Fail Safe Mechanisms
C513AO
User’s Manual
8-5
05.99
8.2
Oscillator Watchdog Unit
The Oscillator Watchdog (OWD) unit is used for three functions:
– Monitoring the on-chip oscillator’s function
The watchdog supervises the on-chip oscillator’s frequency. If the frequency 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 (that
is, if the on-chip oscillator has a higher frequency than the RC oscillator), the device executes
a final reset phase of typically 1 ms to allow the oscillator to stabilize. Then, the oscillator
watchdog reset is released and the device resumes program execution.
– Fast internal reset after power-on
The oscillator watchdog unit provides a clock supply for reset before the on-chip oscillator has
started. The oscillator watchdog unit reset works identically to the monitoring function.
– Control of external wake-up from software power-down mode
When power-down mode is terminated by a low level at the INT0 pin, the oscillator watchdog
unit ensures that the microcontroller resumes operation (execution of the power-down wake-
up interrupt) with the nominal clock rate. In power-down mode, the RC oscillator and the on-
chip oscillator are stopped. Both oscillators are started again when power-down mode is
terminated. When the on-chip oscillator has a frequency higher than the RC oscillator, the
microcontroller starts operation after a final delay of typ. 1 ms to allow the on-chip oscillator to
stabilize.
Note: The Oscillator Watchdog unit is always enabled.
