ST72E121 datasheet(15/92 Pages) STMICROELECTRONICS

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ST72E121 ST72T121

3 CLOCKS, RESET, INTERRUPTS & POWER SAVING MODES

3.1 CLOCK SYSTEM

3.1.1 General Description

The MCU accepts either a crystal or ceramic reso-

nator, or an external clock signal to drive the inter-

nal oscillator. The internal clock (fCPU) is derived

from the external oscillator frequency (fOSC). The

external Oscillator clock is first divided by 2, and

an additional division factor of 2, 4, 8, or 16 can be

applied, in Slow Mode, to reduce the frequency of

the fCPU; this 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 9 is recommended when

using a crystal, and Table 4 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.

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.

Table 4 Recommended Values for 16 MHz

Crystal Resonator (C0 < 7pF)

RSMAX: Parasitic series resistance of the quartz

crystal (upper limit).

C0: Parasitic shunt capacitance of the quartz crys-

tal (upper limit 7pF).

COSCOUT,COSCIN: Maximum total capacitance on

pins OSCIN and OSCOUT (the value includes the

external capacitance tied to the pin plus the para-

sitic capacitance of the board and of the device).

Figure 8. External Clock Source Connections

Figure 9. Crystal/Ceramic Resonator

Figure 10. 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

% 2,4,8, 16

fCPU

to CPU and

Peripherals

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