How should I connect the microcontroller pins in a system with a split ground plane?
If complete isolation between an 'Analog' and 'Digital' section of a board is required, the microcontroller should be placed entirely on one of the two ground planes. Which ground plane to place the microcontroller on depends on how it is being used in the system:
For devices with both an AGND and DGND pin (like the C8051F12x family):
Although AGND is the return-current path for the analog supplies (AV+) and DGND is the return-current path for the digital supplies (VDD), the AGND and DGND pins of a device are not isolated internally. Both signals are connected electrically to the substrate of the device, and must be connected on the PCB as well. In a system with a spilt ground plane, it is recommended that the AGND and DGND signals should be connected together at a single point on the PCB close to the controller.
Do not isolate the AGND and DGND pins on the device from each other. This includes using an inductor, ferrite bead, or any other form of isolation between these pins. Doing so will cause current to flow through the substrate, which can lead to poor analog performance and damage to the device.
See application note AN203 for more details. Application notes may be downloaded from https://www.silabs.com/products/mcu/Pages/ApplicationNotes.aspx