I have code running on custom hardware using a EFM32GG arm device, and while attempting to use tickless , coupled with EM2 to achieve max current saving I’ve come across a problem when returning from EM2 sleep. As part of the main loop a simple on/off led toggling with delay is used, a running counter is incremented and output via serial port running at 57600. After return from EM2 the first two or three bytes of the serial message are corrupt, but following bytes are correct. The code for EM2 does a restore to the main HF clock which in this case is a 16MHz external XTAL. It still looks like it could be a clock speed issue, any thoughts?

The EFM parts wake from sleep very quickly by temporarily running on a slower clock instead of waiting for the PLL to lock. Sounds like that could be your issue.