C508_01 Datasheet, PDF (233/267 Pages) Infineon Technologies AG – 8-Bit CMOS Microcontroller

C508
Fail Save Mechanisms
8.2
Oscillator Watchdog Unit
The Oscillator Watchdog unit serves for three functions:
– Monitoring 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 (that is, the on-chip oscillator has a higher frequency than the
RC oscillator), the part executes a final reset phase of typically 1 ms to allow the
oscillator to stabilize; then, the Oscillator Watchdog reset is released and the part
starts program execution again.
– Fast internal reset after power-on
The Oscillator Watchdog unit provides a clock supply for the reset before the on-
chip oscillator and the PLL have started. This is described in Chapter 5.2.
– Control of external wake-up from software power-down mode
When the software power-down mode is terminated by a low level at pins P3.2/INT0
or P5.7/INT7, the Oscillator Watchdog unit ensures that the microcontroller
resumes operation (execution of the power-down wake-up interrupt) with the
nominal clock rate. The RC oscillator, the on-chip oscillator, and the PLL are
stopped in power-down mode. They are started again when power-down mode is
terminated. After the on-chip oscillator is stable and the PLL has been locked, the
microcontroller starts program execution.
Note: The Oscillator Watchdog unit is always enabled.
Special Function Register IP0 (Address A9_H)
Reset Value: 00_H
MSB
LSB
Bit No. 7
6
5
4
3
2
1
0
A9_H OWDS WDTS IP0.5 IP0.4 IP0.3 IP0.2 IP0.1 IP0.0 IP0
The shaded bits are not used for fail save control.
Bit
OWDS
Function
Oscillator Watchdog Status flag
Set by hardware when an Oscillator Watchdog reset occurrs.
Can be set and cleared by software.
User’s Manual
8-7
2001-05