C515A_9708 Datasheet, PDF (144/182 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Fail Safe Mechanisms
C515A
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. If PE/SWD is low and V_AREF is low the
oscillator watchdog is disabled (testmode) !
Semiconductor Group
8-6
1997-08-01