C505_9708 SIEMENS [Siemens Semiconductor Group], C505_9708 Datasheet - Page 194

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

8.2
The oscillator watchdog unit serves for three functions:
Note: The oscillator watchdog unit is always enabled.
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. It also shows the additional provisions for integration of the wake-up from power
down mode.
Special Function Register IP0 (Address A9 H )
Bit
OWDS
Semiconductor Group
– Monitoring of the on-chip oscillator's function
– Fast internal reset after power-on
– Control of external wake-up from software power-down mode
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, in order to allow the
oscillator to stabilize, executes a final reset phase of typ. 1 ms; then the oscillator watchdog
reset is released and the part starts program execution from address 0000 H again.
The oscillator watchdog unit provides a clock supply for the reset before the on-chip oscillator
has started. The oscillator watchdog unit also works identically to the monitoring function.
When the power-down mode is left by a low level at the P3.2/INT0 pin or the P4.1/RXDC pin,
the oscillator watchdog unit assures that the microcontroller resumes operation (execution of
the power-down wake-up interrupt) with the nominal clock rate. In the power-down mode the
RC oscillator and the on-chip oscillator are stopped. Both oscillators are started again when
power-down mode is released. When the on-chip oscillator has a higher frequency than the
RC oscillator, the microcontroller starts program execution by processing a power down
interrupt after a final delay of typ. 1 ms in order to allow the on-chip oscillator to stabilize.
Bit No.
Oscillator Watchdog Unit
A9 H
The shaded bits are not used for fail save control.
OWDS WDTS
MSB
7
6
Function
Oscillator Watchdog Status Flag.
Set by hardware when an oscillator watchdog reset occured. Can be
set and cleared by software.
IP0.5
5
IP0.4
4
8-6
IP0.3
3
IP0.2
2
IP0.1
1
Fail Save Mechanisms
IP0.0
LSB
0
Reset Value : 00 H
C505 / C505C
IP0
1997-08-01