ST62T35B datasheet(19/82 Pages) STMICROELECTRONICS

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ST62T35B/E35B

CLOCK SYSTEM (Cont’d)

Turning on the main oscillator is achieved by re-

setting the OSCOFF bit of the OSCR Register or

by resetting the MCU. Restarting the main oscilla-

tor implies a delay comprising the oscillator start

up delay period plus the duration of the software

instruction at fLFAO clock frequency.

3.1.2 Low

Frequency

Auxiliary

Oscillator

(LFAO)

The Low Frequency Auxiliary Oscillator has three

main purposes. Firstly, it can be used to reduce

power consumption in non timing critical routines.

Secondly, it offers a fully integrated system clock,

without any external components. Lastly, it acts as

a safety oscillator in case of main oscillator failure.

This oscillator is available when the OSG ENA-

BLED option is selected. In this case, it automati-

cally starts one of its periods after the first missing

edge from the main oscillator, whatever the rea-

son (main oscillator defective, no clock circuitry

provided, main oscillator switched off...).

User code, normal interrupts, WAIT and STOP in-

structions, are processed as normal, at the re-

duced fLFAO frequency. The A/D converter accu-

racy is decreased, since the internal frequency is

below 1MHz.

At power on, the Low Frequency Auxiliary Oscilla-

tor starts faster than the Main Oscillator. It there-

fore feeds the on-chip counter generating the

POR delay until the Main Oscillator runs.

The Low Frequency Auxiliary Oscillator is auto-

matically switched off as soon as the main oscilla-

tor starts.

OSCR

Address: 0DBh

—

Read/Write

Bit 7-1= These bits are not used and must be kept

cleared after reset.

Bit 0 = OSCOFF.

Main oscillator turn-off. When

low, this bit enables main oscillator to run. The

main oscillator is switched off when OSCOFF is

high.

3.1.3 Oscillator Safe Guard

The Oscillator Safe Guard (OSG) affords drasti-

cally increased operational integrity in ST62xx de-

vices. The OSG circuit provides three basic func-

tions: it filters spikes from the oscillator lines which

would result in over frequency to the ST62 CPU; it

gives access to the Low Frequency Auxiliary Os-

cillator (LFAO), used to ensure minimum process-

ing in case of main oscillator failure, to offer re-

duced power consumption or to provide a fixed

frequency low cost oscillator; finally, it automati-

cally limits the internal clock frequency as a func-

tion of supply voltage, in order to ensure correct

operation even if the power supply should drop.

The OSG is enabled or disabled by choosing the

relevant OSG option. It may be viewed as a filter

whose cross-over frequency is device dependent.

Spikes on the oscillator lines result in an effective-

ly increased internal clock frequency. In the ab-

sence of an OSG circuit, this may lead to an over

frequency for a given power supply voltage. The

OSG filters out such spikes (as illustrated inFig-

ure 9.). In all cases, when the OSG is active, the

maximum internal clock frequency, fINT, is limited

to fOSG, which is supply voltage dependent. This

relationship is illustrated inFigure 12..

When the OSG is enabled, the Low Frequency

Auxiliary Oscillator may be accessed. This oscilla-

tor starts operating after the first missing edge of

the main oscillator (see Figure 10.).

Over-frequency, at a given power supply level, is

seen by the OSG as spikes; it therefore filters out

some cycles in order that the internal clock fre-

quency of the device is kept within the range the

particular device can stand (depending on VDD),

and below fOSG: the maximum authorised fre-

quency with OSG enabled.

Note. The OSG should be used wherever possi-

ble as it provides maximum safety. Care must be

taken, however, as it can increase power con-

sumption and reduce the maximum operating fre-

quency to fOSG.

---

OSC

OFF

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