In most cases, the internal VDD monitor is sufficient and an external VDD monitor is not needed, unless the application has special requirements that call for an external VDD monitor. Whenever the internal VDD monitor is used, care should be taken with the 8-bit devices to make sure the monitor is always enabled as a reset source and is never accidentally disabled by a write to RSTSRC.
The C8051F12x or C8051F36x devices operating above 50 MHz have a minimum supply voltage specification of 3.0 V. However, the internal VDD monitor is calibrated to 2.7 V typical. In this instance, an external VDD monitor may be required.
External VDD Monitor