C515C_9711 Datasheet, PDF (213/268 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Fail Save Mechanisms
C515C
8.2 Oscillator Watchdog Unit
The oscillator watchdog unit serves for four 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 typ. 1 ms in order 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
has started. The oscillator watchdog unit also works identically to the monitoring function.
– 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.
– Control of external wake-up from software power-down mode
When the power-down mode is left by a low level at the P3.2/INT0 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 operation after a final delay of typ. 1 ms in order to allow the on-chip
oscillator to stabilize.
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 modifications which have been made for integration of the wake-
up from power down mode capability.
Special Function Register IP0 (Address A9H)
Reset Value : 00H
MSB
LSB
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
The shaded bits are not used for fail save control.
Bit
OWDS
Function
Oscillator Watchdog Timer Status Flag.
Set by hardware when an oscillator watchdog reset occured. Can be
set and cleared by software.
Semiconductor Group
8-6
1997-11-01