C541U_99 Datasheet, PDF (161/192 Pages) Siemens Semiconductor Group – 8-BIT CMOS MICROCONTROLLER

Fail Safe Mechanisms
C541U
8.2.1 Functionality 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 the comparison with the frequency of the on-
chip oscillator.
EWPD
(PCON1.7)
WS
(PCON1.4)
Activity on
USB Bus
P3.2 / INT0
Control
Logic
XTAL1
XTAL2
RC
Oscillator
On-Chip
Oscillator
Start /
Stop
f _RC
3 MHz
Start /
Stop
Power - Down
Mode Activated
Control
Logic
Power-Down Mode
Wake - Up Interrupt
Internal Reset
10 ^{f 1}
Frequency
f ₂ ^Comparator
f₂<f₁
Delay
OWDS
>1
WDCON (C0 _H)
Int. Clock
MCD03385
Figure 8-2
Functional Block Diagram of the Oscillator Watchdog
The frequency coming from the RC oscillator is divided by 10 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 C541U in its defined reset state. The reset
is performed because a clock is available from the RC oscillator. This internal watchdog reset has
the same effects as an externally applied reset signal with the following exceptions: The watchdog
timer status flag WDTS 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 occured.
Semiconductor Group
8-6
1997-10-01