AT89C51RD2 (Based on these datasheet extracts), seems to have all three abilities of a supervisor IC. Moreover we have that PCA WDT feature for those who are allergic to noise/glitch/ESD. Now, Except for the fact that the internal watchdog is 'software programmed' and supposedly not as good as a hardware one* is there any other reason not to replace the supervisor IC in an AT89C51RD2 system with an RC?(Tantalium C)

. Brownout detection:
_{... Integrated Power Monitor (POR/PFD) to Supervise Internal Power Supply...
... The POR/PFD function monitors the internal power-supply of the CPU core memories and the peripherals, and if needed, suspends their activity when the internal power supply falls below a safety threshold. This is achieved by applying an internal reset to them....
... If the power falls below the Power Fail Detect threshold level, the Reset will be applied immediately....
... If the internal power supply falls below a safety level, a reset is immediately asserted...
... The Power fail detect monitor the supply generated by the voltage regulator and generate a reset if this supply falls below a safety threshold...}

. Reset generation:
_{... The reset sources are: Power Management, Hardware Watchdog, PCA Watchdog and Reset input...
... By generating the Reset the Power Monitor insures a correct start up when AT89C51RD2/ED2 is powered up. ...
... When the power is applied, the Power Monitor immediately asserts a reset. Once the internal supply after the voltage regulator reach a safety level, the power monitor then looks at the XTAL clock input. The internal reset will remain asserted until the Xtal1 levels are above and below VIH and VIL. Further more. An internal counter will count 1024 clock periods before the reset is de-asserted...}

. Watchdog timer:
<sub>... Hardware Watchdog Timer (One-time Enabled with Reset-Out)...*

* Never use programmable watchdogs or brownout monitors. Because programmable devices can have their programs corrupted by interference, programmable devices must not be used for watchdog or power monitor functions.
Design Techniques for EMC – Part 1
** Don't know why Atmel calls it "hardware" watchdog!</sub>