C509-L_97 Datasheet, PDF (225/290 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Fail Save Mechanisms
C509-L
Figure 8-3
Functional Block Diagram of the Oscillator Watchdog
The frequency coming from the RC oscillator is divided by 5 and compared to the on-chip oscillator's
frequency. If the frequency coming from the on-chip oscillator is found lower than the frequency
derived from the RC oscillator the watchdog detects a failure condition (the oscillation at the on-chip
oscillator could stop because of crystal damage etc.). In this case it switches the input of the internal
clock system to the output of the RC oscillator. This means that the part is being clocked even if the
on-chip oscillator has stopped or has not yet started. At the same time the watchdog activates the
internal reset in order to bring the part in its defined reset state. The reset is performed because
clock is available from the RC oscillator. This internal watchdog reset has the same effects as an
externally applied reset signal with the following exception: The watchdog timer status flag WDTS
(IP0.6) is not reset (the Watchdog Timer however is stopped) and bit OWDS is set. This allows the
software to examine error conditions detected by the watchdog timer even if meanwhile an oscillator
failure occurred.
The oscillator watchdog is able to detect a recovery of the on-chip oscillator after a failure. If the
frequency derived from the on-chip oscillator is again higher than the reference the watchdog starts
a final reset sequence which takes typ. 1 ms. Within that time the clock is still supplied by the RC
oscillator and the part is held in reset. This allows a reliable stabilization of the on chip oscillator.
After that, the watchdog toggles the clock supply back to the on-chip oscillator and releases the
reset request. If no external reset is applied in this moment the part will start program execution. If
an external reset is active, however, the device will keep the reset state until also the external reset
request disappears.
Furthermore, the status flag OWDS is set if the oscillator watchdog was active. The status flag can
be evaluated by software to detect that a reset was caused by the oscillator watchdog. The flag
OWDS can be set or cleared by software. An external reset request, however, also resets OWDS
(and WDTS).
Semiconductor Group
8-7
1997-10-01