C517A_99 Datasheet, PDF (196/218 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Fail Safe Mechanisms
C517A
8.2 Oscillator Watchdog Unit
The oscillator watchdog unit serves for four 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.
– Restart from the hardware power down mode.
If the hardware power down mode is terminated the oscillator watchdog has to control the
correct start-up of the on-chip oscillator and to restart the program. The oscillator watchdog
function is only part of the complete hardware power down sequence; however, the watchdog
works identically to the monitoring function.
Note: The oscillator watchdog unit is always enabled.
Semiconductor Group
8-6