Integrating a low voltage 3V MCU EFM8 from Silicon Labs to 5 volt sytem. Link here (PDF)
When using a 3 V device in a 5 V system, the user must consider:
• A 3 V power supply must be provided.
• A 5 V device driving a 3 V input.
• A 3 V device driving a 5 V input.