then you may have to resort to an external watchdog. Some safety standards insist on an external watchdog, because a Failure Mode Effects Analysis (FMEA) will show you that a watchdog integrated with a processor may have a common failure mode. A typical example is a watchdog which counts clock cycles, so if the processor inadvertantly stops the clock (sleep mode etc), the watchdog is disabled. In addition, an external watchdog provides a signal which can be used to put the whole system into a known, fail-safe state.

These issues are reasons why designers will continue to use external watchdogs, irrespective of how good the internal ones are.

As for brownout detectors, a similar arguement applies. A chip may be specified to run from 2V to 5.5V, so the on-chip brownout detector may be set to 1.99V. It's no use though if the whole system stops working below 3.1V.