7643 datasheet(60/121 Pages) RENESAS

60 / 121 page

Rev.2.00

Aug 28, 2006

page 60 of 119

7643 Group

REJ03B0054-0200

CLOCK GENERATING CIRCUIT

The 7643 group has two built-in oscillation circuits. An oscillation

circuit can be formed by connecting a resonator between XIN and

XOUT (XCIN and XCOUT). Use the circuit constants in accordance

with the resonator manufacturer’s recommended values. No exter-

nal resistor is needed between XIN and XOUT since a feed-back

resistor exists on-chip. (An external feed-back resistor may be

needed depending on conditions.) However, an external feed-back

resistor is needed between XCIN and XCOUT.

When using an external clock, input the clocks to the XIN or XCIN

pin and leave the XOUT or XCOUT pin open.

Immediately after power on, only the XIN oscillation circuit starts

oscillating, and XCIN and XCOUT pins function as I/O ports.

Frequency Control

The internal system clock can be selected among fSYN, f(XIN),

f(XIN)/2, and f(XCIN). The internal clock

φ is half the frequency of

internal system clock.

(1) fSYN clock

This is made by the frequency synthesizer. f(XIN) or f(XCIN) can be

selected as its input clock. See also section “FREQUENCY SYN-

THESIZER”.

(2) f(XIN) clock

The frequency of internal system clock is the frequency of XIN pin.

(3) f(XIN)/2 clock

The frequency of internal system clock is half the frequency of XIN

pin.

(4) f(XCIN) clock

The frequency of internal system clock is the frequency of XCIN

pin.

sNote

If you switch the oscillation between XIN - XOUT and XCIN - XCOUT,

stabilize both XIN and XCIN oscillations. The sufficient time is re-

quired for the XCIN oscillation to stabilize, especially immediately

after power on and at returning from the stop mode.

Fig. 56 Ceramic resonator or quartz-crystal oscillator exter-

nal circuit

Fig. 57 External clock input circuit

XCIN

XCOUT

XIN

XOUT

CIN

COUT

CCIN

CCOUT

Notes : Insert a damping resistor if required. The

resistance will vary depending on the oscillator and

the oscillation drive capacity setting. Use the value

recommended by the maker of the oscillator. Also,

if the oscillator manufacturer's data sheet specifies

that a feedback resistor be added external to the

chip though a feedback resistor exists on-chip,

insert a feedback resistor between XIN and XOUT

following the instruction.

Rd (Note)

VCC

VSS

XCIN

XCOUT

XIN

XOUT

Open

External oscillation

circuit

External oscillation

circuit

VCC

VSS

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