After WKUP I want configure PA0 for output, then at some point configure for WKUP and go into STANDBY deep sleep mode. It appears that once the EWUP bit is set in RCC_CSR a) the EWUP bit cannot be reset unless there is a power on reset, and b) coming out of reset configuring PA0 for output has no effect (PA0 remains low). For a power on reset PA0 configures for output and works correctly, and the setup and execution to go to STANDBY works correctly, except that PA0 no longer seems to be available for output even though it is setup as before. The only difference is that the EWUP bit is now on and I haven't found a way to turn it off (though RM0008 says it is a software rw bit). This makes me think that dual use of PA0 for WKUP and gpio output is not possible.