C515A_9708 Datasheet, PDF (147/182 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Fail Safe Mechanisms
C515A
8.2.2 Fast Internal Reset after Power-On
The C515A can use the oscillator watchdog unit for a fast internal reset procedure after power-on.
Normally the members of the 8051 family (e. g. SAB 80C52) 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 time 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 C515A 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
microseconds). In the following the watchdog circuitry detects a failure condition for the on-chip
oscillator because 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 clock source for the chip. This allows correct resetting of the part and
brings all ports to the defined state (see also chapter 5 of this manual). The delay time between
power-on and correct reset state is max 34 µs (more details see chapter 5.2).
Semiconductor Group
8-9
1997-08-01