M38C89MB datasheet(38/55 Pages) RENESAS

38 / 55 page

SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER

MITSUBISHI MICROCOMPUTERS

38C8 Group

CLOCK GENERATING CIRCUIT

The 38C8 group has two built-in oscillation circuits: main clock XIN-

XOUT oscillation circuit and sub-clock XCIN-XCOUT oscillation circuit.

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

tween XIN and XOUT (XCIN and XCOUT). RC oscillation is available for

XIN-XOUT.

Immediately after reset is released, the XIN-XOUT oscillation circuit

starts oscillating, and XCIN and XCOUT pins go to high impedance

state.

Main Clock

An oscillation circuit by a resonator can be formed by setting the

OSCSEL pin is set to “L” level and connecting a resonator between

XIN and XOUT. Use the circuit constants in accordance with the reso-

nator manufacturer’s recommended values. No external resistor is

needed between XIN and XOUT since a feed-back resistor exists on-

chip. To supply a clock signal externally, make the XOUT pin open in

the “L” level state of the OSCSEL pin, and supply the clock from the

XIN pin. The RC oscillation circuit can be formed by setting the

OSCSEL pin to “H” level and connecting a resistor between the XIN

pin and the XOUT pin. At this time, the feed-back resistor is cut off.

The frequency of the RC oscillation changes owing to a parasitic

capacitance or the wiring length etc. of the printed circuit board. Do

not use the RC oscillation in the usage which the frequency accuracy

of the main clock is needed.

Sub-clock

Connect a resonator between XCIN and XCOUT. An external feed-

back resistor is needed between XCIN and XCOUT since a feed-back

resistor does not exist on-chip. The sub-clock XCIN-XCOUT oscillation

circuit cannot directly input clocks that are externally generated. Ac-

cordingly, be sure to cause an external resonator to oscillate.

Frequency Control

(1) Middle-speed Mode

The internal clock

φ is the frequency of XIN divided by 8. After reset is

released, this mode is selected.

(2) High-speed Mode

The internal clock

φ is the frequency of XIN divided by 2.

(3) Low-speed Mode

The internal clock

φ is the frequency of XCIN divided by 2.

A low-power consumption operation can be realized by stopping the

main clock XIN in this mode. To stop the main clock, set bit 5 of the

CPU mode register to “1”. When the main clock XIN is restarted, set

enough time for oscillation to stabilize by programming.

sNotes on clock generating circuit

If you switch the mode between middle/high-speed and low-speed,

stabilize both XIN and XCIN oscillations. The sufficient time is required

for the sub-clock to stabilize, especially immediately after power on

and at returning from stop mode. When switching the mode between

middle/high-speed and low-speed, set the frequency on condition

that f(XIN) > 3•f(XCIN).

Oscillation Control

(1) Stop Mode

If the STP instruction is executed, the internal clock

φ stops at an “H”

level, and XIN and XCIN oscillators stop. Timer 1 is set to “FF16” and

timer 2 is set to “0116.”

Either XIN divided by 16 or XCIN divided by 16 is input to timer 1 as

count source, and the output of timer 1 is connected to timer 2. The

bits except bit 4 of the timer 123 mode register are cleared to “0.” Set

the interrupt enable bits of timer 1 and timer 2 to disabled (“0”) before

executing the STP instruction.

Oscillator restarts at reset or when an external interrupt is received,

but the internal clock

φ is not supplied to the CPU until timer 2

underflows. This allows time for the clock circuit oscillation to stabi-

lize.

(2) Wait Mode

If the WIT instruction is executed, the internal clock

φ stops at an “H”

level. The states of XIN and XCIN are the same as the state before

executing the WIT instruction. The internal clock

φ restarts at reset

or when an interrupt is received. Since the oscillator does not stop,

normal operation can be started immediately after the clock is re-

started.

Fig. 36 RC oscillation circuit

Fig. 37 Resonator circuit

CCIN

XCIN

XCOUT OSCSEL

XIN

XOUT

Rosc

XCIN XCOUT

OSCSELXIN

XOUT

CIN

COUT

CCIN

CCOUT

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