The values in the enumeration am_hal_clkgen_clkout_e in SDK2.1.0 do not match Apollo3's datasheet 0.9.1. Thus the CLKOUT does not function as expected. How can this be worked around?
Setting "AM_HAL_CLKGEN_CLKOUT_LFRC_512 = 0x23" and "AM_HAL_CLKGEN_CLKOUT_LFRC_0_0004 = 0x31" in the enumeration will correct all settings. These fixes will be in the next SDK release.
This fix and other intended fixes are listed in the KB article AmbiqSuite SDK - Known Defects and Fixes.