SAB80C517-M16 SIEMENS [Siemens Semiconductor Group], SAB80C517-M16 Datasheet - Page 145

• Manufacturer Part Number: SAB80C517-M16
• Description: 8-Bit CMOS Single-Chip Microcontroller
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

On-Chip Peripheral Components
7.8.2
Oscillator Watchdog
What happens in a microcontroller system it the controller’s on-chip oscillator stops working? This
failure e.g. caused by a broken crystal, an open connection to the crystal, or a long-term disturbance
normally leaves the system in a random, undetermined state. The SAB 80C517 provides a "fail-
safe" reaction upon an oscillator failure. lf the on-chip oscillator frequency falls below a certain limit
due to a hardware defect, the oscillator watchdog initiates an internal reset. This reset state is
maintained until the on-chip oscillator is working again. This ensures a maximum of system
protection with a minimum of susceptibility to distortion or to operating errors.
In the reset state all port pins of the SAB 80C517 show a ’1’.
The oscillator watchdog consists of an integrated RC oscillator combined with a frequency
comparator. lf the on-chip oscillator’s frequency falls below the frequency of the RC oscillator, the
comparator generates a signal which initiates a reset.
The RC oscillator runs with a frequency of typically 300 kHz and works without any external
components. lt also determines, as long as it is used, the lower limit of the SAB 80C517’s operating
frequency, which is therefore specified at 1 MHz.
Since the frequency comparator of the oscillator watchdog takes its inputs directly from the on-chip
oscillator, the minimum frequency of 1 MHz does not restrict the use of the slow-down mode. In this
mode the CPU runs with one eighth of the normal clock rate (see section 7.7).
The oscillator watchdog circuitry can be enabled externally. lf the OWE pin (oscillator watchdog
enable) is pulled low, the oscillator watchdog function is off. lf the pin is left unconnected or has a
logic high level, the watchdog oscillator is activated. Thus, the watchdog is enabled even if the pin
or the path to the pin is broken.
Like the watchdog timer circuitry, the oscillator watchdog circuitry contains a status flip-flop. This
flip-flop is set when an oscillator failure is detected and it is cleared by an external HW reset or by
software (see figure 7-62).
The block diagram in figure 7-64 illustrates the function of the oscillator watchdog. Note that the
OWD reset request is held for at least three additional cycles after the on-chip oscillator returns to
normal operation. This is done to ensure a proper oscillator startup.
Semiconductor Group
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