ST72101G1M1 datasheet(16/85 Pages) STMICROELECTRONICS

16 / 85 page

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ST72101/ST72212/ST72213

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 2,

and division factor of 32 can be applied if Slow

Mode is selected by setting the SMS bit in the Mis-

cellaneous Register. This reduces the frequency

of the fCPU; the clock signal is also routed to the

on-chip peripherals.

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 13 is recommended when

using a crystal, and Table 6 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 6. Recommended Values for 16 MHz

Crystal Resonator (C0<7pF)

C0: parasitic shunt capacitance of the quartz crys-

tal.

RSMAX: equivalent serial resistor of the crystal (up-

er limit, see crystal specification).

COSCOUT, COSCIN: maximum total capacitance on

OSCIN and OSCOUT, including the external ca-

pacitance plus the parasitic capacitance of the

board and the device.

Figure 12. External Clock Source Connections

Figure 13. Crystal/Ceramic Resonator

Figure 14. Clock Prescaler Block Diagram

RSMAX

Ω

150

Ω

COSCIN

56pF

47pF

22pF

COSCOUT

56pF

47pF

22pF

OSCIN

OSCOUT

EXTERNAL

CLOCK

OSCIN

OSCOUT

COSCIN

COSCOUT

OSCIN

OSCOUT

COSCIN

COSCOUT

% 16

fCPU

to CPU and

Peripherals

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