Fresenius Medical Care 4008 E Technical Manual page 471

Oscillator
For being externally connected, the oscillator requires a crystal, two capacitors and a resistor.
Since the internal timers must be set to the correct basic pulse rate, the frequency of the crystal
must amount to 4.096 MHz.
Watchdog
The watchdog output of the gate array is set to H level by the program. Every millisecond, the
integrated watchdog circuit sets the output level to L level, thus releasing an interrupt at the input
INT0 of the microcontroller.
During normal operation, the microcontroller now performs an interrupt routine, during which the
watchdog circuit in the gate array is reset. This causes the watchdog output to return to H level.
The base of FET T2 is likewise connected to the watchdog signal via a lowpass filter. The time
constant of the lowpass filter is selected such that, during normal operation, the 1-ms interrupt
causes transistor T1 to become conductive.
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Fresenius Medical Care 4008 4/09.03 (TM)