Datasheet

ST6200C/ST6201C/ST6203C
20/100
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 f
INT
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, f
INT
, is limited to
f
OSG
, which is supply voltage dependent.
5.1.2.2 Management of 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 V
DD
),
and below f
OSG
: 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 f
OSG
(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 V
DD
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
f
OSC
f
OSG
f
INT
f
OSC<
f
OSG
f
OSC>
f
OSG
MAIN OSCILLATOR
STOPS
MAIN OSCILLATOR
RESTARTS
INTERNAL CLOCK DRIVEN BY LFAO
f
OSC
f
INT
f
LFAO
1