C505_9708 SIEMENS [Siemens Semiconductor Group], C505_9708 Datasheet - Page 195

• Manufacturer Part Number: C505_9708
• Description: 8-Bit CMOS Microcontroller
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

Figure 8-3
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 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 is, however, stopped); and bit OWDS is
set. This allows the software to examine error conditions detected by the Watchdog unit 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
Semiconductor Group
P4.1 / RXDC
P3.2 / INT0
XTAL1
XTAL2
EWPD
(PCON1.7)
Oscillator
Oscillator
On-Chip
Control
Logic
RC
WS
(PCON1.4)
f
3 MHz
Start /
Stop
Start /
Stop
RC
10
Mode Activated
Power - Down
f
OWDS
f
2
1
Comparator
Frequency
8-7
f
2
f <
1
Control
Delay
Logic
Fail Save Mechanisms
Power-Down Mode
Wake - Up Interrupt
Internal Reset
>1
IP0 (A9 )
C505 / C505C
Int. Clock
H
1997-08-01
MCB03308