User’s Manual
8-6
05.99
Fail Safe Mechanisms
C513AO
8.2.1 Detailed Description of the Oscillator Watchdog Unit
Figure 8-2 shows the block diagram of the Oscillator Watchdog unit. It consists of an internal RC
oscillator which provides the reference frequency for comparison with the frequency of the on-chip
oscillator.
Figure 8-2
Functional Block Diagram of the Oscillator Watchdog
The frequency coming from the RC oscillator is divided by five and compared to the on-chip
oscillator’s frequency. If the frequency coming from the on-chip oscillator is lower than that derived
from the RC oscillator, the watchdog detects a failure condition (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 device 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 to bring the device into its defined reset state. The reset is performed because a clock
is available from the RC oscillator. This internal watchdog reset has the same effect as an externally
applied reset signal with the following exceptions: The Watchdog Timer Status flag WDTS is not
reset (however, the Watchdog Timer is stopped); and bit OWDS is set. This allows the software to
examine error conditions detected by the Watchdog Timer if an oscillator failure occurs.