ST6208CB3 datasheet(14/70 Pages) STMICROELECTRONICS

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ST62T08C/T09C ST62T10C/T20C/E20C

3 CLOCKS, RESET, INTERRUPTS AND POWER SAVING MODES

3.1 CLOCK SYSTEM

The MCU features a Main Oscillator which can be

driven by an external clock, or used in conjunction

with an AT-cut parallel resonant crystal or a suita-

ble ceramic resonator, or with an external resistor

(RNET). In addition, a Low Frequency Auxiliary Os-

cillator (LFAO) can be switched in for security rea-

sons, to reduce power consumption, or to offer the

benefits of a back-up clock system.

The Oscillator Safeguard (OSG) option filters

spikes from the oscillator lines, provides access to

the LFAO to provide a backup oscillator in the

event of main oscillator failure and also automati-

cally limits the internal clock frequency (fINT) as a

function of VDD, in order to guarantee correct oper-

ation. These functions are illustrated in Figure 9,

Figure 10, Figure 11 and Figure 12.

Figure 8 illustrates various possible oscillator con-

figurations using an external crystal or ceramic res-

onator, an external clock input, an external resistor

(RNET), or the lowest cost solution using only the

LFAO. CL1 an CL2 should have a capacitance in the

range 12 to 22 pF for an oscillator frequency in the

4-8 MHz range.

The internal MCU clock frequency (fINT) is divided

by 12 to drive the Timer, the A/D converter and the

Watchdog timer, and by 13 to drive the CPU core,

as may be seen in Figure 11.

With an 8 MHz oscillator frequency, the fastest ma-

chine cycle is therefore 1.625µs.

A machine cycle is the smallest unit of time needed

to execute any operation (for instance, to increment

the Program Counter). An instruction may require

two, four, or five machine cycles for execution.

3.1.1 Main Oscillator

The oscillator configuration may be specified by se-

lecting the appropriate option. When the CRYSTAL/

RESONATORoptionisselected,itmustbeusedwith

a quartz crystal, a ceramic resonator or an external

signal providedon the OSCin pin.When the RCNET-

WORK option is selected, the system clock is gen-

erated by an external resistor.

The main oscillator can be turned off (when the

OSG ENABLED option is selected) by setting the

OSCOFF bit of the ADC Control Register. The

Low Frequency Auxiliary Oscillator is automatical-

ly started.

Figure 8. Oscillator Configurations

INTEGRATED CLOCK

CRYSTAL/RESONATOR option

OSG ENABLED option

OSCin

OSCout

CL1n

CL2

ST6xxx

CRYSTAL/RESONATOR CLOCK

CRYSTAL/RESONATOR option

OSCin

OSCout

ST6xxx

EXTERNAL CLOCK

CRYSTAL/RESONATOR option

OSCin

OSCout

ST6xxx

OSCin

OSCout

RNET

ST6xxx

RC NETWORK

RC NETWORK option

102

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