ST62T35B datasheet(18/82 Pages) STMICROELECTRONICS

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ST62T35B/E35B

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. In addition, a Low Frequen-

cy Auxiliary Oscillator (LFAO) can be switched in

for security reasons, to reduce power consump-

tion, or to offer the benefits of a back-up clock sys-

tem.

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 op-

eration. 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 or the lowest cost so-

lution using only the LFAO. CL1 an CL2 should have

a capacitance in the range 12 to 22 pF for an os-

cillator frequency in the 4-8 MHz range.

The internal MCU clock frequency (fINT) is divided

by 12 to drive the Timer and the Watchdog timer,

and by 13 to drive the CPU core, while the A/D

converter is driven by fINT divided either by 6 or by

12 as may be seen inFigure 11..

With an 8MHz oscillator frequency, the fastest

machine cycle is therefore 1.625

µs.

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 main oscillator can be turned off (when the

OSG ENABLED option is selected) by setting the

OSCOFF bit of the OSCR Control Register. The

Low Frequency Auxiliary Oscillator is automatical-

ly started.

Figure 8. Oscillator Configurations

INTEGR ATED CLOCK

OSG ENABLED option

OSCin

OSCout

CL1n

CL2

ST6xxx

CRYSTAL/RESON ATOR CLOCK

OSCin

OSCout

ST6xxx

EXTER NAL CLOCK

OSCin

OSCout

ST6xxx

VA0016

VA0015A

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