Q: 

When I write to a control register in a peripheral and try to read back the results, I only get 0's back. What is wrong?

A:

By default all peripheral clocks are disabled in the EFM32. To use a peripheral, you must first enable the clock for it in the CMU_HFPERCLKEN0, HFCORECLKEN0, LFACLKEN or LFBCLKEN. If you are using one of the low frequency peripherals, remember to also enable the LE clock in CMU_HFCORECLKEN0 in addition to the LF clock for the peripheral.