C508_01 Datasheet, PDF (236/267 Pages) Infineon Technologies AG – 8-Bit CMOS Microcontroller

C508
Fail Save Mechanisms
8.2.2 Fast Internal Reset after Power-On
The C508 can use the Oscillator Watchdog unit for a fast internal reset procedure after
power-on.
Normally, the members of the 8051 family (for example: SAB 80C52) do not enter their
default reset state before the on-chip oscillator starts. The reason is that the external
reset signal must be internally synchronized and processed to bring the device into the
correct reset state. The startup time of the oscillator can be relatively long (typ. 10 ms),
especially if a crystal is used. During this time period, the pins have an undefined state
which could have severe effect on such things as actuators connected to port pins.
The Oscillator Watchdog unit avoids this situation in the C508. After power-on, the
oscillator Watchdog’s RC oscillator starts working within a very short startup time (typ.
less than 2 µs). The Watchdog circuitry then detects a failure condition for the on-chip
oscillator because it has not yet started (a failure is always recognized if the Watchdog’s
RC oscillator runs faster than the gated PLL clock output, as described in the previous
section). As long as this condition is valid, the Watchdog uses the RC oscillator output
as the clock source for the chip. This allows the chip to be correctly reset and brings all
ports to the defined state. The exception is Port 1, which will be at its default state when
external reset is active (see also Chapter 5 of this manual).
User’s Manual
8-10
2001-05