R5F3640MNFB datasheet(103/398 Pages) RENESAS

103 / 398 page

REJ09B0392-0064 Rev.0.64

Oct 12, 2007

Page 82 of 373

Under development

Preliminary Specification

Specification in this preliminary version is subject to change.

M16C/64 Group

10. Clock Generation Circuit

10.1.2

Sub Clock

The sub clock is generated by the sub clock oscillation circuit. This clock is used as the clock source for

the CPU clock, as well as the timer A and timer B count sources. In addition, an fC clock with the same

frequency as that of the sub clock can be output from the CLKOUT pin.

The sub clock oscillation circuit is configured by connecting a crystal resonator between pins XCIN and

XCOUT. The sub clock oscillation circuit contains a feedback resistor, which is disconnected from the

oscillation circuit during stop mode in order to reduce the amount of power consumed in the chip. The

sub clock oscillation circuit may also be configured by feeding an externally generated clock to the

XCIN pin.

Figure 10.8 shows the Examples of Sub Clock Connection Circuit.

After reset, the sub clock is turned off. At this time, the feedback resistor is disconnected from the oscil-

lation circuit.

To use the sub clock for the CPU clock, set the CM07 bit in the CM0 register to 1 (sub clock) after the

sub clock becomes oscillating stably.

During stop mode, all clocks including the sub clock are turned off. Refer to 10.4 “Power Control” for

details.

Figure 10.8

Examples of Sub Clock Connection Circuit

NOTE :

1. Place a damping resistor if required. The resistance will vary depending on the

oscillator and the oscillation drive capacity setting. Use the value recommended by

each oscillator manufacturer.

When the oscillation drive capacity is set to low, check if oscillation is stable at low.

Also, place a feedback resistor between XCIN and XCOUT if the oscillator

manufacturer recommends placing the resistor externally.

External clock

Open

VCC1

VSS

XCIN

XCOUT

RCd (1)

CCIN

CCOUT

VSS

Oscillator

Microcomputer

(Built-in feedback resistor)

XCIN

XCOUT

Microcomputer

(Built-in feedback resistor)

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