SAB80C515A-5 Datasheet, PDF (32/108 Pages) Siemens Semiconductor Group – 8-Bit CMOS Single-Chip Microcontroller Family

System Reset
After the on-chip oscillator finally has started, the oscillator watchdog detects the correct function;
then the watchdog still holds the reset active for a time period of 768 cycles of the RC oscillator in
order to allow the oscillation of the on-chip oscillator to stabilize (figure 4-4, II). Subsequently the
clock is supplied by the on-chip oscillator and the oscillator watchdog’s reset request is released
(figure 4-4, III). However, an externally applied reset still remains active (figure 4-4, IV) and the
device does not start program execution (figure 4-4, V) before the external reset is also released.
Although the oscillator watchdog provides a fast internal reset it is additionally necessary to apply
the external reset signal when powering up. The reasons are as follows:
– Termination of Hardware Power Down Mode (a HWPD signal is overridden by reset)
– Termination of Software Power Down Mode
– Reset of the status flag OWDS that is set by the oscillator watchdog during the power up
sequence.
The external reset signal must be hold active at least until the on-chip oscillator has started and the
internal watchdog reset phase is completed. An external reset time of more than 50 µs should be
sufficient in typical applications. If only a capacitor at pin Reset is used a value of less than 100 nF
provides the desired reset time.
Semiconductor Group
4-9