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Device Specification

Semiconductor Group 7-58

Oscillator Watchdog

The unit serves three functions:

– Monitoring of the on-chip oscillator’s function.

The watchdog supervises the on-chip oscillator’s frequency; if it is lower than the

frequency of the auxiliary RC oscillator in the watchdog unit, the internal clock is

supplied by the RC oscillator and the device is forced into reset; if the failure condition

disappears (i.e. the on-chip oscillator has again a higher frequency than the RC oscillator),

the part executes a final reset phase of appr. 0.25 ms in order to allow the oscillator

to stabilize; then the oscillator watchdog reset is released and the part starts

program execution again.

– Restart from the Hardware Power Down Mode.

If the Hardware Power Down Mode is terminated the oscillator watchdog has to control

the correct start-up of the on-chip oscillator and to restart the program. The oscillator

watchdog function is only part of the complete Hardware Power Down sequence; however,

the watchdog works identically to the monitoring function.

– Fast internal reset after power-on.

In this function the oscillator watchdog unit provides a clock supply for the reset before

the on-chip oscillator has started. In this case the oscillator watchdog unit also

works identically to the monitoring function.

If the oscillator watchdog unit is to be used it must be enabled (this is done by applying high

level to the control pin OWE).

Figure 12 shows the block diagram of the oscillator watchdog unit. It consists of an internal RC

oscillator which provides the reference frequency of the on-chip oscillator. The RC oscillator

can be enabled/disabled by the control pin OWE. If it is disabled the complete unit has no

function.

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Siemens EF 88H Series Bedienungsanleitung Seite 120