SAF-C502-2R20N Siemens Semiconductor Group, SAF-C502-2R20N Datasheet - Page 31

• Manufacturer Part Number: SAF-C502-2R20N
• Description: 8-Bit CMOS Microcontroller
• Manufacturer: Siemens Semiconductor Group

Fast Internal Resest after Power-On
The SAB-C502 can use the oscillator watchdog unit for a fast internal resert procedure after power-
on.
Normally members of the 8051 family enter their default reset state not before the on-chip oscillator
starts. The reason is that the external reset signal must be internally synchronized and processed
in order to bring the device into the correct reset state. Especially if a crystal is used the start up
timed of the oscillator is relatively long (typ. 1 ms). During this time period the pins have an
undefined state which could have severe effects e.g. to actuators connected to port pins.
In the SAB-C502 the oscillator watchdog unit avoids this situation. After power-on the oscillator
watchdog’s RC oscillator starts working within a very short start-up time (typ. less than 2 s). In the
following the watchdog circuitry detects a failure condition for the on-chip oscillator this has not yet
started (a failure is always recognized if the watchdog’s RC oscillator runs faster than the on-chip
oscillator). As long as this condition is valid the watchdog uses the RC oscillator output as a clock
source for the chip rather than the on-chip oscillator’s 16 output. This allows correct resetting of the
part and brings also all ports to the defined state.
Delay between power-on and correct reset state:
Typ:
Max:
Semiconductor Group
18 s
34 s
30
C502