ST72371 datasheet(15/94 Pages) STMICROELECTRONICS

15 / 94 page

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ST72371/ST72372

3 CLOCKS, RESET, INTERRUPTS & POWER SAVING MODES

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).

The external Oscillator clock is first divided by 3,

and an additional division factor of 2 can be ap-

plied if Slow Mode is selected by resetting the

SMS bit in the Miscellaneous Register. This reduc-

es the frequency of the fCPU; the clock signal 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 crystal resona-

tor in the frequency range specified for fosc.The

circuit shown in Figure 8 is recommended when

using a crystal, and Table 5 lists the recommend-

ed capacitance and feedback resistance values.

The crystal and associated components should be

mounted as close as possible to the input pins in

order to minimize output distortion and start-up

stabilisation time.

Use of an external CMOS oscillator is recom-

mended when crystals outside the specified fre-

quency ranges are to be used.

Table 5

. Recommended Crystal Values

Legend:

CL1,CL2 = Maximum total capacitance on pins

OSCIN and OSCOUT (the value includes the ex-

ternal capacitance tied to the pin plus the parasitic

capacitance of the board and of the device).

RSMAX = Maximum series parasitic resistance of

the quartz allowed.

Note: The tables relate to the quartz crystal only

(not ceramic resonator).

3.1.2 External Clock

An external clock may be applied to the OSCIN in-

put with the OSCOUT pin not connected. The tOX-

OV specifications does not apply when using an

external clock input. The equivalent specification

of the external clock source should be used in-

stead of tOXOV (see Electrical Characteristics).

Figure 8. Crystal/Ceramic Resonator

Figure 9. Clock Prescaler Block Diagram

24 Mhz

Unit

RSMAX

Ohms

CL1

CL2

OSCIN

OSCOUT

COSCIN

COSCOUT

OSCIN

OSCOUT

COSCIN

COSCOUT

% 2

CPUCLK

to CPU and

Peripherals

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