38K2 datasheet(116/358 Pages) RENESAS

116 / 358 page

FUNCTIONAL DESCRIPTION

Rev.2.00

Oct 15, 2006

page 97 of 130

38K2 Group

REJ09B0338-0200

HARDWARE

CLOCK GENERATING CIRCUIT

An oscillation circuit can be formed by connecting a resonator be-

tween XIN and XOUT. Use the circuit constants in accordance with

the resonator manufacturer’s recommended values. No external

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

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

depending on conditions.)

Frequency Control

Either fSYN or f(XIN) can be selected as an internal system clock.

Furthermore, the frequency of internal clock

φ can be selected by

the system clock division ratio selection bit.

(1) fSYN clock

fSYN clock is generated by the PLL circuit. f(XIN) or fVCO can be

selected as an input clock. When using as an internal system

clock, there is restriction on use. Refer to the clause of “PLL CIR-

CUIT”.

(2) f(XIN) clock

The frequency applied to the XIN pin is used as an internal system

clock frequency.

Oscillation Control

(1) Stop mode

If the STP instruction is executed, the internal clock

φ stops at an

“H” level, and the XIN oscillator stops. When the oscillation stabi-

lizing time set after STP instruction released bit is “0,” the

prescaler 12 is set to “FF16” and timer 1 is set to “0116.” When the

oscillation stabilizing time set after STP instruction released bit is

“1,” set the sufficient time for oscillation of used oscillator to stabi-

lize since nothing is set to the prescaler 12 and timer 1. XIN

divided by 16 is compulsorily connected to the input of the

prescaler 12. Oscillator restarts when an external interrupt (includ-

ing USB resume interrupt) is received, but the internal clock

remains at “H” until timer 1 underflows. The internal clock

φ is not

supplied until timer 1 underflows. Because the sufficient time is re-

quired for the oscillation to stabilize when a ceramic resonator etc.

is used. When the oscillator is restarted by reset, apply “L” level to

the RESET pin until the oscillation is stable since a wait time will

not be generated automatically.

(2) Wait mode

If the WIT instruction is executed, the internal clock

φ stops at an

“H” level, but the oscillator does not stop. The internal clock

φ re-

starts at reset or when an interrupt is received. Since the oscillator

does not stop, normal operation can be started immediately after

the clock is restarted.

To ensure that the interrupts will be received to release the STP or

WIT state, their interrupt enable bits must be set to “1” before ex-

ecuting of the STP or WIT instruction.

When releasing the STP state, the prescaler 12 and timer 1 will

start counting the clock XIN divided by 16. Accordingly, set the

timer 1 interrupt enable bit to “0” before executing the STP instruc-

tion.

sNote

When using the oscillation stabilizing time set after STP instruction

released bit set to “1”, evaluate time to stabilize oscillation of the

used oscillator and set the value to the timer 1 and prescaler 12.

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