C509-L_97 Datasheet, PDF (224/290 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Fail Save Mechanisms
C509-L
8.2 Oscillator Watchdog
The C509-L has a oscillator watchdog unit that is fully compatible with the SAB 80C517A's oscillator
watchdog.
The oscillator watchdog unit serves three functions:
– Monitoring of the on-chip oscillator's function.
The watchdog supervises the on-chip oscillator's frequency; if it is lower than the frequency
of the auxiliary RC oscillator in the watchdog unit, the internal clock is supplied by the RC
oscillator and the device is brought into reset; if the failure condition disappears (i.e. the on-
chip oscillator has a higher frequency than the RC oscillator), the part executes a final reset
phase of appr. 0.5 ms in order to allow the oscillator to stabilize; then the oscillator watchdog
reset is released and the part starts program execution again.
– Restart from the hardware power down mode.
If the hardware power down mode is terminated the oscillator watchdog has to control the
correct start-up of the on-chip oscillator and to restart the program. The oscillator watchdog
function is only part of the complete hardware power down sequence; however, the watchdog
works identically to the monitoring function.
– Fast internal reset after power-on.
In this function the oscillator watchdog unit provides a clock supply for the reset before the on-
chip oscillator has started. In this case the oscillator watchdog unit also works identically to
the monitoring function.
If the oscillator watchdog unit shall be used it must be enabled (this is done by applying high level
to the control pin OWE).
Figure 8-3 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. The RC oscillator can be enabled/disabled by the control pin OWE. If it is disabled
the complete unit has no function.
The SFR IP0 contains a status flag of the oscillator watchdog as shown below.
Special Function Register IP0 (Address A9H)
Reset Value : 00H
Bit No. 7
6
5
4
3
2
1
0
A9H OWDS WDTS IP0.5 IP0.4 IP0.3 IP0.2 IP0.1 IP0.0 IP0
Bit
OWDS
The shaded bits are not used in controlling the oscillator watchdog.
Function
Oscillator watchdog timer status flag
Set by hardware when an oscillator watchdog reset occurred.
Can be cleared or set by software.
Semiconductor Group
8-6
1997-10-01