Siemens EF 88H Series Manual de usuario descargar pdf (Pagina 33)

Semiconductor Group 4 - 8

4.3 Fast internal Reset after Power-On

The SAB 80C517A can use the oscillator watchdog unit for a fast internal reset procedure after

power-on.

Figure 4-4 shows the power-on sequence under control of the oscillator watchdog.

Normally the devices of the 8051 family (like the SAB 80C517) 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. 1ms). During this time period

the pins have an undefined state which could have severe effects especially to actuators connected

to port pins.

In the SAB 80C517A the oscillator watchdog unit can avoid this situation. For doing this, the

oscillator watchdog must be enabled. In this case, 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 detected the watchdog uses the RC oscillator output

as clock source for the chip rather than the on-chip oscillator’s output. This allows correct resetting

of the part and brings also all ports to the defined state (see figure 4-4, I). The time period from

power-on till reaching the reset state at the ports adds from the following terms:

– RC oscillator start-up < 2 µs

– synchronization of the RC oscillators divider-by-5 < 6 T

– synchronization of the state and cycle counters < 6 T

– reset procedure till correct port states are reached < 12T

Delay between power-on and correct reset state:

Typ.: 18 µs

Max.: 34 µs

System Reset

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Siemens EF 88H Series Manual de usuario Pagina 33