I did not see VDD ramp up time requirement for some EFM8 device in the datasheet, do you made some change.
Could you summarize some changes for these new EFM8 devices?
The VDD ramp up time limitation was removed for EFM8UB1/EFM8LB/EFMBB1/EFMBB2 /EFMBB3/C8051F85x. But the delay time was prolonged. EFM8UB2/EFM8SB1/EFM8SB2 still need VDD ramp up to a specific level within specific time. You could see the detailed requirement in the datasheet.
The watchdog peripheral was also changed. EFM8UB1/EFM8LB/EFMBB1/EFMBB2 /EFMBB3/C8051F85x use a separate watchdog peripheral and the initial timeout period was prolonged. You may not encounter the issue listed in this KB for these devices.
For EFM8SB1/EFM8UB1/EFM8LB/EFMBB1/EFMBB2 /EFMBB3/C8051F85x device, DEVICEID/DERIVID was added as SFR register and firmware could access them. For old 8-bit MCU like EFM8UB2/EFM8SB2 these information could only be accessed by C2 interface.
UUID was also added for EFM8LB1/EFM8BB2/EFM8BB3/EFM8UB1 devices. The UUID resides in the read-only area of flash memory, which cannot be erased or written by the end application. The UUID can be read by firmware or through the debug interface at flash locations 0xFFC0-0xFFCF.