AN975 Datasheet, PDF (4/11 Pages) STMicroelectronics – As part of a process of continuous improvement

UPGRADING FROM ST625X/6XB TO ST625X/6XC
Figure 2. OSG Filtering Function
f_OSC>f_OSG
f_OSC
f_OSG
f_OSC<f_OSG
f_INT
1.1.2.1 Low Frequency Auxiliary Oscillator (LFAO)
The Low Frequency Auxiliary Oscillator has three main purposes. Firstly, it can be used to re-
duce power consumption in non timing critical routines. Secondly, it offers a fully integrated
system clock, without any external components. Lastly, it acts as a backup oscillator in case of
main oscillator failure.
This oscillator is available when the OSG option is selected in the option byte. In this case, it
automatically starts one of its periods after the first missing edge of the main oscillator, what-
ever the reason for the failure (main oscillator defective, no clock circuitry provided, main os-
cillator switched off...). See Figure 3.
Please note that the A/D converter accuracy is decreased, since the internal frequency is
below 1.2 MHz.
Figure 3. LFAO Oscillator Function
f_OSC
f_LFAO
f_INT
MAIN OSCILLATOR
STOPS
MAIN OSCILLATOR
RESTARTS
INTERNAL CLOCK DRIVEN BY LFAO
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