M32CSLASH8A datasheet(97/372 Pages) RENESAS

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M32C/8A Group

9. Clock Generation Circuits

Rev.1.00

Jul 15, 2007

Page 80 of 352

REJ09B0385-0100

9.1.3

On-Chip Oscillator Clock

On-chip oscillator generates the 1-MHz on-chip oscillator clock. The on-chip oscillator clock is used as the

clock source for the CPU clock and peripheral function clocks.

The on-chip oscillator clock is stopped after reset. When the CM21 bit in the CM2 register is set to 1 (on-chip

oscillator clock), the on-chip oscillator starts oscillating and becomes the clock source for the CPU clock and

peripheral function clocks in place of the main clock.

Table 9.2 lists on-chip oscillator start conditions.

Table 9.2

On-Chip Oscillator Start Condition

9.1.3.1

Oscillation Stop Detect Function

When the main clock is terminated running by an external factor, the on-chip oscillator automatically starts

oscillating.

When the CM 20 bit in the CM2 register is set to 1 (oscillation stop detect function used), an oscillation stop

detection interrupt request is generated as soon as the main clock is lost. Simultaneously, the on-chip oscillator

starts oscillating. The on-chip oscillator clock takes the place of the main clock as the clock source for the CPU

clock and peripheral function clocks. Associated bits in the CM2 register are changed as follows:

• CM21 bit becomes 1 (on-chip oscillator clock becomes the CPU clock)

• CM22 bit becomes 1 (loss of main clock stop is detected)

• CM23 bit becomes 1 (main clock stops)

The oscillation stop detection interrupt shares the vector with the watchdog timer interrupt and the Vdet4

detection interrupt. When these interrupts are used simultaneously, verify the CM22 bit within an interrupt

routine to determine if an oscillation stop detection interrupt request has been generated.

When the main clock resumes its operation after a loss of the main clock is detected, the main clock can be

selected as the clock source for the CPU clock and peripheral function clocks by program. Figure 9.11 shows

the procedure to switch the clock source from the on-chip oscillator clock to the main clock.

In low-speed mode, when the main clock is lost while the CM20 bit is set to 1, an oscillation stop detection

interrupt request is generated, and the on-chip oscillator starts oscillating. The sub clock remains as the source

for the CPU clock. The on-chip oscillator clock becomes the source for the peripheral function clocks.

When the peripheral function clocks are stopped, the oscillation stop detect function cannot be used. To enter

wait mode while using the oscillation stop detect function, set the CM02 bit in the CM0 register to 0 (peripheral

clocks do not stop in wait mode).

The oscillation stop detect function is a precaution against the unintended termination of the main clock by an

external factor. Set the CM20 bit to 0 (oscillation stop detect function not used) when the main clock is stopped

by program, i.e., entering stop mode or setting the CM05 bit in the CM0 register to 1 (main clock stops).

When the main clock frequency is 2 MHz or lower, the oscillation stop detect function is not available. In this

case, set the CM20 bit to 0.

CM2 Register

PM2 Register

Applications

CM21

PM22

PM27, PM26

00b

Clock source for the CPU clock and peripheral function clock

00b

Count source for the watchdog timer

(The clock keeps running in stop mode.)

10b

Clock source for f2n

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