AN975 Datasheet, PDF (3/11 Pages) STMicroelectronics – As part of a process of continuous improvement

UPGRADING FROM ST625X/6XB TO ST625X/6XC
Figure 1. Low Voltage Detector Reset
V_DD
V_IT+
V_IT-
V_hyst
RESET
1.1.2 Oscillator Safeguard (OSG)
The Oscillator Safeguard (OSG) feature is a means of dramatically improving the operational
integrity of the MCU.The OSG function is enabled by setting the OSG option bit in the Option
Byte (refer to the Option Bytes section in the device data sheet).
The OSG acts as a filter whose cross-over frequency 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 oscillator failure or for low power consumption)
– Automatically limiting the f_INT clock frequency as a function of supply voltage, to ensure cor-
rect operation even if the power supply drops.
The OSG function is illustrated in Figure 2. For more details on it, please refer to the Clock,
Supply and Reset section in the device data sheet.
Please note that the OSG should be used wherever possible as it provides maximum security
for the application. It should be noted however, that it can increase power consumption and re-
duce the maximum operating frequency to f_OSG (refer to the Electrical Characteristics section
of the device data sheet).
Caution: Care has to be taken when using the OSG, as the internal frequency is defined be-
tween a minimum and a maximum value and may vary depending on both V_DD and tempera-
ture. For precise timing measurements, it is not recommended to use the OSG.
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