ST7294 datasheet(10/69 Pages) STMICROELECTRONICS

10 / 69 page

ST7294

10/69

3 CLOCKS, RESET, INTERRUPTS & POWER SAVING MODES

3.1 CLOCK SYSTEM

3.1.1 General Description

The MCU accepts either a Crystal/Ceramic reso-

nator or an external clock to provide the internal

oscillator. The internal clock (CPU Clock) is de-

rived by a divide-by-2 from the external oscillator

frequency (f

OSC).

The slow mode function allows under software

control to further slow down the internal clock,

thus reducing power consumption. This feature is

particular useful in WAIT mode.

The slow mode is entered by setting the SM bit in

the Miscellaneous Register (0Ch). This mode af-

fects all functions, including timer and EEPROM.

The slow mode is exited by clearing SM or by en-

tering the HALT mode.

3.1.2 Crystal

The internal oscillator is designed to interface with

an AT-cut parallel resonant quartz crystal resona-

tor in the frequency range specified for fosc. The

circuit shown on Figure 8 is recommended when

using a crystal. The table lists the recommended

capacitance and feedback resistance values.

Figure 6. External Clock Source Connections

OSCin

OSCout

EXTERNAL

CLOCK

VR02047A

Use of an external CMOS oscillator is recom-

mended when crystals outside the specified rang-

es are to be used.

The crystal and components should be mounted

as close as possible to the input pins to minimize

output distortion and start-up stabilization time.

Figure 7. Equivalent Crystal Circuit

Figure 8. Crystal/Ceramic Resonator

OSCin

OSCout

VR02047B

VR02047C

OSCin

OSCout

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