ST72631K4B0 datasheet(17/109 Pages) STMICROELECTRONICS

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3 CLOCKS AND RESET

3.1 CLOCK SYSTEM

3.1.1 General Description

The MCU accepts either a Crystal or Ceramic res-

onator, or an external clock signal to drive the in-

ternal oscillator. The internal clock (fCPU) is de-

rived from the external oscillator frequency (fOSC),

which is divided by 3 (and by 2 or 4 for USB, de-

pending on the external clock used).

By setting the CLKDIV bit in the Miscellaneous

ing an internal frequency of 4 MHz while maintain-

ing a 6 MHz for USB (refer to Figure 10).

The internal clock signal (fCPU) is also routed to

the on-chip peripherals. The CPU clock signal

consists of a square wave with a duty cycle of

50%.

The internal oscillator is designed to operate with

an AT-cut parallel resonant quartz or ceramic res-

onator in the frequency range specified for fosc.

The circuit shown in Figure 9 is recommended

when using a crystal, and Table 5 Recommended

Values for 24 MHz Crystal Resonator lists the rec-

ommended capacitance. The crystal and associat-

ed components should be mounted as close as

possible to the input pins in order to minimize out-

put distortion and start-up stabilisation time.

Table 5. Recommended Values for 24 MHz

Crystal Resonator

Note: RSMAX is the equivalent serial resistor of the

crystal (see crystal specification).

3.1.2 External Clock

An external clock may be applied to the OSCIN in-

put with the OSCOUT pin not connected, as

shown on Figure 8. The tOXOV specifications does

not apply when using an external clock input. The

equivalent specification of the external clock

source should be used instead of tOXOV (see Sec-

tion 6.5 CONTROL TIMING).

Figure 8. External Clock Source Connections

Figure 9. Crystal/Ceramic Resonator

Figure 10. Clock block diagram

RSMAX

Ω

COSCIN

56pF

47pF

22pF

COSCOUT

56pF

47pF

22pF

1-10 M

Ω

1-10 M

Ω

1-10 M

Ω

OSCIN

OSCOUT

EXTERNAL

CLOCK

OSCIN

OSCOUT

COSCIN

COSCOUT

CPU and

8 or 4 MHz

6 MHz (USB)

24 or

peripherals)

CLKDIV

12 MHz

Crystal

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