Siemens EF 88H Series Manual de usuario descargar pdf (Pagina 119)

Device Specification

Semiconductor Group 7-57

Watchdog Units

The SAB 80C517A offers two enhanced fail safe mechanisms, which allow an automatic

recovery from hardware failure or software upset:

– programmable watchdog timer (WDT), variable from 512 µs up to appr. 1.1 s time-out

period @12 MHz. Upward compatible to SAB 80515 watchdog.

– oscillator watchdog (OWD), monitors the on-chip oscillator and forces the micro-

controller into reset state, in case the on-chip oscillator fails, controls the restart from

the Hardware Power Down Mode and provides clock for a fast internal reset after power-on.

Programmable Watchdog Timer

The WDT can be activated by hardware or software.

Hardware initialization is done when pin PE/SWD (Pin 4) is held high during RESET. The

SAB 80C517A then starts program execution with the WDT running. Since Pin PE/SWD is

only sampled during Reset (and hardware power down at parts with stepping code AD and

later) dynamical switching of the WDT is not possible.

Software initialization is done by setting bit SWDT.

A refresh of the watchdog timer is done by setting bits WDT and SWDT consecutively.

A block diagram of the watchdog timer is shown in figure 11.

When a watchdog timer resest occurs, the watchdog timer keeps on running, but a status flag

WDTS is set. This flag can also be cleared by software.

Figure 11

Block Diagram of the Programmable Watchdog Timer

1 2 ... 114 115 116 117 118 119 120 121 122 123 124 ... 136 137

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Siemens EF 88H Series Manual de usuario Pagina 119