C515C_9711 Datasheet, PDF (214/268 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Fail Save Mechanisms
C515C
EWPD
(PCON1.7)
P3.2/
Control
INT0
Logic
RC
Oscillator
Start/
Stop
f _RC
3 MHz
XTAL1
XTAL2
Start/
Stop
On-Chip
Oscillator
Power-Down
Mode Activated
Control
Logic
Power-Down Mode
Wake-Up Interrupt
Internal
Reset
÷2 ÷5 ^{f 1}
Frequency
Comparator
f ₂ < f _{1 Delay}
1
f₂
OWDS
IP0 (A9H)
Internal
Clock
MCB02757
Figure 8-3
Functional Block Diagram of the Oscillator Watchdog
The frequency coming from the RC oscillator is divided by 2 and 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 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.
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.
Semiconductor Group
8-7
1997-11-01