

RA.

When the WAIT instruction is executed, the clocks fOCO does not stop.

10.3.5

fOCO40M

fOCO40M is used as the count source for the timer RD. The fOCO40M is generated by the high-speed on-chip

oscillator and provided by setting the FRA00 bit to 1.

When the WAIT instruction is executed, the clock fOCO40M does not stop.

fOCO40M can be used with supply voltage VCC = 3.0 to 5.5V.

10.3.6

fOCO-F

fOCO-F is used as the count source for the AD converter. The fOCO-F is generated by the high-speed on-chip

oscillator and provided by setting the FRA00 bit to 1.

When the WAIT instruction is executed, the clock fOCO-F does not stop.

10.3.7

fOCO-S

fOCO-S is an operating clock for the watchdog timer and voltage detection circuit. When setting the CM14 bit

to 0 (low-speed on-chip oscillator on) using the clock generated by the low-speed on-chip oscillator, the fOCO-

S can be provided. When the WAIT instruction is executed or in count source protect mode of the watchdog

timer, the clock fOCO-S does not stop.

10.3.8

fOCO128

fOCO128 is generated by fOCO divided-by-128. The clock fOCO128 is used for capture signal of timer RD

(channel 0).

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