C541U Datasheet, PDF (35/63 Pages) Siemens Semiconductor Group – 8 BIT CMOS MICROCONTROLLER

C541U
Oscillator Watchdog
The oscillator watchdog unit serves for three functions:
– Monitoring of the on-chip oscillator’s function
The watchdog supervises the on-chip oscillator's frequency; if it is lower than the frequency of
the auxiliary RC oscillator in the watchdog unit, the internal clock is supplied by the RC
oscillator and the device is brought into reset; if the failure condition disappears (i.e. the on-
chip oscillator has a higher frequency than the RC oscillator), the part executes a final reset
phase of typ. 1 ms in order to allow the oscillator to stabilize; then the oscillator watchdog reset
is released and the part starts program execution again.
– Fast internal reset after power-on
The oscillator watchdog unit provides a clock supply for the reset before the on-chip oscillator
has started. The oscillator watchdog unit also works identically to the monitoring function.
– Control of external wake-up from software power-down mode (description see chapter 9)
When the power-down mode is left by a low level at the INT0 pin or by the USB, the oscillator
watchdog unit assures that the microcontroller resumes operation (execution of the power-
down wake-up interrupt) with the nominal clock rate. In the power-down mode the RC
oscillator and the on-chip oscillator are stopped. Both oscillators are started again when
power-down mode is released. When the on-chip oscillator has a higher frequency than the
RC oscillator, the microcontroller starts operation after a final delay of typ. 1 ms in order to
allow the on-chip oscillator to stabilize.
Semiconductor Group
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