ST62T15C datasheet(21/105 Pages) STMICROELECTRONICS

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ST6215C/ST6225C

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CLOCK SYSTEM (Cont’d)

5.1.2 Oscillator Safeguard (OSG)

The Oscillator Safeguard (OSG) feature is a

means of dramatically improving the operational

integrity of the MCU. It is available when the OSG

ENABLED option is selected in the option byte (re-

fer to the Option Bytes section of this document).

The OSG acts as a filter whose cross-over fre-

quency is device dependent and provides three

basic functions:

– Filtering spikes on the oscillator lines which

would result in driving the CPU at excessive fre-

quencies

– Management of the Low Frequency Auxiliary

Oscillator (LFAO), (useable as low cost internal

clock source, backup clock in case of main oscil-

lator failure or for low power consumption)

– Automatically limiting the fINT clock frequency as

a function of supply voltage, to ensure correct

operation even if the power supply drops.

5.1.2.1 Spike Filtering

Spikes on the oscillator lines result in an effectively

increased internal clock frequency. In the absence

of an OSG circuit, this may lead to an over fre-

quency for a given power supply voltage. The

OSG filters out such spikes (as illustrated in Figure

10). In all cases, when the OSG is active, the max-

imum internal clock frequency, fINT, is limited to

fOSG, which is supply voltage dependent.

5.1.2.2

Management

Supply

Voltage

Variations

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 frequen-

cy with OSG enabled.

5.1.2.3 LFAO Management

When the OSG is enabled, the Low Frequency

Auxiliary Oscillator can be used (see Section

5.1.3).

Note: The OSG should be used wherever possible

as it provides maximum security for the applica-

tion. It should be noted however, that it can in-

crease power consumption and reduce the maxi-

mum operating frequency to fOSG (see Electrical

Characteristics section).

Caution: Care has to be taken when using the

OSG, as the internal frequency is defined between

a minimum and a maximum value and may vary

depending on both VDD and temperature. For pre-

cise timing measurements, it is not recommended

to use the OSG.

Figure 10. OSG Filtering Function

Figure 11. LFAO Oscillator Function

fOSC

fOSG

fINT

fOSC<fOSG

fOSC>fOSG

MAIN OSCILLATOR

STOPS

MAIN OSCILLATOR

RESTARTS

INTERNAL CLOCK DRIVEN BY LFAO

fOSC

fINT

fLFAO

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